promoting stock recovery through the … promoting stock recovery through the standardisation of...
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i
Promoting stock recovery through the standardisation
of fishing gear: streamlining the hauling net sector of
South Australia’s Garfish Fishery.
M.A. Steer, R. McGarvey, A.J. Fowler, W.B. Jackson and M.T Lloyd
SARDI Publication No. F2011/000412-1 SARDI Research Report Series No. 578
SARDI Aquatic Sciences PO Box 120 Henley Beach SA 5022
September 2011
Report to PIRSA Fisheries and Aquaculture
ii
Promoting stock recovery through the standardisation
of fishing gear: streamlining the hauling net sector of
South Australia’s Garfish Fishery.
Report to PIRSA Fisheries and Aquaculture
M.A. Steer, R. McGarvey, A.J. Fowler, W.B. Jackson and M.T Lloyd
SARDI Publication No. F2011/000412-1 SARDI Research Report Series No. 578
September 2011
iii
This Publication may be cited as: Steer, M.A., McGarvey, R., Fowler, A.J., Jackson, W.B. and Lloyd, M.T (2011). Promoting stock recovery through the standardisation of fishing gear: streamlining the hauling net sector of South Australia‟s Garfish Fishery. Report to PIRSA Fisheries and Aquaculture. South Australian Research and Development Institute (Aquatic Sciences), Adelaide. SARDI Publication No. F2011/000412-1. SARDI Research Report Series No. 578. 55pp.
South Australian Research and Development Institute SARDI Aquatic Sciences 2 Hamra Avenue West Beach SA 5024 Telephone: (08) 8207 5400 Facsimile: (08) 8207 5406 http://www.sardi.sa.gov.au
DISCLAIMER
The authors warrant that they have taken all reasonable care in producing this report. The report has been through the SARDI Aquatic Sciences internal review process, and has been formally approved for release by the Chief, Aquatic Sciences. Although all reasonable efforts have been made to ensure quality, SARDI Aquatic Sciences does not warrant that the information in this report is free from errors or omissions. SARDI Aquatic Sciences does not accept any liability for the contents of the report or for any consequences arising from its use or any reliance placed upon it.
© 2011 SARDI
This work is copyright. Apart from any use as permitted under the Copyright Act 1968 (Cth), no part may be reproduced by any process, electronic or otherwise, without the specific written permission of the copyright owner. Neither may information be stored electronically in any form whatsoever without such permission. Printed in Adelaide: October 2011 SARDI Publication No. F2011/000412-1 SARDI Research Report Series No. 578 Author(s): M.A. Steer, R. McGarvey, A.J. Fowler, W.B. Jackson and M.T Lloyd Reviewer(s): A. Linnane and C. Dixon Approved by: Assoc Prof T. Ward Principal Scientist – Wild Fisheries Signed: Date: 7 October 2011 Distribution: PIRSA Fisheries and Aquaculture, SAASC Library, University of Adelaide Library,
Parliamentary Library, State Library and National Library Circulation: Public Domain
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Table of Contents
Table of Contents ..................................................................................................... iv List of Figures ............................................................................................................ v List of Tables ............................................................................................................ vi Acknowledgements .................................................................................................. vii Executive Summary ................................................................................................. viii 1 General Introduction .......................................................................................... 1
1.1 Description of the fishery ............................................................................. 1 1.2 Hauling net sector........................................................................................ 2 1.3 Need............................................................................................................ 3 1.4 Aims and objectives ..................................................................................... 4
2 Methods ............................................................................................................. 6 2.1 Experimental design .................................................................................... 6 2.2 Mesh measurements ................................................................................... 9 2.3 Age analysis .............................................................................................. 10 2.4 Fisher survey ............................................................................................. 10 2.5 Statistical analysis ..................................................................................... 10 2.6 Model simulations ...................................................................................... 11
3 Results ............................................................................................................. 13 3.1 Sample details ........................................................................................... 13 3.2 Variation in mesh size ............................................................................... 13 3.3 Hauling net characteristics (wing vs. pocket) ............................................. 15 3.4 Garfish size and age selectivity ................................................................. 15
3.4.1 Variation in the size and age structures .............................................. 15 3.4.2 Variation in growth and condition ........................................................ 16 3.4.3 Selectivity of the 30 mm standard knot pocket (30SK) ........................ 18 3.4.4 Selectivity of the 32 mm standard knot pocket (32SK) ........................ 20 3.4.5 Selectivity of the 34 mm knotless pocket (34KL) ................................. 22 3.4.6 Comparison of the three pocket types. ............................................... 24
3.5 Model simulations ...................................................................................... 26 3.5.1 Market value ....................................................................................... 26 3.5.2 Model output....................................................................................... 26 3.5.3 Model extension ................................................................................. 29
3.6 Non-targeted catch .................................................................................... 32 3.6.1 Australian herring (Arripis georgianus) ................................................ 35 3.6.2 Weeping toadfish (Torquigener pleurogramma) .................................. 36 3.6.3 Western striped grunter (Pelates octolineatus) ................................... 37 3.6.4 King George whiting (Sillaginodes punctatus)..................................... 38 3.6.5 Southern calamary (Sepioteuthis australis) ........................................ 39 3.6.6 Snook (Sphyraena novaehollandiae) .................................................. 40 3.6.7 Yellowfin whiting (Sillago schomburgkii) ............................................. 41 3.6.8 Blue crab (Portunus armatus) ............................................................. 42 3.6.9 Australian salmon (Arripis truttaceus) ................................................. 43
3.7 Fisher survey ............................................................................................. 44 4 Discussion ....................................................................................................... 46
4.1 Mesh selectivity ......................................................................................... 46 4.2 Model simulations ...................................................................................... 48 4.3 Non-targeted catch .................................................................................... 49 4.4 Fisher survey ............................................................................................. 51 4.5 Implications for management ..................................................................... 52 4.6 Future considerations ................................................................................ 52
5 References ...................................................................................................... 54 6 Appendix .......................................................................................................... 55
v
List of Figures
Figure 1.1. Map of South Australian coastal waters divided into Marine Fishing Areas (MFA), showing the distribution of the average annual catch per MFA of garfish from 2005/06 – 2007/08. ........................ 1 Figure 1.2. Schematic illustration of a typical single power-hauled ringshot identifying the pocket and lateral wing sections. ................................................................................................................................... 2 Figure 1.3. (A.) 32 mm standard knot (32SK) and (B.) 34 mm knotless (34KL) construction. ................... 4 Figure 2.1 Standard power-hauled ring-shot. (A.) shoot net around a school of fish, or in an area likely to contain fish; (B.) join the two ends of the net to completely encircle a school of fish; (C.) gradually haul (retrieve) the wing end of the net herding the fish into the pocket; (D.) pull the pocket up to the side of the vessel; (E. & F.) the smaller fish generally escape through the pocket once it is „bunted up‟; (G.) Prop open the pocket; (H.) manually brail fish out of the pocket. ............................................................... 7 Figure 2.2. Experimental design involving a single power-hauled ring- shot............................................. 8 Figure 2.3. Experimental design involving a double drain-off shot. ............................................................ 8 Figure 2.4. Hierarchical experimental design. ........................................................................................... 8 Figure 2.5. Standardised weighted callipers used to measure mesh size (Note: Knotless mesh). ............ 9 Figure 3.1. The average mesh size of the nine pockets used in the selectivity trials. ..............................13 Figure 3.2. Relative catch (total biomass and garfish) from the wing and pocket of each of the three net types. The relative proportions (%) of wing catch are provided. ...............................................................15 Figure 3.3. Size and age structures of garfish caught in GSV and SG. Age structures are based on regional age/length key generated from sub-sampled garfish. .................................................................16 Figure 3.4. Von Bertalanffy growth curves for garfish collected from GSV and SG. ................................17 Figure 3.5. Length – weight relationships, as an estimate of relative condition, for garfish caught in (A.) each gulf (B.) seasonal extremes. Model parameters are provided. .......................................................17 Figure 3.6. Size structure of garfish caught in the 30SK and control nets in each trial. ..........................19 Figure 3.7. Combined size and age structures of garfish caught in the 30SK trial. ..................................19 Figure 3.8. Length at 50% selectivity (L50%) for each separate 30SK pocket trials. ................................19 Figure 3.9. Size structure of garfish caught in the 32SK and control nets in each trial. ..........................21 Figure 3.10. Combined size and age structures of garfish caught in the 32SK trial. ................................21 Figure 3.11. Length at 50% selectivity (L50%) for each separate 32SK pocket trials. ...............................21 Figure 3.12. Size structure of garfish caught in the 34KL and control nets in each trial. .........................23 Figure 3.13. Combined size and age structures of garfish caught in the 32SK trial. ................................23 Figure 3.14. Length at 50% selectivity (L50%) for each separate 34KL pocket trials. ................................23 Figure 3.15. The seasonal relationships between pocket mesh size and their respective lengths at 50% selection (L50%). Linear relationships are provided. .................................................................................24 Figure 3.16. Comparison of the mean (A.) % of undersize garfish retained in the pocket; (B.) % of legal garfish that escaped the pocket; (C.) % of age 1+ garfish retained in the pocket; and (D.) % of age 2+ garfish retained in the pocket, for each of the three pocket types. Means with the same lower case letter are not significantly different from each other, as determined using a Hochberg‟s GT2 post hoc test. ............................................................................................................................................................25 Figure 3.17. Approximate market value ( 95% confidence limits) of garfish by average size. ...............26 Figure 3.18. Simulated GarEst model output of the response of the garfish fishery to the theoretical standardisation of the fishing fleet to using of one of the three pocket types. Simulations were hind-cast back to 2001 and run through to 2007. The fishery parameters of interest were: (A.) biomass; (B.) catch; (C) value; and (D.) egg production. ................................................................................................28 Figure 3.19. Simulated GarEst model output of the response of the garfish fishery to the theoretical standardisation of the fishing fleet to include a hypothetical 38 mm mesh pocket. The fishery parameters of interest were: (A.) biomass; (B.) catch; (C.) value; and (D.) egg production. .......................................30 Figure 3.20. Simulated GarEst model output of the response of the garfish fishery to the theoretical reductions in fishing effort by 15%, 30% and 45%. The fishery parameters of interest were: (A.) biomass; (B.) catch; (C.) value; and (D.) egg production..........................................................................31 Figure 3.21. Non-parametric MDS plots that assess the effects of (A.) season, (B.) region, and (C.) hauling net pocket type on the multi-species catches when targeting garfish. .........................................34 Figure 3.22. The size selectivity of each of the three test pockets for Australian herring. .......................35 Figure 3.23. The size selectivity of each of the three test pockets for weeping toadfish. .........................36 Figure 3.24. The size selectivity of each of the three test pockets for western striped grunter. ...............37 Figure 3.25. The size selectivity of each of the three test pockets for King George whiting. ...................38 Figure 3.26. The size selectivity of each of the three test pockets for southern calamary. ......................39 Figure 3.27. The size selectivity of each of the three test pockets for snook. ..........................................40 Figure 3.28. The size selectivity of each of the three test pockets for yellowfin whiting. ..........................41 Figure 3.29. The size selectivity of each of the three test pockets for blue crabs. ...................................42 Figure 3.30. The size selectivity of each of the three test pockets for Australian salmon. .......................43
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Figure 3.31. The number of nets per fisher used to target garfish. ..........................................................45 Figure 3.32. The relative proportion (%) of fishers who use nets that differ in; (A.) operation; (B.) material; (C.) ply; and (D.) construction across each of the three pocket mesh size grades. ..................45
List of Tables
Table 3.1. A summary of the sample details. ..........................................................................................14 Table 3.2. Comparison of von Bertalanffy growth curves for garfish collected from GSV and SG using Kimura‟s (1980) likelihood ratio test. .........................................................................................................16 Table 3.3. Summary of the species captured throughout the study. The table shows the common and scientific names as described in Gomon et al. (2008), the number of individuals caught, their total weight (kg), and relative proportion (%) of total catch by number (n) and by weight (wt). * Listed commercial MSF species. ^ The species has a regulated legal minimum length (LML). .........................33 Table 4.1. Summary of major findings for each of the three pocket types trialled in this study. * The capacity of large fish to evade capture by the experimental pocket (e.g. swimming under the lead line) precludes an accurate estimate of „% escaped legal‟. ..............................................................................53
vii
Acknowledgements
The Marine Fishers Association (MFA) has been pro-active in their aim to promote the
recovery of South Australia‟s garfish fishery. They were the driving force behind the
development of this project and were ably supported by a team of dedicated commercial
fishers. Particular thanks are extended to Peter Welch and Mike Fooks of the MFA. The field
work that was an integral part of this project would not have been possible without the expert
assistance of Bart Butson, Paul „The General‟ Murray, Robert Butson, Clive Bradwell, David
Wilks, Simon Grenfell, Daniel McGregor, Mark Brevi, Giovanni Brevi, Shannon Gill, David Gill,
June Gill, Todd Twelftree, Wade Wheeler, Darren Wade, Richard James, Amanda Wheeler,
Ian Degilio, Shane Degilio, Jeff Wait, Mike Slattery, Brenton Bock, Benjamin Barnes, Andrew
Pisani, Bart Pisani, and Mike Pennington. The Gill family kindly provided the control pocket
that was used throughout the entire study. Thanks also to Mark Brevi who graciously leant us
his new 34KL pocket.
Thanks also to Paul Faithow, Alex McKay and Roger Stenning from PIRSA Fisheries
(Compliance) for on the ground support, especially when we were operating in the various
closed areas around the State. We gratefully acknowledge the PIRSA Marine Scalefish
Managers, Michelle Besley and Adriana Montoya for their on-going commitment and
enthusiasm for the project.
Funds for this research were provided by PIRSA, obtained through commercial licence fees.
This report was reviewed by Dr Adrian Linnane, Mr Cameron Dixon and Michelle Besley
(PIRSA) and formally approved for release by Assoc. Prof. Tim Ward, Principle Scientist of
SARDI‟s Wild Fisheries Program.
viii
Executive Summary
1. The most recent stock assessment of South Australia‟s southern garfish fishery
indicated that in 2007/08 the performance of the fishery had declined to its lowest level
since records began in 1983/84 (McGarvey et al. 2009).
2. The Garfish Working Group (GWG) stressed the need to identify a standard hauling
net pocket that will reduce the capture of undersized garfish and promote the recovery
of the fishery. Three pocket types were identified: (1.) 30 mm mesh, standard knot
(30SK), (2.) 32 mm mesh, standard knot (32SK); and (3.) 34 mm mesh, knotless
(34KL).
3. There was a general linear relationship between estimates of length at 50% selection
(L50%) and mesh size, however, the nature of this relationship did not remain consistent
throughout the year and was influenced by season. All three pocket types retained
proportionately more small garfish during the summer with seasonal differences in
estimates of L50% ranging from 5.2 mm to 17.2 mm for the 34KL and 32SK pockets,
respectively.
4. The 30SK pocket retained 16.5% of undersized garfish, whereas the 32SK and 34KL
retained 5.9% and 2.6%, respectively.
5. GarEst model simulations indicated that all three pocket types each demonstrated the
capacity to promote stock recovery, through rapid increases in biomass, value and egg
production. The extent of this recovery, however, was relatively minor, as there was a
marginal <5% improvement in all of the fishery parameters modelled over the 7-year
timeframe.
6. The simulated exclusive use of the 32SK pocket yielded the most positive results,
increasing biomass by ~3% and marginally out-performing the 34KL by 0.7%. A similar
trend was also evident for egg production and value.
7. Increasing the mesh size from 30 mm to 34 mm appeared to have a marginal effect on
the relative catch of undersize King George Whiting (KGW), however, given the small
catches of KGW in general, the overall effect of the „small-mesh‟ hauling net sector
remains an inconsequential risk to the State-wide KGW fishery.
8. The results from a fisher survey indicated that there is a wide variety of net types used
to target garfish, differing in mesh size, material, construction and configuration. Most
fishers (56%) preferred to use the 32SK pocket, approximately 40% continued to use
the 30SK pocket and the remaining 4% have adopted the new 34KL pocket.
ix
9. Mesh nets, particularly those constructed from polypropylene, have a propensity to
shrink up to ~4 mm. Therefore, it is likely that there are numerous fishers who are
currently unintentionally using „illegal‟ (<30 mm) gear to target garfish.
10. In terms of the relative capture of undersize garfish, seasonal estimates of L50%,
retention of 1 and 2 year-old fish and model simulations, the performance of the 30SK
pocket was sub-optimal and hence the current regulated minimum mesh size is out-
dated.
11. The similarity in the selective properties and resultant model output of the 32SK and
34KL pockets makes it difficult to differentiate them from a management perspective.
Future work should extend the experiment to include a 34 mm standard knot pocket to
investigate whether a comparable 2 mm increase in mesh size is a practical
management alternative.
1
1 General Introduction
1.1 Description of the fishery
The Southern Garfish (Hyporhamphus melanochir) is one of the most significant
inshore fish species of southern Australia, with fisheries in Victoria, Tasmania, South
Australia and Western Australia. Historically, the national commercial catch for this
species has been dominated by that from South Australia where the catch has
usually exceeded 400 t per annum, with an approximate value of AUD$2 million
(ABARE 2008). This species is also heavily targeted by the recreational sector and it
was estimated that in 2007/08 South Australian recreational anglers harvested 75 t,
accounting for approximately 20% of the total State-wide catch (Jones 2009).
In South Australia, the garfish fishery is principally located in Spencer Gulf and Gulf
St. Vincent (Figure 1.1) and is managed as part of the multi-species, multi-gear
Marine Scalefish Fishery through a series of input and output controls. Licensed
commercial fishers target garfish using hauling nets and dab nets. Hauling net
fishers account for the majority (~90%) of the commercial catch even though their
fishing activities are restricted by regulation to waters <5m depth. There are also
numerous areas around the State that are either permanently or seasonally closed to
net fishing. Recreational fishers are also permitted to use dab nets but
predominantly use traditional hook and line as they fish from boats and shore-based
platforms throughout the State. Current output controls for garfish caught in South
Australia include a legal minimum length (LML) of 230 mm total length (TL) and a
recreational bag and boat limit of 60 and 180 fish, respectively. Commercial catches
from both gulfs are similar, whereas recreational landings are higher in Gulf St.
Vincent as a consequence of a greater number of recreational fishers residing in
metropolitan Adelaide (McGarvey et al. 2009).
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Figure 1.1. Map of South Australian coastal waters divided into Marine Fishing Areas (MFA), showing the distribution of the average annual catch per MFA of garfish from 2005/06 – 2007/08.
2
1.2 Hauling net sector
The hauling net sector of the garfish fishery is almost exclusively confined to the
northern gulfs of South Australia (i.e. Gulf St. Vincent and Spencer Gulf) (Steer
2009). Although garfish are targeted by the hauling net fishers throughout the year,
catch and fishing effort typically peaks during late summer and through early autumn
(McGarvey et al. 2009).
The hauling nets that are used to target garfish in the South Australian Marine
Scalefish Fishery consist of a „pocket‟ end and lateral „wings‟ (Figure 1.2). The mesh
size of the wings is generally smaller than that of pocket and typically constructed of
different material. This is because the wings are specifically designed to herd fish
into the pocket of the net, rather than enmesh them. Fish that accumulate within the
pocket are manually brailed out with a hand-held brailing net and are released or
retained at the discretion of the fisher. The size-selective property of the net is,
therefore, largely determined by the dimensions and construction of the pocket.
Netting regulations have been enforced in this fishery since 1980 where hauling nets
can only be used in specific areas of the State, cannot exceed 600 m in length or 10
m depth, and have a minimum mesh size of 30 mm. Initially the legal minimum
length (LML) for garfish was 210 mm TL but this was increased to 230 mm in 2001.
At that time, no corresponding amendments were made to the regulated mesh size of
the hauling nets despite a previous mesh selectivity experiment indicating that the 30
mm mesh nets catch a large number of undersize (<230 mm) garfish and a 32 mm
mesh size would be more appropriate (Jones 1982). Furthermore, most commercial
fishers have modified the construction and configuration of their nets within the
boundaries of the regulations to suit their individual fishing preferences. The hauling
net sector has subsequently evolved to comprise of a wide diversity of net types (e.g.
knotless nets, braided nylon nets, variable ply nets, 30 – 34 mm mesh sizes, etc.)
with each individual net configuration potentially selecting for different sized garfish.
PocketWing PocketWing
Figure 1.2. Schematic illustration of a typical single power-hauled ringshot identifying the pocket and lateral wing sections.
3
1.3 Need
The most recent stock assessment of South Australia‟s southern garfish fishery
indicated that in 2007/08 the performance of the fishery had slumped to its lowest
level since records began in 1983/84 (McGarvey et al. 2009). In that assessment
four principle features of the fishery were highlighted to be of greatest concern: (1.)
Model estimates (GarEst) have indicated that exploitation rates were extremely high,
with annual harvest fractions exceeding 70%, i.e. substantially higher than for any
other South Australian species for which harvest rate estimates are available; (2.)
The most recent data suggest that there has been minimal evidence of a recovery
since a significant decline in 2001 – 2003 despite the implementation of an enhanced
management framework in 2005 which involved a voluntary buy-back of net fishing
endorsements and permanent spatial netting closures; (3.) The size and age
structures of the harvestable biomass, which was once dominated by four- and five-
year-old fish, were considerably truncated to consist of primarily one- and two-year-
old fish indicating that the fishery was largely based on a single year class (Fowler et
al. 2008; Fowler and Ling 2010); and (4.) the release mortality of undersized garfish
is generally estimated to be close to 100% (Knuckey et al. 2002, Fowler et al. 2009),
which represents both wasted catch and compromises sustainability as there is
reduced capacity for garfish to reach legal size and contribute to egg production.
Although it may be too soon to assess whether the management arrangements
implemented in 2005 have contributed to rebuilding the fishery‟s harvestable
biomass, there is an immediate need to address the sustainability issues associated
with garfish release mortality. This is particularly relevant in the hauling net sector of
the fishery as it accounts for ~90% of the total annual commercial garfish catch
(McGarvey et al. 2009). Given the concerning status of the garfish fishery and the
complexity of hauling net gear within the net sector, the commercial fishers have
instigated and stressed, through the Garfish Working Group (GWG), the need to
identify a standard net configuration that will reduce the capture of undersized
garfish. This restructure will have immediate flow-on benefits to the fishery as there
will be an instant reduction in discard mortality, which will consequently promote
stock rebuilding and lead to a concomitant increase in the profitability of the
commercial harvest. Furthermore, gear-related compliance issues relating to the net
sector will be simplified.
Any potential changes in by-catch resulting from streamlining the hauling net sector
will also need to be addressed to fulfil the fishery‟s obligation to comply with
principles of ecologically sustainable development (ESD). This will satisfy
4
government policy and legislation at both the Commonwealth and State levels that
specifies that any fishery that exports its product must undergo ecological
assessment to ensure the fishery is managed in an ecologically sustainable way.
Assessing potential changes in by-catch as part of this project will partially address
the recommendation from the Commonwealth Department of Sustainability,
Environment, Water, Population and Communities (SEWPAC) that states „PIRSA
should develop and implement a system for the quantitative monitoring of by-catch in
the MSF fishery, sufficient to identify changes in composition and quantity of by-catch
in each sector of the fishery‟.
An initial screening of the various types of hauling nets used by commercial fishers
was undertaken at a GWG meeting in February 2010. Out of a wide diversity of
hauling net configurations, three were unanimously agreed upon as preferred garfish
nets to be included in the mesh selectivity experiment. These test nets were chosen
on the basis of their availability within the fishery, their ease of construction and
perceived size-selective properties. They were: (1.) 30 mm mesh, standard knot
(30SK), (2.) 32 mm mesh, standard knot (32SK); and (3.) 34 mm mesh, knotless
(34KL) (Figure 1.3). A fourth control net (28 mm mesh, standard knot, 24 ply, nylon
(28SK)) was also identified and included in the experimental design.
A. B.
Figure 1.3. (A.) 32 mm standard knot (32SK) and (B.) 34 mm knotless (34KL) construction.
1.4 Aims and objectives
The overall objective of this study is to identify the most appropriate hauling net to
maximise the safe escapement of smaller fish and promote stock recovery. To
achieve this, the specific aims were:
to undertake field selectivity trials with different types of pocket configurations
in hauling nets;
5
to document the species diversity and size selectivity of non-targeted catch
associated with the changes in fishing gear;
to provide a census of the various net configurations that are currently used to
target southern garfish in the Marine Scalefish Fishery;
to undertake modelling work to estimate the benefits to the fishery in terms of
biomass, catch, value and egg production from each tested gear configuration
whose selectivity curves are obtained from measurements in the previous
aim. This would take into consideration economic information such as the
value of the different sized fish as well as the biological considerations such
as size-at-first-maturity and fecundity schedules.
6
2 Methods
2.1 Experimental design
This experiment examined the size selective properties of each of the three test nets
(30SK, 32SK and 34KL) against the control net (28SK). Each test net was trialled up
to three times to ensure that a wide size range of garfish were sampled. Each trial
involved two independent vessels. The first vessel deployed the test net as per the
usual commercial fishing practice. This either involved a single power-hauled ring-
shot, or a double drain-off shot. For a power-haul ring shot the pocket end of the net
is anchored and the rest of the net is deployed in a large semi-circle. The wing end
of the net is then slowly towed by the vessel around to the anchored pocket to form a
complete circle. The wing end is then retrieved either by hand or mechanically, as
the vessel goes astern, reducing the area inside the circle of the net and herding the
fish into the pocket (Figures 2.1 & 2.2). A double drain-off shot is carried out by two
fishers who join the pocket ends of the two nets together and then deploy them in
opposite directions parallel to the shore. The deployment of the nets is timed to
coincide with the movement of fish off inter-tidal banks with the ebbing tide. At the
completion of the shot the nets are hauled in a similar fashion to the power-hauling
method (Figure 2.3).
As the test net was being hauled, the second vessel encircled the entire test net shot
with the control net (Figures 2.2 & 2.3). This methodology ensured that any fish that
escaped or „sieved‟ through the test net were captured in the control net. All fish,
regardless of size and species, captured in each of the two nets, including fish that
were enmeshed in the net wings or were brailed from the pocket of the net, were
retained, identified and measured. The size, weight and species composition of the
two nets were then compared. In situations where catches were large (i.e. > 100 kg),
representative sub-samples were taken and scaled up appropriately. A complete
mesh selectivity trial generally comprised nine separate fishing events (i.e. 3 „test‟ net
trials x 3 replicates).
To account for any seasonal variation within the fishery and to ensure that the results
from the experiment were representative of typical fishing practices, separate mesh
selectivity trials were undertaken in both gulfs (GSV and SG) and during the summer
(October – March) and winter (April – September) periods of the fishery. The overall
study, therefore consisted of 36 separate fishing events (i.e. 3 „test‟ net trials x 3
replicates x 2 gulfs x 2 seasons) (Figure 2.4).
7
The construction of the wing section of a typical hauling net generally differs to that of
the pocket in terms of material, ply and mesh size. Given that this study aimed to
determine the selective properties of the various pocket types it was necessary to
differentiate the catch throughout the entire net.
Figure 2.1 Standard power-hauled ring-shot. (A.) shoot net around a school of fish, or in an area likely to contain fish; (B.) join the two ends of the net to completely encircle a school of fish; (C.) gradually haul (retrieve) the wing end of the net herding the fish into the pocket; (D.) pull the pocket up to the side of the vessel; (E. & F.) the smaller fish generally escape through the pocket once it is „bunted up‟; (G.) Prop open the pocket; (H.) manually brail fish out of the pocket.
8
Shoot test net as per usual power-
haul shot. Control net vessel
remains close by.
Control net shoots away as test net
is being hauled.
Control net encircles test net while
fish are being brailed from the
pocket.
Test vessel completes fishing and
exits the area as control net is
hauled.
A.
B.
C.
D.
Control net
vessel
Test net
vessel
Fish
PocketWing
Shoot test net as per usual power-
haul shot. Control net vessel
remains close by.
Control net shoots away as test net
is being hauled.
Control net encircles test net while
fish are being brailed from the
pocket.
Test vessel completes fishing and
exits the area as control net is
hauled.
A.
B.
C.
D.
Shoot test net as per usual power-
haul shot. Control net vessel
remains close by.
Control net shoots away as test net
is being hauled.
Control net encircles test net while
fish are being brailed from the
pocket.
Test vessel completes fishing and
exits the area as control net is
hauled.
A.
B.
C.
D.
Control net
vessel
Test net
vessel
Fish
PocketWing
Figure 2.2. Experimental design involving a single power-hauled ring- shot.
Control net shoots away as test net
is being hauled.
Control and test net vessels
connect pockets and shoot away in
opposite directions.
Both vessels meet to form a large
circle. The control net is retrieved.
The control net vessel separates
from the test net vessel.
A.
B.
C.
D.
Control net encircles test net while
fish are being brailed from the
pocket.
Test net vessel completes fishing
and exits the area as control net is
hauled.
E.
F.
Control net shoots away as test net
is being hauled.
Control and test net vessels
connect pockets and shoot away in
opposite directions.
Both vessels meet to form a large
circle. The control net is retrieved.
The control net vessel separates
from the test net vessel.
A.
B.
C.
D.
Control net encircles test net while
fish are being brailed from the
pocket.
Test net vessel completes fishing
and exits the area as control net is
hauled.
E.
F.
Figure 2.3. Experimental design involving a double drain-off shot.
Gulf St. Vincent
30SK 32SK 34KL
1.
2.
3.
1.
2.
3.
1.
2.
3.
30SK 32SK 34KL
1.
2.
3.
1.
2.
3.
1.
2.
3.
Spencer Gulf
30SK 32SK 34KL
1.
2.
3.
1.
2.
3.
1.
2.
3.
30SK 32SK 34KL
1.
2.
3.
1.
2.
3.
1.
2.
3.
Summer Winter Summer Winter
Gulf St. Vincent
30SK 32SK 34KL
1.
2.
3.
1.
2.
3.
1.
2.
3.
30SK 32SK 34KL
1.
2.
3.
1.
2.
3.
1.
2.
3.
Gulf St. Vincent
30SK 32SK 34KL
1.
2.
3.
1.
2.
3.
1.
2.
3.
30SK 32SK 34KL
1.
2.
3.
1.
2.
3.
1.
2.
3.
30SK 32SK 34KL
1.
2.
3.
1.
2.
3.
1.
2.
3.
1.
2.
3.
1.
2.
3.
1.
2.
3.
30SK 32SK 34KL
1.
2.
3.
1.
2.
3.
1.
2.
3.
30SK 32SK 34KL
1.
2.
3.
1.
2.
3.
1.
2.
3.
30SK 32SK 34KL
1.
2.
3.
1.
2.
3.
1.
2.
3.
1.
2.
3.
1.
2.
3.
1.
2.
3.
Spencer Gulf
30SK 32SK 34KL
1.
2.
3.
1.
2.
3.
1.
2.
3.
30SK 32SK 34KL
1.
2.
3.
1.
2.
3.
1.
2.
3.
Spencer Gulf
30SK 32SK 34KL
1.
2.
3.
1.
2.
3.
1.
2.
3.
30SK 32SK 34KL
1.
2.
3.
1.
2.
3.
1.
2.
3.
30SK 32SK 34KL
1.
2.
3.
1.
2.
3.
1.
2.
3.
1.
2.
3.
1.
2.
3.
1.
2.
3.
30SK 32SK 34KL
1.
2.
3.
1.
2.
3.
1.
2.
3.
30SK 32SK 34KL
1.
2.
3.
1.
2.
3.
1.
2.
3.
30SK 32SK 34KL
1.
2.
3.
1.
2.
3.
1.
2.
3.
1.
2.
3.
1.
2.
3.
1.
2.
3.
Summer Winter Summer Winter
Figure 2.4. Hierarchical experimental design.
9
2.2 Mesh measurements
Numerous commercial hauling net fishers were involved in the field component of
this study. In most cases these fishers supplied their own mesh pocket to be used
as part of the selectivity trials. Although attempts were made to standardize the
mesh size of the pockets used in each of the trials, they invariably differed in terms of
their construction material, ply, weight, and how they were slung on the head line.
Furthermore, many of the nets used were relatively old and it is generally accepted
amongst the netting community that over time mesh nets have a propensity to shrink,
particularly those made from polypropylene. In order to account for some of this
variation, it was necessary to accurately measure the mesh size of the pockets that
were used and document the nets‟ specifications.
The mesh size of each of the nets used in this study was measured according to the
methods stipulated in Section 3.3 of the Fisheries Management (General)
Regulations 2007. For the purpose of these regulations, the mesh size of a net is
determined as the average size of 10 meshes. The mesh size is defined as the inner
distance between the diagonally opposite corners of the mesh. The regulations
stipulate that the part of the net containing the mesh to be measured must be soaked
in water for at least 5 minutes. Immediately after soaking, a weight of 1.5 kg must be
slung to one corner of the mesh to be measured. This weight provides a
standardised means of stretching the mesh. In this study each mesh was measured
using a set of weighted (1.5 kg) callipers which had been calibrated and certified by
Fisheries Compliance (Figure 2.5).
1.5 Kg
Stretched
mesh
1.5 Kg
Stretched
mesh
Figure 2.5. Standardised weighted callipers used to measure mesh size (Note: Knotless mesh).
10
2.3 Age analysis
A representative sample of approximately 30 garfish was selected from each net
shot for age analysis. Each garfish was measured for both total length (TL) and
standard length (SL) to the nearest mm, and weighed (to the nearest 0.01 g). The
sagittae, i.e. the largest pair of otoliths were removed and subsequently used for age
determination. One otolith for each garfish was embedded in resin and sectioned
using a diamond saw to produce a thin transverse section. This section was
mounted on a glass microscope slide using super glue and then examined using a
low power microscope. The number of opaque zones, the characteristics of the edge
of the otolith, the sample date and the universal birth date (1st January) were
recorded and used to estimate fish age. The birth date of 1st January was used as
this falls in the middle of the spawning season (Ye et al. 2002). Each otolith was
assigned a grade of 1 – 4 which indicated, on an increasing scale, its overall clarity
and relative interpretability. All otoliths that scored a grade =1 were rejected as
unreadable.
Additional region and season specific age data were extracted from SARDI‟s garfish
database and incorporated into the analysis. This was done to increase the
resolution of the age/length keys that were generated for each of the netting trials.
These keys were used to convert the garfish size frequency distributions into age
structures.
2.4 Fisher survey
Given the importance of this study to the management of the fishery it was
necessary to obtain a baseline understanding of the net configurations that are
currently used by commercial netters. Fishers who targeted or caught garfish in
2009 were identified from SARDI‟s commercial catch and effort database. A „garfish
netters survey‟ was sent to each of these fishers. The survey requested specific
information relating to the type of net(s) that each fisher currently uses to catch
garfish, including whether they are floating or sinking hauling nets, their pocket mesh
size, construction material, ply, whether the mesh was knotted or knotless, and if
more than one net was used, their relative proportion of use (see Appendix I).
2.5 Statistical analysis
The experimental design used to assess mesh selectivity in this study is a variant of
the „covered codend‟ design typically used to assess the selective properties of
towed fishing gear (e.g. trawlers). With the covered cod-end design, the small-mesh
cover captures all the fish that pass through the experimental cod-end mesh, so all
11
fish contacting the gear are observed (Millar and Fryer 1999). This, however, is not
the case in this experiment as not all fish that are caught in the control net would
have necessarily passed through the experimental pocket. Some fish may have
avoided the gear by swimming under the lead line, through the wing section of the
net, or may be caught in the gap between the two nets. Traditional mesh selectivity
studies fit logistic curves to the data to determine the fishes‟ probability of capture
(Pretain) on the basis of its size (l). The escapement, or evasion, of larger fish from
the experimental net may bias the results and overestimate the probability of capture
of the small fish and subsequently produce an inaccurate estimate of the size at
which 50% of the fish are retained (L50%). This study adapted the traditional logistic
function to account for the loss (evasion) of large fish from the experimental pocket
by producing a „disjointed‟ logistic function which fits logistic functions to the top and
bottom halves of the data separately and these two halves converge at L50%. The
disjointed logistic function can be written as;
50
50 50
50 50 50
50
50 50
1, if
1 expP̂ ; , ,
1, if
1 exp
l
retain l l
l
l lr l l
l l r r
l lr l l
.
Non-metric, multi-dimensional scaling was used to compare the multi-species
catches between each of the pocket types and to determine the influence of region
and season. The statistical program Primer (v5.2.9) was used to run the analyses.
For each analysis, the species abundance data were arranged into a matrix with a
row for each of the mesh selectivity trials and a column for each species. Prior to the
analysis, the data matrix was transformed using the fourth root transformation, and
then standardised, after which a similarity matrix comparing between mesh
selectivity trials was generated using the Bray-Curtis similarity coefficient. The
ordination was then done on the similarity matrix to identify those mesh selectivity
trials that were most similar to each other. The analysis of similarity test (ANOSIM)
was then used to test hypotheses about differences between the multi-species
catches from each of the mesh selectivity trials, with respect to pocket type, region
and season.
2.6 Model simulations
The GarEst model, which is the key tool used to assess South Australia‟s garfish
fishery (McGarvey and Feenstra 2004, McGarvey et al. 2007), was used to simulate
the performance of the garfish fishery in terms of its total biomass, total catch, egg
12
production and relative value for each of the three pocket types. This was achieved
by retrospectively homogenising the fishery to consist of a single gear type. Three
separate scenarios were simulated to correspond with each of the pocket types
examined in this study (i.e. 30SK, 32SK and 34KL). These simulations were hind-
cast back to 2001 to align with the most recent changes to the LML for garfish and
were extended to 2007 to correspond with the last stock assessment cycle
(McGarvey et al. 2009). Estimates of mesh selectivity (L50%) for each of the pocket
types were integrated into the model. These estimates were also seasonally
adjusted to account for the temporal variation in mesh selectivity. The simulated
output was then compared against a baseline model which assumes that there was
100% selectivity of all captured legal size garfish. The difference between the
simulated and baseline model outputs was expressed as a percentage. Estimates of
biomass, catch, and egg production were routinely generated from the pre-existing
GarEst model, however, estimates of value required the input of additional „market
price‟ information.
Fishers typically sort their garfish catch into arbitrary size grades i.e., small, medium,
large and extra-large, as the relative size of garfish has a considerable influence on
market price. Price, however, also fluctuates on the basis of fish condition,
availability and season, and as such, there is considerable variation in the market
value of garfish throughout the year. In order to investigate the economic projections
of standardising hauling net gear within the fishery it was necessary to firstly
determine the relative value of the different size grades of garfish.
SARDI is currently involved in an ongoing market sampling program for garfish,
where a small research team visits Adelaide‟s central fish market facility (SAFCOL)
on a weekly basis to sample and process fish prior to the morning auction. The
details of the sampling protocol can be found in Ye et al. (2002). During each trip
multiple small quantities (~1kg) of garfish are purchased from a range of catches.
These garfish are strategically sampled so they are representative of the catch and
returned to the laboratory for biological processing (see McGarvey et al. 2009). The
price of these fish is determined on the day via the auction. Through comparing the
daily purchase price of the garfish with their respective average sizes over 12
months of market sampling it was possible to estimate a relative value for each of the
size grades. A number of commercial fishers were also consulted to verify that our
price estimates were realistic and appropriate for the economic analysis component
of the model.
13
3 Results
3.1 Sample details
A total of 29 hauling net shots were carried out by 11 commercial fishers during this
study (Table 3.1). Of these, 13 were single power-hauled ring-shots and 16 were
combined double drain-off shots. Nine shots were carried out in areas that are
closed to net fishing (MFAs 33, 34 & 11) and were done in accordance with a
Ministerial Exemption (Section 115 of the Fisheries Management Act 2007). Time
constraints prevented the completion of the 30SK trial in Spencer Gulf during winter.
3.2 Variation in mesh size
Nine different hauling net pockets were used over the course of this study; two 30SK,
four 32SK, and three 34KL. Both of the 30SK pockets were constructed from a mix
of polypropylene and nylon, 18 ply twine. Of the four 32SK pockets, two were
constructed from polypropylene, 18 ply twine; one with a combination of nylon and
polypropylene, 18 ply twine; and one with nylon, 24 ply twine (Table 3.1). All three of
the 34KL pockets were recently purchased from the same net supplier and were all
constructed from nylon, 24 ply twine (Table 3.1).
A 2-way analysis of variance confirmed that despite the variation in construction
material and ply amongst the mesh pockets used in the selectivity trials, no
significant differences were detected in the average size of the mesh within each of
the three pocket types (F3,101 = 4.03, p = 0.10). The analysis further confirmed that
all three pocket types had average mesh sizes that were significantly different from
each other (F2,101 = 469.4, p <0.05) (Figure 3.1).
30SK 32SK 34KLPocket type
30
32
34
Av
era
ge m
es
h s
ize (
mm
s
e)
Pockets used
Target mesh size
Figure 3.1. The average mesh size of the nine pockets used in the selectivity trials.
14
Table 3.1. A summary of the sample details.
SEASON NET TRIAL GULF Date # shots MFAClosed
Area?Net Owner
Ave. Mesh
Size (mm)Construction Ply Fishers Shot Type
SUMMER
30SK
GSV
SG
32SK
GSV
SG
34KL
GSV
SG
34KL
GSV
SG
WINTER
GSV
SG
30SK
32SK
GSV
SG
Nylon/Poly 18
- -
301-02 Sept 10 35 No B. Butson 29.6 ± 0.3
Nylon/Poly 18
B. Butson / R. ButsonSingle pow er-
hauled ring-shots
- - - - - -
Poly 18
- -
09-10 Aug 10 3 35 No R. Butson 31.6 ± 0.2
Nylon 24
B. Butson / R. ButsonSingle pow er-
hauled ring-shots
21-21 Jul 10 3 11 & 21C Yes (MFA11) M. Brevi 31.8 ± 0.1
M. Brevi / D. WilksDouble drain-off
shots
M. Brevi / D. WilksDouble drain-off
shots
10&17 May 11 3 35 No B. Butson 34.4 ± 0.2
04-05 May 11 11 Yes M. Brevi
B. Butson / R. ButsonSingle pow er-
hauled ring-shots
2 34.4 ± 0.3 Nylon 24
Nylon/Poly 18 R. Butson / I. Degilio22-Feb-10 3 35 No B. ButsonSingle pow er-
hauled ring-shots
02-03 Nov 10 3 23 No M. Slattery 30.1 ± 0.6 Nylon/Poly 18
29.6 ± 0.3
M. Slattery / B. Barnes
2 Double drain-off
& 1 pow er-hauled
ring shot
19-20 Jan 11 2 34 Yes A. Pisani 31.2 ± 0.1 Poly 18 A. Pisani / S. GillDouble drain-off
shots
28-29 Mar 11 2 33 Yes S. Gill 31.0 ± 0.3 Nylon 24 S. Gill / D. GillDouble drain-off
shots
12,17 & 18
Jan 113 35 No J. Wait 34.1 ± 0.1 Nylon 24
13&18 Mar 11 2 33 Yes
J. Wait / I. DegilioDouble drain-off
shots
M. Brevi 34.4 ± 0.3 S. Gill / D. GillDouble drain-off
shotsNylon 24
15
3.3 Hauling net characteristics (wing vs. pocket)
The biomass retrieved from the wing sections of the hauling nets represented a
minor proportion (<7%) of the total catch and was relatively consistent for each of the
three net types (Figure 3.2). The relative proportion of garfish retrieved from the
wings ranged from 7% in the 30SK net to 13.6% in the 34KL net (Figure 3.2). The
remaining analysis only considered fish that were retained in the pockets of the test
nets and ignored the wing component of the catch.
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
6.8
5.4 6.7
30SK 32SK 34KL
TOTAL BIOMASS
0
250
500
750
1,000
1,250
1,500
1,750
2,000
Wing
7.05
12.2
13.6
30SK 32SK 34KL
GARFISH
We
igh
t (k
gs
)
Pocket Type
Figure 3.2. Relative catch (total biomass and garfish) from the wing and pocket of each of the three net types. The relative proportions (%) of wing catch are provided.
3.4 Garfish size and age selectivity
3.4.1 Variation in the size and age structures
A total of 74,784 garfish were caught over the course of this study. These fish
ranged in size from 117 to 409 mm TL and from 0+ to 6+ years of age. Although the
overall size ranges were comparable between the two gulfs, their respective size
distributions were different (Kolmogorov-Smirnov Test: z = 15.59, p <0.001). Both
the mode and average size of garfish were larger in GSV compared with SG (Figure
3.3). There was also proportionately more small (<220 mm) and large (>280 mm)
garfish caught in GSV. Most (75%) of the garfish caught from SG ranged in size
from 231 – 266 mm (interquartile range = 35 mm) whereas 75% of garfish caught in
GSV ranged from 230 – 276 mm (interquartile range = 46 mm).
There was very little difference in the age structures of garfish between the two gulfs.
In both gulfs >90% of the catch was dominated by the 1+ and 2+ age classes (Figure
3.3). Of these, the 1+ garfish were the most prevalent accounting for 54% and 64%
of the catch in SG and GSV, respectively. The oldest garfish (6+) were caught from
SG, however, they only accounted for a negligible proportion (0.1%) of the total catch
from that gulf.
16
0.0
0.5
1.0
1.5
2.0
2.5
3.0
GULF ST. VINCENT
0.0
0.5
1.0
1.5
2.0
2.5
3.0
150
160
170
180
190
200
210
220
230
240
250
260
270
280
290
300
310
320
330
340
350
360
TL (mm)
SPENCER GULF
Re
lati
ve
fre
qu
en
cy
(%
)Mode:
263 mm
Ave:252 mm
Mode: 242 mm
Ave: 248 mm
0
10
20
30
40
50
60
70
80
GULF ST. VINCENT
0
10
20
30
40
50
60
70
80
0+ 1+ 2+ 3+ 4+ 5+ 6+
Age (years)
SPENCER GULF
Figure 3.3. Size and age structures of garfish caught in GSV and SG. Age structures are based on regional age/length key generated from sub-sampled garfish.
3.4.2 Variation in growth and condition
The von Bertalanffy growth functions for garfish from the two gulfs were significantly
different (χ2 = 14.7, df = 3, p = 0.002) using Kimura‟s likelihood ratio test (Table 3.2).
Garfish from GSV grew considerably faster (K) and attained a smaller asymptotic
length (L ) compared with SG garfish (Table 3.2, Figure 3.4). The estimate of t0 was
also appreciably smaller for SG garfish, however, this parameter may be obscured by
an under-representation of juveniles (<12 months) within the sample.
The relative condition of garfish were the same from both gulfs (F1, 2,046 = 0.0, p =
1.0). The length-weight relationships for both gulfs were congruent (Figure 3.5).
Variation, however, was detected between seasons (F1, 2,046 = 4.3x10-17, p < 0.001),
where garfish collected in summer were heavier at any given length compared with
winter-caught garfish (Figure 3.5).
Table 3.2. Comparison of von Bertalanffy growth curves for garfish collected from GSV and SG using Kimura‟s (1980) likelihood ratio test.
Test Hypothesis GSV SG χ2 df p
H0 vs H1 L GSV = L SG 421.15 491.18 3.36 1 0.067*
H0 vs H2 ΚGSV = ΚSG 0.021 0.009 2.83 1 0.093*
H0 vs H3 t0GSV = t0SG -23.4 -53.53 2.60 1 0.107
H0 vs H4 VBGSV = VBSG 14.73 3 0.002**
17
100
150
200
250
300
350
400
450
TL
(m
m)
GSV
SG
0
10
20
30
40
50
60
70
80
90
100
Estimated age (months)
Figure 3.4. Von Bertalanffy growth curves for garfish collected from GSV and SG.
0
50
100
150
200
250
300 GSV (Wt =7.5x10-6*TL2.9)
SG (Wt = 7.5x10-6*TL2.9)
100
125
150
175
200
225
250
275
300
325
350
375
400
425
0
50
100
150
200
250
300 Summer (Wt =7.7x10-6*TL2.9)
Winter (Wt = 2.3x10-6*TL3.1)
100
125
150
175
200
225
250
275
300
325
350
375
400
425
TL (mm)
We
igh
t (g
)
A.
B.
Figure 3.5. Length – weight relationships, as an estimate of relative condition, for garfish caught in (A.) each gulf (B.) seasonal extremes. Model parameters are provided.
18
3.4.3 Selectivity of the 30 mm standard knot pocket (30SK)
A total of 8,663 garfish were caught across all three trials, of which 5,639 (65.1%)
were retained in the 30SK pocket. Garfish size consistently ranged from 160 mm to
350 mm in each of the trials. Small garfish (190 – 230 mm) constituted a
considerable proportion of the catch from GSV during winter and were poorly
represented in the other trials (Figure 3.6). The average size of garfish retained in
the 30SK pocket ranged from 243.0 mm in GSV during summer to 256.4 mm in SG
during summer (Figure 3.6). The relative proportion of undersize garfish retained in
the 30SK pocket varied from 7.5% in SG during summer to 28.9% in GSV during
summer (Figure 3.6). No trials were carried out in SG during winter.
The average size of garfish retained in the 30SK pocket combined across all three
trials was 253.1 mm (Figure 3.7). Overall, 16.5% of garfish retained in the 30SK
pocket were undersize. In total 16.6% of legal size garfish escaped the 30SK pocket.
The retention rate of 1 and 2 year old garfish was 60.9% and 85%, respectively
(Figure 3.7). Retention rates exceeded 88% for 3+ and older garfish.
All three trials yielded length at 50% selectivity (L50%) estimates that were less than
the legal minimum length (LML) of 230 mm. The GSV summer trial had the smallest
L50% estimate at 202.0 mm. The selective properties of the 30SK pockets used in the
remaining two trials (GSV winter and SG summer) were similar, each yielding L50%
estimates of ~226 mm (Figure 3.8).
19
0
10
20
30
40
50
60
70
80
90
100
110
120
GULF ST. VINCENTSUMMER
n = 1,581
0
10
20
30
40
50
60
70
80
90
100
110
120
SPENCER GULFSUMMER
n =3,108
150
160
170
180
190
200
210
220
230
240
250
260
270
280
290
300
310
320
330
340
350
0
10
20
30
40
50
60
70
80
90
100
110
120
30SK
Control
GULF ST. VINCENTWINTER
n = 3,974
150
160
170
180
190
200
210
220
230
240
250
260
270
280
290
300
310
320
330
340
350
0
10
20
30
40
50
60
70
80
90
100
110
120
SPENCER GULFWINTER
n = 0
Ave: 243.0 mm
Ave. 256.4 mm
Ave: 254.8 mm
Fre
qu
en
cy
TL (mm)
LM
L
LM
L
Figure 3.6. Size structure of garfish caught in the 30SK and control nets in each trial.
150
160
170
180
190
200
210
220
230
240
250
260
270
280
290
300
310
320
330
340
350
0
25
50
75
100
125
150
175
200
225
250
ALL 30SKn = 8,663
<LMLretained = 16.5%>LMLescaped = 16.6%
Ave: 253.1 mm
Fre
qu
en
cy
TL (mm)
LM
L
0+ 1+ 2+ 3+ 4+ 5+ 6+
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
30SK
Control
27.5
60.9
85.0
88.192.4 95.4
Estimated age (years)
Figure 3.7. Combined size and age structures of garfish caught in the 30SK trial.
120
130
140
150
160
170
180
190
200
210
220
230
240
250
260
270
280
290
300
310
320
330
340
350
360
TL (mm)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Pro
po
rtio
n r
eta
ined
GSV_Summer 202.0 mm
GSV_Winter 225.5 mm
SG_Summer 227.1 mm
LM
L
Figure 3.8. Length at 50% selectivity (L50%) for each separate 30SK pocket trials.
20
3.4.4 Selectivity of the 32 mm standard knot pocket (32SK)
A total of 27,581 garfish were caught across all four trials, of which 15,688 (56.9%)
were retained in the 32SK pocket. There were marked spatial and temporal
differences in the size distribution of garfish caught using this pocket. There was a
clear absence of small garfish (<230 mm) caught during summer in GSV, which was
also reflected in SG, but to a lesser extent (Figure 3.9). In both of these trials the
average size of garfish retained in the 32SK pocket exceeded 260 mm. These trials
were carried out in areas that had been closed to commercial hauling netting since
2005. A greater proportion of small garfish (>230 mm) were caught during winter in
both gulfs. The size distribution of garfish in GSV appeared to consist of two cohorts,
with modal sizes of 222 mm and 292 mm, respectively (Figure 3.9). Spencer Gulf
garfish, however, were distributed around a single mode of 222 mm. The average
size of garfish retained in the 32SK pockets during winter was 280.9 mm and 253.1
mm, in GSV and SG, respectively (Figure 3.9).
The relative proportion of undersize garfish retained in the 32SK pocket ranged from
0.3% in GSV during summer to 9.5% in SG during winter (Figure 3.10). Overall,
5.9% of garfish retained in the 32SK pocket were undersize. In total 25.6% of legal
size garfish escaped the 32SK pocket. The retention rate of 1 and 2 year old garfish
was 45.0% and 74.6%, respectively (Figure 3.10). Retention rates exceeded 80% for
3+ and older garfish.
There appeared to be strong seasonal differences in the relative selectivity of the
32SK pocket. The selectivity ogives generated from the summer trials across both
gulfs exhibited L50% estimates that were up to 18.8 mm smaller than the
corresponding winter estimates (Figure 3.11). The L50% estimate generated from the
GSV summer trial was 226.7 mm, whereas the estimate from the SG trial was 12.0
mm shorter, both of which were <LML (Figure 3.11). The shape of the selectivity
ogives for the winter trials were more defined as a greater proportion of small to
medium size (180 – 240 mm) garfish were sampled. Estimates of L50% exceeded the
LML for both the GSV and SG winter trials at 233.6 mm and 233.5 mm, respectively
(Figure 3.11).
21
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
GULF ST. VINCENTSUMMER
n = 3,741
0
25
50
75
100
125
150
175
200
225
250
275
300
SPENCER GULFSUMMER
n =7,400
150
160
170
180
190
200
210
220
230
240
250
260
270
280
290
300
310
320
330
340
350
0
10
20
30
40
50
60
70
80
90
100
110
120
32SK
Control
GULF ST. VINCENTWINTER
n = 3,791
150
160
170
180
190
200
210
220
230
240
250
260
270
280
290
300
310
320
330
340
350
0
50
100
150
200
250
300
350
400
SPENCER GULFWINTERn = 12,647
Ave: 269.3 mm
Ave. 268.8 mm
Ave: 280.9 mm
Fre
qu
en
cy
TL (mm)
LM
L
LM
L
Ave. 253.1 mm
Figure 3.9. Size structure of garfish caught in the 32SK and control nets in each trial.
150
160
170
180
190
200
210
220
230
240
250
260
270
280
290
300
310
320
330
340
350
0
100
200
300
400
500
600
ALL 32SKn = 27,581
<LMLretained = 5.9%>LMLescaped = 25.6%
Ave: 265.4 mm
Fre
qu
en
cy
TL (mm)
LM
L
0+ 1+ 2+ 3+ 4+ 5+ 6+
0
2,000
4,000
6,000
8,000
10,000
12,000
14,000
16,000
18,000
20,000
32SK
Control
17.1
45.0
74.6
83.8
90.7 92.6
Estimated age (years)
Figure 3.10. Combined size and age structures of garfish caught in the 32SK trial.
120
130
140
150
160
170
180
190
200
210
220
230
240
250
260
270
280
290
300
310
320
330
340
350
360
TL (mm)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Pro
po
rtio
n r
eta
ined
GSV_Summer 226.7 mm
GSV_Winter 233.6 mm
SG_Summer 214.7 mm
SG_Winter 233.5 mm
LM
L
Figure 3.11. Length at 50% selectivity (L50%) for each separate 32SK pocket trials.
22
3.4.5 Selectivity of the 34 mm knotless pocket (34KL)
A total of 31,507 garfish were caught across all four trials, of which 19,490 (61.9%)
were retained in the 34KL pocket. All size classes of garfish were relatively well
represented, with the exception of a lack of small garfish within the 200 – 220 mm
size range in SG during summer (Figure 3.12). There was also considerable
representation of large (>310 mm) garfish in the SG summer trial. The average size
of garfish retained in the 34KL pocket ranged from 255.5 mm in SG during winter to
274.8 mm in GSV during winter (Figure 3.12). The combined average size was
264.7 mm.
The relative proportion of undersize garfish retained in the 34KL pocket ranged from
0.3% in GSV during winter to 4.5% in GSV during summer (Figure 3.13). Overall,
2.6% of garfish retained in the 30SK pocket were undersize. In total 25.1% of legal
size garfish escaped the 32SK pocket. The retention rate of 1 and 2 year old garfish
was 50.5% and 77.5%, respectively (Figure 3.13). Retention rates exceeded 85% for
3+ and older garfish.
The 34KL pocket tended to select for smaller garfish during summer with L50%
estimates being up to 6.5 mm shorter than the corresponding winter estimates for
each of the two gulfs (Figure 3.14). This was the only pocket type that had length at
50% selectivity estimates that consistently exceeded the LML.
23
0
25
50
75
100
125
150
175
200
225
250
GULF ST. VINCENTSUMMER
n = 11,697
0
20
40
60
80
100
120
140
160
180
200
SPENCER GULFSUMMER
n =4,376
150
160
170
180
190
200
210
220
230
240
250
260
270
280
290
300
310
320
330
340
350
0
20
40
60
80
100
120
140
160
180
200
34KL
Control
GULF ST. VINCENTWINTER
n = 5,276
150
160
170
180
190
200
210
220
230
240
250
260
270
280
290
300
310
320
330
340
350
0
50
100
150
200
250
300
350
400
SPENCER GULFWINTERn = 10,156
Ave: 265.0 mm
Ave. 269.3 mm
Ave: 274.8 mm
Fre
qu
en
cy
TL (mm)
LM
L
LM
L
Ave. 255.5 mm
Figure 3.12. Size structure of garfish caught in the 34KL and control nets in each trial.
150
160
170
180
190
200
210
220
230
240
250
260
270
280
290
300
310
320
330
340
350
0
100
200
300
400
500
600
700
ALL 34KLn = 31,507
<LMLretained = 2.6%>LMLescaped = 25.1%
Ave: 264.7 mm
Fre
qu
en
cy
TL (mm)
LM
L
0+ 1+ 2+ 3+ 4+ 5+ 6+
0
2,000
4,000
6,000
8,000
10,000
12,000
14,000
16,000
18,000
20,000
34KL
Control
18.3
50.5
77.5
86.8
92.2 94.4
Estimated age (years)
Figure 3.13. Combined size and age structures of garfish caught in the 32SK trial.
120
130
140
150
160
170
180
190
200
210
220
230
240
250
260
270
280
290
300
310
320
330
340
350
360
TL (mm)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Pro
po
rtio
n r
eta
ine
d
GSV_Summer 234.9 mm
GSV_Winter 241.4 mm
SG_Summer 230.8 mm
SG_Winter 232.94 mm
LM
L
Figure 3.14. Length at 50% selectivity (L50%) for each separate 34KL pocket trials.
24
3.4.6 Comparison of the three pocket types.
There was a general linear relationship between estimates of L50% and mesh size,
however, the nature of this relationship did not remain consistent throughout the year
and appeared to be influenced by season (Figure 3.15). All three pocket types
trialled during this study retained proportionately more small garfish during the
summer with seasonal differences in estimates of L50% ranging from 5.2 mm to 17.2
mm for the 34KL and 32SK pockets, respectively (Figure 3.15).
The greatest proportion of undersize fish was retained in the 30SK pocket. On
average, the catch from this pocket consisted of 18.7 6.2% of <LML garfish which
was significantly more (F2,10 = 7.22, p = 0.02) than for both the 32SK and 34KL
pockets at 4.7 12.1% and 2.1 0.9%, respectively (Figure 3.16A). The relative
proportion of undersize garfish in the 32SK and 34KL pockets were similar. The
relative quantities of escaped legal-size garfish from the experimental pockets were
statistically similar for all three pocket types (F2,10 = 0.80, p = 0.48) (Figure 3.16B).
Similarly, the proportions of garfish from the 1+ and 2+ age classes retained by each
of the pockets remained relatively consistent, at ~50% (p = 0.3) and ~80% (p = 0.5),
respectively (Figures 3.16B & C).
30 31 32 33 34
Pocket mesh size (mm)
210
220
230
240
250
L5
0% (
mm
)
Summer
Y = 5.1 * X + 60.97
r2 = 0.98
Winter
Y = 3.7 * X + 116.83
r2 = 0.77
Figure 3.15. The seasonal relationships between pocket mesh size and their respective lengths at 50% selection (L50%). Linear relationships are provided.
25
30SK 32SK 34KL
0
5
10
15
20
25
30
Me
an
%>
LM
Les
cap
ed
se
30SK 32SK 34KL
0
5
10
15
20
25
30
35
40
Me
an
%<
LM
Lre
tain
ed
se
30SK 32SK 34KL
0
10
20
30
40
50
60
70
80
90
100
Me
an
% A
ge
1+
s
e
30SK 32SK 34KL
0
10
20
30
40
50
60
70
80
90
100
Me
an
% A
ge
2+
s
e
aab
b
a
b
b
a
a a
a
a a
A. B.
C. B.
a a
Figure 3.16. Comparison of the mean (A.) % of undersize garfish retained in the pocket; (B.) % of legal garfish that escaped the pocket; (C.) % of age 1+ garfish retained in the pocket; and (D.) % of age 2+ garfish retained in the pocket, for each of the three pocket types. Means with the same lower case letter are not significantly different from each other, as determined using a Hochberg‟s GT2 post hoc test.
26
3.5 Model simulations
3.5.1 Market value
A total of 2,863 garfish, representing 61 individual catches, were measured at the
SAFCOL market from 30th June 2010 to 18 May 2011. A small sub-sample of garfish
(~1kg) was purchased from each of the measured catches and ranged in price from
$4.50 - $14.50 kg-1. Each sample was accompanied by an average of 48.5 4.3
length measurements. There was a weak, yet significant positive relationship (F1,60 =
13.26, p = 0.001), between market value and average garfish size, with size
accounting for ~18% of the variation in price (Figure 3.17). Clearly, there are other
factors that contribute to market fluctuations, however, for the purpose of this study
this result was considered an appropriate baseline and was used in the subsequent
economic analysis (conferred at the Marine Fishers Association AGM 24th June
2011).
230 240 250 260 270 280 290 300 310 320 330 340
Average TL (mm)
0
2
4
6
8
10
12
14
16
18
20
$ k
g-1
Y = 0.087 * X - 15.12
r2= 0.18
Figure 3.17. Approximate market value ( 95% confidence limits) of garfish by average size.
3.5.2 Model output
The model output indicated that standardising hauling net mesh size across the
entire fishing fleet would promote the recovery of the garfish fishery, however, the
overall extent of this recovery can be considered minor. Model output indicated that
the exclusive use of the 32SK pocket simulated would increase garfish biomass by
3.3% over a six year period, which out-performed the 34KL and 30SK pockets by
0.7% and 1.5%, respectively (Figure 3.18A). The response rate to the gear change
was rapid with all three pocket types eliciting the greatest increases in biomass within
the first year. The relative rates of increase, however, did not sequentially
correspond to increases in mesh size, with the 32SK pocket consistently yielding
27
higher estimates of biomass than the 34KL pocket (Figure 3.18A). The instant
increase in biomass would likely be due to a higher proportion of „small‟ garfish
sieving through the mesh pockets and being left in the population to grow. Many of
these „small‟ garfish would be of legal size and therefore can be considered lost
catch. This is clearly reflected in the immediate decrease in both catch and value
estimates. Catch declined within the first year of the simulation from -1.0% for the
30SK to -3.0% for the 32SK (Figure 3.18B). This translated to an initial loss of value
ranging from -0.5% to -2.0%, for the 30SK and 32SK pockets, respectively (Figure
3.18C). Catches, however, fully recovered within two years and with the exception of
a moderate decline in 2004 through the exclusive use of the 32SK pocket, catches
remained relatively stable throughout the duration of the simulation ( 1% of the
baseline). Similarly, the relative value of the fishery recovered within the first year
and generated a profit in subsequent years. By 2007 the simulated increase in value
ranged from 0.8% for the 30SK to 1.7% for the 32SK (Figure 3.18C).
Egg production also increased as a function of standardising the fishing fleet.
Simulated estimates indicated that there was an immediate increase in egg
production for each of the three pocket types, which increased in the first year by
1.1% for the 30SK pocket to 2.7% for the 32SK pocket (Figure 3.18D). The 32SK
pocket demonstrated the most beneficial long-term performance as egg production
increased by ~4% over six years. This result was marginally better than the 34KL
pocket which yielded an increase of 3.1% over the same time period, whereas the
30SK pocket increased egg production by ~2% (Figure 3.18D).
The moderate decline in all four of the simulated outputs (i.e. biomass, catch, value,
and egg production) observed in 2004 for the 32SK pocket is an artefact of the
logistic selectivity function used within the model. Given that few undersize garfish
were caught by the 32SK pocket during the summer trials (Figure 3.9), the estimates
of selection for fish within the 200 – 230 mm size range is exaggerated and therefore
overly sensitive to the model simulations.
28
0
1
2
3
4
5
A. Biomass
-3
-2
-1
0
1
2
3
B. Catch
-3
-2
-1
0
1
2
3
C. Value
2000 2001 2002 2003 2004 2005 2006 2007
0
1
2
3
4
5
30SK
32SK
34KL
D. Egg Production
% D
iffe
ren
ce
Year
Figure 3.18. Simulated GarEst model output of the response of the garfish fishery to the theoretical standardisation of the fishing fleet to using of one of the three pocket types. Simulations were hind-cast back to 2001 and run through to 2007. The fishery parameters of interest were: (A.) biomass; (B.) catch; (C) value; and (D.) egg production.
29
3.5.3 Model extension
Because the expected improvements in garfish biomass, total catch, landed value
and egg production were small when increasing the pocket mesh size from 30SK to
34KL, as reported in the previous section, two additional strategies were simulated.
Using the same GarEst-based management strategy evaluation tool, we further
considered „theoretical‟ scenarios of:
(1.) An increase in pocket mesh size to 38 mm, and
(2.) Reductions in total fishing effort (by all sectors and gears) by 15%, 30%
and 45%.
The exclusive use of the „nominal‟ 38 mm mesh pocket yielded striking improvements
in garfish biomass and egg production, increasing each by >30% within a three-year
period (Figure 3.19). The magnitude of these increases dwarfed the respective
simulated output from the three other trialled pocket types. Although there were
long-term gains in adopting a standard 38 mm pocket, the fishery would suffer an
immediate 55% reduction in catch which would translate to a 43% loss in landed
value. These losses, however, were simulated to be short-term, with fishers turning
a profit of ~8% within three years, which further increased to ~13% within seven
years, assuming larger garfish landed continue to command a higher price (Figure.
3.19).
Disregarding the use of any standardised pocket to target garfish and concentrating
purely on theoretical reductions in fishing effort also appeared to benefit the fishery.
Reducing effort by at least 15% produced an immediate improvement in biomass and
egg production (Figure 3.20). Reducing effort by 15% resulted in a 10% increase in
biomass within three years and a 14% increase in egg production over the same time
period. Reducing effort by 30% and 45% produced corresponding increases by at
least 20% and 30%, respectively (Figure 3.20). Each of these three effort reduction
simulations resulted in an immediate decline in catch ranging from -10% for the 15%
reduction in effort to -25% for the 30% reduction in effort (Figure 3.20). This naturally
translated to corresponding loss in value of up to 46%. The model output indicated
that fishers continued to harvest reduced catches throughout the six year period, and
each of the three scenarios yielded marginal increases (ranging from 0.1% to 1.8%)
in landed value within four years (Figure 3.20).
30
0
5
10
15
20
25
30
35
40
45
50
A. Biomass
-60-55-50-45-40-35-30-25-20-15-10
-505
1015
B. Catch
-50
-40
-30
-20
-10
0
10
20
30
C. Value
2000 2001 2002 2003 2004 2005 2006 2007
0
5
10
15
20
25
30
35
40
45
50
30SK
32SK
34KL
38 mm
D. Egg Production
% D
iffe
ren
ce
Year
Figure 3.19. Simulated GarEst model output of the response of the garfish fishery to the theoretical standardisation of the fishing fleet to include a hypothetical 38 mm mesh pocket. The fishery parameters of interest were: (A.) biomass; (B.) catch; (C.) value; and (D.) egg production.
31
0
5
10
15
20
25
30
35
40
45
50
A. Biomass
-30
-25
-20
-15
-10
-5
0
5
10
15
B. Catch
-50
-40
-30
-20
-10
0
10
20
30
C. Value
2000 2001 2002 2003 2004 2005 2006 2007
0
5
10
15
20
25
30
35
40
45
50
15%
30%
45%
D. Egg Production
% D
iffe
ren
ce
Year Figure 3.20. Simulated GarEst model output of the response of the garfish fishery to the theoretical reductions in fishing effort by 15%, 30% and 45%. The fishery parameters of interest were: (A.) biomass; (B.) catch; (C.) value; and (D.) egg production.
32
3.6 Non-targeted catch
An estimated total of 129,288 individuals, representing 44 different species were
caught over the course of this study (Table 3.3). The estimated total biomass caught
was ~12,735 kgs. Approximately half (47.7%) of the species were represented by
<12 individuals. The target species, garfish, comprised the bulk of the catch,
accounting for 57.8% of the number caught and 38.6% of the total biomass.
Australian herring ranked second both in number (28.4%) and biomass (32.9%).
With the exception of weeping toadfish, which ranked third (6.4% in number, 7.8% in
biomass), all of the top 10 most prevalent species have commercial value in the
Marine Scalefish Fishery (Table 3.3). Of these, southern garfish, Australian herring,
King George whiting, snook, yellowfin whiting, blue crab and Western Australian
salmon all have legal minimum lengths (LMLs) and with the exception of blue crabs
can be sold by commercial hauling net fishers.
The multi-species catch composition remained relatively similar across all of the
mesh selectivity trials, regardless of the type of pocket used, region or season fished
(Figure 3.21). In each case, ANOSIM did not detect any differences at the
significance level (p) of 0.05 (Figure 3.21). Of the three comparisons, the greatest
level of dissimilarity in multi-species catch was detected between the two gulfs (p =
0.058). An analysis of the similarity percentages of each of the contributing species
(SIMPER analysis) indicated that this level of dissimilarity was predominantly driven
by large catches of Australian herring in SG. The average dissimilarity in Australian
herring catches between the two gulfs was 5.62%. All of the remaining contributing
species differed by <3.5%.
33
Table 3.3. Summary of the species captured throughout the study. The table shows the common and scientific names as described in Gomon et al. (2008), the number of individuals caught, their total weight (kg), and relative proportion (%) of total catch by number (n) and by weight (wt). * Listed commercial MSF species. ^ The species has a regulated legal minimum length (LML).
Common name Scientific name n weight (kg) %n %wt
1 Southern Garfish*^ Hyporhamphus melanochir 74,784 4,914 58 39
2 Australian Herring* Arripis georgianus 36,726 4,193 28 33
3 Weeping toadfish Torquigener pleurogramma 8,271 997 6 8
4 Western Striped Grunter* Pelates octolineatus 2,824 219 2 2
5 King George Whiting*^ Sillaginodes punctatus 2,371 698 2 5
6 Southern Calamary* Sepioteuthis australis 1,304 333 1 3
7 Snook*^ Sphyraena novaehollandiae 999 408 1 3
8 Yellow fin Whiting*^ Sillago schomburgkii 502 100 0 1
9 Blue Crab*^ Portunus armatus 273 31 0 0
10 Western Australian Salmon*^ Arripis truttaceus 232 58 0 0
11 Spinytail Leatherjacket Acanthaluteres brownii 207 11 0 0
12 Blue Weed Whiting Haletta semifasciata 142 5 0 0
13 Yellow eye Mullet*^ Aldrichetta forsteri 133 30 0 0
14 Globefish Diodon nicthemerus 108 80 0 1
15 Prickly Toadfish Contusus brevicaudus 89 6 0 0
16 Bridled Leatherjacket Acanthaluteres spilomelanurus 70 1 0 0
17 Southern Eagle Ray* Myliobatis australis 44 378 0 3
18 Sixspine Leather Jacket Meuschenia freycineti 33 9 0 0
19 Southern Fiddler Ray* Trygonorrhina fasciata 31 62 0 0
20 Soldierf ish Gymnapistes marmoratus 25 0 0 0
21 Bronze Whaler*^ Carcharhinus brachyurus 21 70 0 1
22 Port Jackson Shark* Heterodontus portjacksoni 12 38 0 0
23 Yellow tail Scad Trachurus novaezelandiae 10 1 0 0
24 Gummy Shark*^ Mustelus antarcticus 9 32 0 0
25 Rock Flathead*^ Platycephalus laevigatus 9 7 0 0
26 Smooth Stingray* Dasyatis brevicaudata 9 26 0 0
27 Ornate Cow fish Aracana ornata 7 1 0 0
28 Western Shovelnose Ray* Aptychotrema vincentiana 7 12 0 0
29 Little Weed Whiting Neodax balteatus 6 0 0 0
30 Dumpling Squid Euprymna tasmanica 5 0 0 0
31 Southern Sand Flathead*^ Platycephalus bassensis 5 2 0 0
32 Rock Crab Nectocarcinus integrifrons 3 0 0 0
33 Estuary Cobbler Cnidoglanis macrocephalus 2 5 0 0
34 Rough Leatherjacket Scobinichthys granulatus 2 0 0 0
35 Shaw 's Cow fish Aracana aurita 2 2 0 0
36 Silver Trevally Pseudocaranx georgianus 2 0 0 0
37 Dusky Morw ong Dactylophora nigricans 2 1 0 0
38 Beaked Salmon Gonorynchus greyi 1 0 0 0
39 Giant Cuttlefish* Sepia apama 1 1 0 0
40 Nova Cuttlefish* Sepia novaehollandiae 1 0 0 0
41 Spotted Pipefish Stigmatopora argus 1 0 0 0
42 Southern Pygmy Leatherjacket Brachaluteres jacksonianus 1 0 0 0
43 Elongate Bullseye Parapriacanthus elongatus 1 0 0 0
44 Southern Bluespotted Flathead* Platycephalus speculator 1 0 0 0
Total 129,288 12,735
34
Summer
Winter
30 SK
32 SK
34 KL
GSV
SG
2D Stress = 0.09
A.
B.
C.
p = 0.624
p = 0.058
p = 0.543
Figure 3.21. Non-parametric MDS plots that assess the effects of (A.) season, (B.) region, and (C.) hauling net pocket type on the multi-species catches when targeting garfish.
35
3.6.1 Australian herring (Arripis georgianus)
Each of the three test pockets caught considerable numbers (>3,000) of Australian
herring that ranged in size from 112 – 270 mm (TL) (Figure 3.22). The size
structures were relatively consistent amongst the trials, with the majority of fish falling
within the 190 – 250 mm size class. Smaller (<190 mm) herring were captured
during the 30SK and 32SK trials. Most of the herring were retained within the test
pockets, with retention rates ranging from 70.4% for the 30SK to 97.8% for the 34KL.
There was little evidence of any size selectivity as the small fish (<180 mm) were
consistently retained in the test pockets and, conversely, large fish (>230 mm) were
present in the control net (Figure 3.22). All size classes of herring sampled had a
>70% probability of being retained in each of the three pocket types.
0
100
200
300
400
500
600
700
test pocket
control pocket
selectivity curve
0
0.2
0.4
0.6
0.8
130SK
n = 3,092
110
120
130
140
150
160
170
180
190
200
210
220
230
240
250
260
270
280
290
300
310
320
330
340
350
0
250
500
750
1000
1250
1500
1750
2000
0
0.2
0.4
0.6
0.8
134KL
n = 10,483
0
250
500
750
1000
1250
1500
1750
2000
0
0.2
0.4
0.6
0.8
132SK
n = 17,754
TL (mm)
Fre
qu
en
cy
Pro
po
rtion
reta
ine
d
Figure 3.22. The size selectivity of each of the three test pockets for Australian herring.
36
3.6.2 Weeping toadfish (Torquigener pleurogramma)
The weeping toadfish ranged in size from 60 – 298 mm (Figure 3.23). In each trial
>90% of toadfish were retained in the test pockets. There was no evidence of any
size selection. All size classes of toadfish caught during the 30SK and 32SK trials
had a >80% probability of being retained in their respective test pockets. Estimates
of selectivity decreased to ~70% for relatively large (>295 mm) toadfish in the 34KL
trial, however, this estimate was strongly influenced by a small proportion of toadfish
that were incidentally caught in the control net and were unlikely to have escaped
through the 34 mm knotless mesh pocket (Figure 3.23).
0
10
20
30
40
50
60
70
80
90
100
test pocket
control pocket
selectivity curve
0.0
0.2
0.4
0.6
0.8
1.0
30SKn = 749
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200
210
220
230
240
250
260
270
280
290
300
0
50
100
150
200
250
300
350
400
450
500
550
600
0.0
0.2
0.4
0.6
0.8
1.0
34KLn = 4,260
0
25
50
75
100
125
150
175
200
225
250
0.0
0.2
0.4
0.6
0.8
1.0
32SKn = 1,181
TL (mm)
Fre
qu
en
cy
Pro
po
rtion
reta
ine
d
Figure 3.23. The size selectivity of each of the three test pockets for weeping toadfish.
37
3.6.3 Western striped grunter (Pelates octolineatus)
Western striped grunters ranged in size from 92 – 325 mm (Figure 3.24). The size
distribution for each net trial was multi-modal, generally consisting of a cohort of
small fish within the 100 – 150 mm size range and a cohort of medium size fish within
160 – 220 mm. Larger grunters (>260 mm) were caught during the 32SK and 34KL
trials (Figure 3.24). Retention rates ranged from 48.5% to 72.4% in the 30SK and
32SK pockets, respectively. Approximately half (52.7%) of the fish caught during the
34KL trial were retained in the test pocket. Each of the three test pockets exhibited a
degree of size selectivity. The length at 50% selection (L50%) was clearly defined at
142 mm in the 30SK pocket as there were relatively few large (>180 mm) grunters
that were retrieved from the control net. Selectivity ogives for the 32SK and 34KL
pockets were not as defined due to the higher proportion of medium to large fish
(>190 mm) evading capture by the test pockets. Estimates of L50% for these two
pockets were 119 mm and 174 mm, respectively.
0
5
10
15
20
25
30
35
40
45
50
test pocket
control pocket
selectivity curve
0.0
0.2
0.4
0.6
0.8
1.030SKn = 388
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200
210
220
230
240
250
260
270
280
290
300
310
320
330
340
0
5
10
15
20
25
30
35
40
45
50
0.2
0.4
0.6
0.8
1.0
34KLn = 647
0
10
20
30
40
50
60
70
80
90
100
0.2
0.4
0.6
0.8
1.0
32SKn = 1,313
Fre
qu
en
cy
TL (mm)
Pro
po
rtion
retia
ne
d
Figure 3.24. The size selectivity of each of the three test pockets for western striped grunter.
38
3.6.4 King George whiting (Sillaginodes punctatus)
Negligible numbers (n = 7) of King George whiting (KGW) were caught using the
30SK pocket. Consequently, it is difficult to make any meaningful inferences
regarding the selective properties of this pocket type. Considerable numbers (>767)
of fish, however, were caught in each of the remaining two pocket types and they
ranged in size from 180 – 499 mm (Figure 3.25). The size composition of KGW
caught during the 32SK trials were distributed around two modes, 210 mm and 390
mm (Figure 3.25). These size classes were also evident during the 34KL trials,
however there was an additional intermediate size class distributed around a mode of
310 mm (Figure 3.25). Retention rates for the 32SK and 34KL pockets were 68.8%
and 85.0%, respectively. The 32SK pocket retained proportionately more undersize
KGW than the 34KL pocket (37.1% cf 16.0%) and lost a smaller proportion of legal
fish (5.2% cf 7.8%) (Figure 3.25). Estimates of L50% for the 32SK and 34KL pockets
were 215 mm and 160 mm, respectively, both well below the LML of 310 mm (Figure
3.25).
0
1
2
3
4
5
6
7
8
9
10
test pocket
control pocket
selectivity curve
0
0.2
0.4
0.6
0.8
130SKn = 7
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
0
10
20
30
40
50
60
70
80
90
100
0
0.2
0.4
0.6
0.8
134KL
n = 1,040
0
10
20
30
40
50
60
70
80
90
100
0
0.2
0.4
0.6
0.8
132SKn = 767
LM
L
Fre
qu
en
cy
TL (mm)
<LML = 100%
<LML = 37.1%
<LML = 16.0%
Pro
po
rtion
reta
ine
d
Figure 3.25. The size selectivity of each of the three test pockets for King George whiting.
39
3.6.5 Southern calamary (Sepioteuthis australis)
Catches of calamary ranged from 17 individuals in the 30SK trial to 622 in the 32SK
trial and ranged in size from 64 – 332 mm mantle length (ML) (Figure 3.26). All of
the calamary caught during the 30SK trial were retained in the test pocket, whereas
~70% were retained in the 32SK and 34KL pockets. There was no evidence of any
size selection for calamary for each of the three pocket types (Figure 3.26).
0
1
2
3
4
5
6
7
8
9
10
test pocket
control pocket
selectivity curve
0.0
0.2
0.4
0.6
0.8
1.0
30SKn = 17
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200
210
220
230
240
250
260
270
280
290
300
310
320
330
340
350
0
10
20
30
40
50
60
70
80
0.0
0.2
0.4
0.6
0.8
1.0
34KLn = 481
0
10
20
30
40
50
60
0.0
0.2
0.4
0.6
0.8
32SKn = 622
Fre
qu
en
cy
ML (mm)
Pro
po
rtion
reta
ine
d
Figure 3.26. The size selectivity of each of the three test pockets for southern calamary.
40
3.6.6 Snook (Sphyraena novaehollandiae)
Catches of snook ranged from nine individuals in the 30SK trial to 545 in the 34KL
trial and ranged in size from 240 – 900 mm (Figure 3.27). All of the snook caught
during the 30SK and 32SK trials were retained in the test pocket, whereas 84.0%
were retained in the 34KL pocket. The relative proportion of undersize snook caught
by the test pockets ranged from 22.2% for the 30SK to 17.6% for the 34KL (Figure
3.27). There was no evidence of any size selection for snook for each of the three
pocket types (Figure 3.27).
0
2
4
6
8
10
test pocket
control pocket
selectivity curve
0.0
0.2
0.4
0.6
0.8
1.0
30SKn = 9
220
250
280
310
340
370
400
430
460
490
520
550
580
610
640
670
700
730
760
790
820
850
880
0
10
20
30
40
50
60
0.0
0.2
0.4
0.6
0.8
1.0
34KLn = 545
0
4
8
12
16
20
0.0
0.2
0.4
0.6
0.8
1.0
32SKn = 143
LM
L
Fre
qu
en
cy
TL (mm)
<LML = 22.2%
<LML = 31.0%
<LML = 17.6%
Pro
po
rtion
retia
ne
d
Figure 3.27. The size selectivity of each of the three test pockets for snook.
41
3.6.7 Yellowfin whiting (Sillago schomburgkii)
Catches of yellowfin whiting (YFW) ranged from 57 individuals in the 32SK trial to
174 in the 34KL trial and ranged in size from 233 - 406 mm (Figure 3.28). The
retention rate was highest (86.0%) for the 32SK pocket, followed by the 34KL
(75.3%) and 30SK (63.8%) pockets. The 34KL pocket was the only one to capture
undersize YFW, however, these fish only constituted 2.3% of the total retained catch
(Figure 3.28). There was no evidence of any size selection for YFW for the 30SK
and 34KL pockets. The selectivity ogive generated for the 32SK pocket estimated a
L50% of 265 mm, however, given the small sample size (n = 57) and the absence of
the fish within the 230 – 250 mm size class, it is difficult to accept that it accurately
reflects the selective properties of this pocket type.
LM
L
0
5
10
15
20
25
30
35
40
test pocket
control pocket
selectivity curve
0.0
0.2
0.4
0.6
0.8
1.0
30SKn = 160
230
240
250
260
270
280
290
300
310
320
330
340
350
360
370
380
390
400
410
0
5
10
15
20
25
30
0.0
0.2
0.4
0.6
0.8
1.0
34KLn = 174
0
4
8
12
16
20
0.0
0.2
0.4
0.6
0.8
1.0
32SKn = 57
Fre
qu
en
cy
TL (mm)
<LML = 0.0%
<LML = 0.0%
<LML = 2.3%
Pro
po
rtion
reta
ine
d
Figure 3.28. The size selectivity of each of the three test pockets for yellowfin whiting.
42
3.6.8 Blue crab (Portunus armatus)
Blue crabs ranged in size from 21 – 148 mm carapace width (CW) (Figure 3.29).
The size distributions of crabs were relatively consistent between the three pocket
trials with the majority measuring within the 60 – 120 mm size range. The retention
of blue crabs by the 30SK and 32SK pockets was relatively low at 41.7% and 28.6%,
respectively, whereas the majority (91.5%) were retained in the 34KL pocket. Most
(>75%) of the retained crabs were undersize across all three pocket types. Although
based on relatively low sample sizes (n <85), the selectivity ogives for each pocket
type indicated that small crabs were more likely to be retained than large crabs
(Figure 3.29).
0
2
4
6
8
10
12
14
16
18
20
test pocket
control pocket
selectivity curve
0.0
0.2
0.4
0.6
0.8
1.0
30SKn = 84
20
30
40
50
60
70
80
90
100
110
120
130
140
150
0
1
2
3
4
5
6
7
8
9
10
0.2
0.4
0.6
0.8
1.0
34KLn = 64
0
1
2
3
4
5
6
7
8
9
10
0.0
0.2
0.4
0.6
0.8
1.0
32SKn = 14
LM
L
Fre
qu
en
cy
CW (mm)
<LML = 100%
<LML = 75%
<LML = 91.5%
Pro
po
rtion
reta
ine
d
Figure 3.29. The size selectivity of each of the three test pockets for blue crabs.
43
3.6.9 Australian salmon (Arripis truttaceus)
Catches of Australian salmon ranged from 48 individuals in the 30SK and 34KL trials
to 135 in the 32SK trial and ranged in size from 211 – 408 mm (Figure 3.30). All
salmon caught during the 34KL trial were caught in the test pocket, whereas 81.3%
and 63.0% were retained in the 30SK and 32SK pockets, respectively. None of the
pockets retained undersize salmon. There was no clear evidence of any size
selection for salmon for each of the three pocket types (Figure 3.30).
LM
L
0
2
4
6
8
10
test pocket
control pocket
selectivity curve
0.0
0.2
0.4
0.6
0.8
1.0
30SKn = 48
200
210
220
230
240
250
260
270
280
290
300
310
320
330
340
350
360
370
380
390
400
410
0
2
4
6
8
10
0.0
0.2
0.4
0.6
0.8
1.0
34KLn = 48
0
4
8
12
16
20
0.0
0.2
0.4
0.6
0.8
32SKn = 135
Fre
qu
en
cy
TL (mm)
<LML = 0.0%
<LML = 0.0%
<LML = 0.0%
Pro
po
rtion
reta
ine
d
Figure 3.30. The size selectivity of each of the three test pockets for Australian salmon.
44
3.7 Fisher survey
A total of 39 commercial hauling net fishers who had either targeted or caught garfish
in 2009 were identified from SARDI‟s commercial catch and effort database and were
sent a „Garfish Netters‟ survey through the mail. Of these, 23 (59.0%) fishers
responded whilst two (5.1%) surveys had been sent to incorrect addresses and were
returned unopened.
A wide variety of net types exist within the garfish fishery, differing in mesh size,
material, construction, and configuration. In most cases (69.6%), commercial hauling
net fishers possessed two or more different nets to target garfish (Figure 3.31).
Fishers generally alternated their nets on a seasonal basis, however, some
alternated their use on the basis of tide, fish availability, time of day, and region.
The mesh size of the pocket sections of the hauling nets ranged from the legal
minimum of 30.0 mm to 34.0 mm. Most fishers (56.5%) used 32.0 – 32.9 mm mesh
pockets, whereas 39.1% used 30.0 – 31.9 mm, and the remaining 4.3% preferred the
34.0 – 34.9 mm mesh size (Figure 3.32). Floating hauling nets were the most
common across each of the pocket grades (Figure 3.32A).
Three different material combinations were used to construct the mesh pockets;
nylon, polypropylene and a combination of both. Overall, nylon was the most
common (62.8%), followed by the polypropylene/nylon combination (23.3%) and
polypropylene (13.9%). All of the 34.0 – 34.9 mm pockets and a large proportion
(77.3%) of the 32.0 – 32.9 mm pockets were constructed from nylon (Figure 3.32B).
Approximately half (47.1%) of the 30.0 – 31.9 mm pockets were constructed from a
combination of polypropylene and nylon, 35.3% nylon and the remaining 17.6%
polypropylene (Figure 3.32B).
The ply of the material also differed amongst the pockets ranging from 15 – 30 ply,
however, 18 and 24 ply twine were the most common. There was a general trend of
increasing ply from the small to the large mesh pockets (Figure 3.32C). The small
30.0 – 31.9 mm pockets were primarily constructed from 18 ply twine, whereas
75.0% of the large 34.0 – 34.9 mm pockets were constructed from 24 ply twine.
There was almost an equal proportion of 32.0 - 32.9 mm pockets constructed from 18
and 24 ply twine (45.5% cf. 50.0%) (Figure 3.32C).
The 34.0 – 34.9 mm pockets are relatively new to the fishery and all have been
constructed from interwoven knotless mesh. Although, there are a few knotless
pockets with mesh sizes <32.9 mm that are currently being used in the fishery, most
45
(89.7%) of the remaining pockets are constructed from the traditional knotted mesh
(Figure 3.32D).
0
10
20
30
40
50
60
70
1 2 3 4
# Nets per fisher
Perc
en
t (%
)
Figure 3.31. The number of nets per fisher used to target garfish.
Mix
Poly
Nylon
0
10
20
30
40
50
60
70
Other
24 ply
18 ply
30.0 - 31.9 32.0 - 32.9 34.0 - 34.9
0
10
20
30
40
50
60
70
Sinking
Floating
Knotless
Knotted
30.0 - 31.9 32.0 - 32.9 34.0 - 34.9
Pocket Mesh Size (mm)
Perc
en
t (%
)
A. B.
C. D.
Figure 3.32. The relative proportion (%) of fishers who use nets that differ in; (A.) operation; (B.) material; (C.) ply; and (D.) construction across each of the three pocket mesh size grades.
46
4 Discussion
The overall objective of this study was to identify the most appropriate hauling net to
maximise the safe escapement of „small‟, undersize garfish and promote stock
recovery in South Australia‟s garfish fishery.
4.1 Mesh selectivity
Mesh selectivity studies generally draw inferences from „snap-shot‟ experiments
where the investigator runs a series of replicated trials in a particular area over a
short time frame (Stewart et al. 2004, Rueda 2007). A major shortfall of these types
of trials is that they do not account for any regional or seasonal variation in the
morphology of fish that may affect their relative catchability. This study addressed
this issue and indeed found that although there was a general linear increase in L50%
with increasing mesh size there was also a strong seasonal component, where each
pocket type consistently selected for smaller garfish during the summer. A
comparison of the length-weight relationships of garfish between the two seasons
indicated that those caught in summer were generally heavier („fatter‟) for their length
and were, therefore, less likely to escape through a mesh pocket than their „skinnier‟
winter equivalents. This morphological difference can be attributed to seasonal
reproductive condition, as garfish spawn from October to March and are typically
laden with mature gonads and accumulated fat reserves (Ye et al. 2002). Anecdotal
information has also intimated that there are regional differences in the relative
condition and, therefore, catchability, of garfish between the two gulfs with the
Spencer Gulf population consisting of a greater proportion of „smaller fatter‟
individuals compared with Gulf St. Vincent garfish. A regional comparison of the
length-weight relationship, however, could not substantiate this, as no significant
difference in relative condition was detected.
The selective performance of fishing nets is typically evaluated in relation to the
target species‟ legal minimum length (Millar and Fryer 1999). Nets that reliably
harvest fish above a prescribed size limit and do not encounter unacceptable losses
of legal size „saleable‟ fish are generally preferred. On the basis of these criteria the
30SK pocket trialled in this study performed sub-optimally, as all estimates of L50%
were less than the LML of 230 mm. The contemporary use of these pockets is
largely a relic of historical management arrangements, as prior to 2001 the LML for
garfish was 210 mm and at that time the regulated minimum 30 mm mesh size was
considered appropriate (Jones 1982). The regulated minimum mesh size was not
adjusted to correspond with the increase in the LML, so fishers were permitted to
47
continue to use this sub-optimal 30SK pocket to target garfish. There was a
moderate improvement with the 32SK pocket with winter L50% estimates exceeding
the LML, whereas the summer estimates continued to catch a larger proportion of
undersize garfish. The 34KL pocket was the only gear type that consistently yielded
estimates of L50% that exceeded the LML for garfish. Furthermore the average
retention rate of undersize garfish in the 34KL pocket was ~2%, which was
moderately better than the 32SK pocket at ~6%. The retention rate of undersize
garfish in the 30SK pocket was an undesirable 19%.
Overall, there was no statistical difference in the average proportion of lost „saleable‟
(>LML) garfish between the three pocket types. In each case, approximately 20% of
legal size garfish avoided capture by the experimental pockets and were retrieved
from the control net. In some instances relatively large (>300 mm TL) garfish were
found to have escaped from the experimental nets. It would be physically impossible
for these large garfish to have sieved through the experimental pocket mesh and it is
more likely that they evaded capture by swimming under the net‟s lead line, through
tears in the wing section, or were simply caught in the gap between the two nets. As
such, the relative loss of legal size garfish for each of the trialled pocket types can
not be confidently assessed from the results obtained in this study.
Although there is a strong emphasis in determining the size selective properties of
each pocket type it is also important to understand their respective age selective
properties. This is particularly relevant in South Australia‟s garfish fishery as there is
compelling evidence that the population‟s age structure has become considerably
truncated to consist of predominantly 1+ and 2+ age classes (Fowler and Ling 2010).
The estimated age of sexual maturity for garfish is 17.5 months (Ye et. al. 2002),
therefore, an individual garfish within the current truncated population has the
capacity to participate in a maximum of two spawning seasons before being caught.
At the current high levels of exploitation (~70%) for the fisheries in both gulfs, this
two-year spawning potential is not considered to be adequate for rebuilding the
population‟s age structure back to historic levels where the fishery was dominated by
3+ and 4+ age classes during the 1950s (Fowler and Ling 2010). Increasing the
pocket mesh size from 30 mm to 32 mm had a positive, yet minor, effect on
increasing the population‟s spawning potential. The 30SK pocket was found to retain
~85% of 2+ garfish, and this was moderately improved to 75% in the 32SK pocket.
The 34KL pocket was slightly worse than the 32SK pocket, retaining 77% of 2+
garfish. On average, however, there was no statistical difference in the mean
capture of 1+ and 2+ garfish for each of the three pocket types. This indicated that
48
although the 34KL and 32SK pockets selected larger garfish than the 30SK pocket,
they were effectively caught from the same age class.
4.2 Model simulations
The GarEst model was the appropriate tool to simulate the response of the garfish
fishery to wholesale changes in fishing gear. This model adopted a retrospective
rather than a predictive approach, as the relative accuracy of the hind-cast „fitted‟
model has been established in the latest stock assessment report (McGarvey et al
2009). Overall, all three pocket types each demonstrated the capacity to promote
stock recovery, through rapid increases in biomass, value and egg production. The
extent of this recovery, however, was relatively minor, as there was a marginal <5%
improvement in all of the fishery parameters modelled over the 7-year timeframe. Of
the three pockets trialled, the 30SK performed the worst, only improving biomass and
value projections by ~1%. The simulated exclusive use of the 32SK pocket yielded
the most positive results, increasing biomass by ~3% and marginally out-performing
the 34KL by 0.7%. A similar trend was also evident for egg production and value.
Despite the 2 mm difference in mesh size between the 32SK and 34KL pockets, they
yielded similar results. The absence of a knot in the construction of the 34KL pocket
appeared to affect its overall selective properties. It is possible that the interwoven
design of the 34KL mesh compromised the ability of the net to maintain a rigid
structure in the water, and as a result increased its capacity to retain smaller garfish
and making it function more like a 32SK pocket.
The relative improvement of egg production for all three pocket types was the most
encouraging aspect of the model output as it did not account for the subsequent
concomitant increases in annual recruitment that would result from the additional
eggs that would be contributed over time. As such, the model only provided a
conservative „baseline‟ projection indicating that there is a greater capacity of the
fishery to rebuild. The relative rate of this compounded increase is likely to be rapid
as garfish grow quickly and are capable of contributing to egg production in their
second year. Despite the marginal, simulated improvements, the model output did
indicate that there is considerable merit in standardising hauling net gear to promote
stock recovery within the garfish fishery.
The model simulation indicated that any standardisation of the hauling net fleet would
initially result in an immediate decrease in catch that would cause a financial loss.
This level of hardship, however, would be expected to be short-lived as catch is
projected to recover within the second year when fishers would be likely to harvest a
49
greater proportion of larger „high-value‟ garfish. Although this economic scenario is
encouraging, it may be an over-simplification, as it assumes that the market value for
the various size grades of garfish will remain constant. Clearly there are other
external factors that contribute to determining the market value of garfish. This
project attempted to encapsulate seasonal variation in estimates of market value by
purchasing garfish over a 12 month period but could not consider market demand or
the relative condition of the product. As such, the economic simulation generated
from the model must be cautiously interpreted.
The extension of the simulation model to include output for a hypothetical 38 mm
mesh pocket yielded striking results, increasing estimates of biomass and egg
production by >30% within three years. This, however, was in response to an initial
short-term loss in catch (-55%) and value (-43%) within the first year. For dedicated
garfish haul netters, the magnitude of the initial economic loss may be too difficult to
withstand, despite the rapid recovery of the fishery and the expected long-term
simulated benefit. It should be emphasised, however, that this simulation exercise
was purely hypothetical and only considered to explore the potential ramifications of
using a larger (>34 mm) pocket mesh size. Clearly, this pocket type would require
dedicated field trials to validate its size selective characteristics and to ascertain
whether there are any associated by-catch issues.
Further extending the model to explore the effects of reducing effort levels to promote
stock recovery, rather than through the wholesale standardisation of fishing gear,
provided positive results. Reducing effort levels by 15% resulted in increases in
biomass and egg production by 10% and 14%, respectively, within three years.
Further reducing effort levels by 30% and 45% improved the projections. Once
again, however, these long-term benefits were accompanied with short-term losses
in catch of garfish (up to -25%) and landed value (up to -46%).
4.3 Non-targeted catch
Encountering by-catch and by-product within hauling net fisheries is unavoidable,
particularly as these types of nets sweep over relatively large fishing areas and
indiscriminately herd mobile fauna into the net pocket. Adjusting the mesh size of the
net is one way of reducing the capture of undersize target species, however, when
making these adjustments the associated flow-on effects towards non-targeted
species needs to be considered. In this study, 43 non-targeted species were caught,
of which 22 are legitimate commercial marine scalefish species and can be sold as
by-product. Overall there were no detectable differences in the composition of the
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multi-species catches between pockets used, regions or seasons fished. The top-ten
species caught represented 99.2% of the total number of individuals caught. Of this,
the targeted species, garfish, dominated (57.8%), followed in descending order by
Australian herring, weeping toadfish, western striped grunter, King George whiting
(KGW), southern calamary, snook, yellowfin whiting, blue crab and Western
Australian salmon.
Since 1979, there have been concerns about the inadvertent capture and
subsequent mortality of undersize KGW by the „small-mesh‟ hauling nets used to
target garfish (Jones 1979). Previous studies that aimed to address these concerns
demonstrated that the use of 30 mm mesh nets resulted in a high rate of mortality of
undersize KGW, however, total mortality was considered insignificant as the relative
proportion of KGW caught by the „small-mesh‟ net sector was considered negligible
in comparison to the Statewide total catch (Jones 1982, Kumar et al. 1995). Catches
of KGW by the 30SK pocket in this study were also negligible and, as such, no
inferences could be made about the selective properties of this pocket type. Both the
32SK and 34KL pockets, however, caught larger numbers (<1,050) of KGW during
their respective trials, with the 32SK pocket retaining proportionately more undersize
KGW than the 34KL pocket (37.1% cf 16.0%). In both cases, approximately 6% of
legal size (>310 mm) KGW had escaped or evaded capture by the experimental
nets. Although not conclusive, these escapees indicate that hauling nets do not
necessarily select and retain all of the „large‟ fish that they encircle. Despite the
capacity of the hauling nets to catch a diversity of non-targeted species, commercial
fishers are generally adept at targeting garfish and avoiding large quantities of
undersize KGW. This is because they target specific habitats known to support large
numbers of garfish, such as shallow zostera (“garweed”) meadows and intertidal
mud-flats, and avoid areas that are known to support undersize KGW, such as sandy
substrates and cystophora (“corkweed”) stands. A recent study that quantified by-
catch within the marine scalefish fishery also identified that catches of KGW by the
„small-mesh‟ hauling net sector were relatively small and averaged in their 10s of fish
per haul (Folwer et al. 2009). Increasing the mesh size from 30 mm to 34 mm
appeared to have a marginal effect on the relative catch of undersize KGW and,
given the small catches of KGW in general, the overall effect of the „small-mesh‟
hauling net sector remains an inconsequential risk to the State-wide KGW fishery.
The relative selectivity of the majority of the other non-targeted species by the three
trialled pocket types was difficult to ascertain. This was because most of these
species were either too large or had morphologies that prevented them from sieving
51
through the various pocket mesh sizes. For example: Australian herring and western
striped grunters possess dorsal and pectoral fin spines that entangle within the mesh;
weeping toados inflate to twice their size once captured; blue crabs tend to grip onto
the net using their claws; and calamary, snook, yellowfin whiting and Australian
salmon are generally too large to escape through „small-mesh‟ pockets. Under
normal fishing operations, these relatively robust species that are not retained as by-
product are either disentangled from the net and discarded or brailed from the pocket
and released in „good‟ condition. Previous studies have identified enmeshment as
the main cause of mortality for undersize KGW (Jones 1982, Kumar et al. 1995)
which is likely to be the case for a number of other species (Fowler et al. 2009).
Indeed, fish were observed to be enmeshed within the wing and pocket sections of
all three experimental nets, however, this study did not quantify specific rates of
enmeshment or distinguish enmeshed fish from those that were brailed from the
pocket. The design of the experiment also prohibited the release of fish caught by
the experimental net as they would have been re-caught in the encircling control net
which would have compromised the results of the selectivity study. As such, the
investigation of post-release survival of non-targeted catch was beyond the scope of
this project.
4.4 Fisher survey
Just over half (59%) of active hauling net fishers responded to the short survey. The
compiled results confirmed that there is indeed a wide variety of net types used to
target garfish, differing in mesh size, material, construction and configuration. Most
fishers (56%) preferred to use the 32SK pocket, approximately 40% continued to use
the 30SK pocket and the remaining 4% have adopted the new 34KL pocket. One of
the most compelling findings of this survey, which was verified through multiple
conversations with commercial fishers and independent inspection of various pocket
types, was the propensity of the net mesh to shrink over time. This was particularly
evident in nets that were constructed with polypropylene twine and initial
measurements using the standardised weighted callipers (Fig. 2.4) indicated that
mesh size can shrink by ~4 mm (Steer unpubl. data). Given this degree of
shrinkage, and the relatively high proportion (37%) of pockets that are either partially
or entirely constructed from polypropylene, it can be speculated that there are
numerous fishers who are currently unintentionally using „illegal‟ (<30 mm) gear to
target garfish. It should be emphasised, however, that these extracurricular findings
are preliminary and further investigation is required to assess the relative rate and
degree of shrinkage for each of the various net types.
52
4.5 Implications for management
The results of this study clearly indicated that in terms of the relative capture of
undersize garfish, seasonal estimates of L50% and retention of 1 and 2 year-old fish,
the performance of the 30SK pocket, which is the current regulated minimum mesh
size, was sub-optimal (Table 4.1). The larger two pocket types, however, were very
similar in their overall performance. This, similarity was further confirmed by the
model output, where respective projected estimates of biomass, catch, value and egg
production for both the 32SK and 34KL pockets were all within 0.7% of each other.
Currently, 39.1% of the commercial hauling net fishers target garfish with the sub-
optimal 30SK pockets and would be immediately affected by changes in gear
regulations. Also some of the 32SK pockets used by the fishers are likely to be
closer to 30 mm based on the estimated rate of shrinkage. If fishery management
decided to increase the minimum mesh size to 34 mm, then a further 56.5% of
fishers would be forced to change their gear (Table 4.1). Given the widespread
variation in pocket construction that currently exists within the fishery, coupled with
the fact that net mesh has propensity to shrink over time, it seems appropriate that
the hauling net sector adopts an agreed, standardised, pocket to target garfish which
in turn is expected to promote stock recovery. These standardised pockets would
require frequent inspection and/or certification to ensure that the dimensions of the
gear remain within the regulations.
Extending the model simulations to include a larger „hypothetical‟ mesh pocket and
reductions in fishing effort indicated alternate scenarios that may promote stock
recovery. Although this information is purely theoretical it does provide managers
with a greater understanding of the relative benefits of managing a fishery either
through the exclusive regulation of fishing gear, or through strategic effort reduction,
or a combination of both strategies.
4.6 Future considerations
The similarity in the selective properties and resultant model output of the 32SK and
34KL pockets makes it difficult to differentiate them from a management perspective.
Future work should extend the experiment to include a 34 mm standard knot pocket
to investigate whether a comparable 2 mm increase in mesh size is a practical
management alternative. Furthermore, the shrinking propensities of the different
mesh materials (i.e. polypropylene and nylon) should be addressed.
53
Table 4.1. Summary of major findings for each of the three pocket types trialled in this study. * The capacity of large fish to evade capture by the experimental pocket (e.g. swimming under the lead line) precludes an accurate estimate of „% escaped legal‟.
30SK 32SK 34KL
GARFISH
Average size (mm) 253.1 265.4 264.7
% undersize (<LML) 16.5 5.9 2.6
% escaped legal (>LML)* 16.6 25.6 25.1
Summer L50% 214.7 222.7 235.1
Winter L50% 225.5 239.9 240.3
% 1 year-old fish (1+) retained 60.9 45.0 50.5
% 2 year-old fish (2+) retained 85.0 74.6 77.5
KGW
% retained undersize (<LML) 100 37.1 16.0
% escaped legal (>LML)* 14.3 5.2 7.8
FISHERS
% use 39.1 56.5 4.3
% polypropylene 17.6 13.6 0.0
% nylon 35.3 77.3 100
% mix 47.1 9.1 0.0
MODEL OUTPUT (2001 - 2007)
Biomass % change 1.52 3.27 2.60
Catch % change -0.01 0.04 0.01
Value % change 0.81 1.72 1.45
Egg Prod. % change 1.96 3.73 3.14
54
5 References
Fowler AJ, Steer MA, Jackson WB, Lloyd MT (2008). Population characterisitics of southern sea garfish (Hyporhamphus melanochir,Hemiramphidae) in South Australia. Marine and Freshwater Research 59: 429-443.
Fowler AJ, Lloyd MT, Schmarr D (2009). A preliminary consideration of by-catch in the Marine Scalefish Fishery of South Australia. SARDI Publication No. F2009/000097-1. SARDI Research Report Series No. 365.
Fowler AJ, Ling JK (2010). Ageing studies done 50 years apart for an inshore fish species from southern Australia – contributions towards determining current stock status. Environmental Biology of Fishes.
Gomon M, Bray D, Kuiter R (2008). Fishes of Australia‟s southern coast. New Holland Publishers (Australia) Pty Ltd. 928 pp.
Jones GK (1982). Mesh selection of hauling nets used in the commercial Marine Scalefish Fishery in South Australian waters. Fisheries Research Papers from the Department of Fisheries (South Australia). Number 5.
Jones GK (2009). South Australian recreational fishing survey. PIRSA Fisheries, Adelaide. South Australian Fisheries Management Series Paper No. 54.
Knuckey IA, Morison AK, Ryan DK (2002). The effects of haul seining in Victorian Bays and inlets. Australian Fisheries Research and Development Corporation Final Report 1997/210.
Kumar MS, Hill R, Partington D (1995). The impact of commercial hauling nets and recreational line fishing on the survival of King George whiting (Silliginodes punctata). SARDI Report.
McGarvey R, Fowler AJ, Feenstra JE, Burch P, Jackson WB (2009). Southern Garfish (hyporhamphus melanochir) Fishery. Fishery assessment report to PIRSA. SARDI Publication No. F2007/000720-2. SARDI Research Report Series No. 397.
Millar RB, Fryer RJ (1999). Estimating the size-selection curves of towed gears, traps, nets and hooks. Reviews in Fish Biology and Fisheries. 9: 89-116.
Rueda M (2007). Evaluating the selective performance of the encircling gillnet used in tropical estuarine fisheries from Colombia. Fisheries Research 87: 28-34.
Steer MA (2009). The dynamics of targeted fishing effort between different species in the Marine Scalefish Fishery. Report to PIRSA. SARDI Aquatic Sciences Publication No. F2009/000446-1. SARDI Research Report Series No. 402.
Stewart J, Walsh C, Reynolds D, Kendall B, Gray, C (2004). Determining an optimum mesh size for use in the lampara net fishery for eastern sea garfish, Hyporhamphus australis. Fisheries Management and Ecology. 11: 403-410.
Ye, Q, Noell C, McGlennon D (2002). Reporductive biology of sea garfish. In „Fishereis biology and habitat ecology of southern garfish (Hyporhamphus melanochir) in southern Australian waters. (eds. Jones GK, Ayvazian S, Coutin P). FRDC Final Report 97/133.
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6 Appendix
A copy of the „Garfish Netters Survey‟ sent to active garfish hauling net fishers.
LICENCE # NAME:
NETPOCKET
MESH SIZEPLY MATERIAL KNOTTED?
FLOATING
or SINKING% USAGE
1
2
3
4
5
6
Please fill out and return to Mike Steer either by:
MAIL SARDI, 2 Hamra Ave, West Beach, SA, 5024
Email [email protected]
Fax (08) 8207 5481
Phone (08) 8207 5435
Feel free to call me directly and I'll fill out the form for you.
GARFISH NETTERS SURVEY
COMMENTS?