assessment of sole (solea solea) in gsa 17 (2005-2007) izmir, turkey, 15-19 september 2008 g. fabi...

ASSESSMENT OF SOLE (Solea solea) IN GSA 17

(2005-2007)

Izmir, Turkey, 15-19 September 2008

G. Fabi1, I. Celic2, M. Despalatovic3, O. Giovanardi2, F. Grati1, I. Isajlović3, A. Jenic4, D. Pehar4, P.

Polidori1, S. Raicevich2, G. Scarcella1, N. Vrgoc3

1 CNR – ISMAR Ancona, Italy

2 ICRAM Chioggia, Italy

3 IOF Split, Croatia

4 FRIS Ljubliana, Slovenia

Scientific Advisory CommitteeSub-Committee on Stock AssessmentWorking Group on Demersal Species

The sole stock in GSA 17 is shared between the Adriatic countries (Italy, Croatia and Slovenia).

This resource is mainly exploited by rapido trawlers and set netters.

Rapido trawl and set nets (gill nets and trammel nets) are used by the Italian fleets, while the trammel net is the most used gear along the eastern coast.

Most of catches come from the Italian rapido trawl fleets (around 80%).

Catches strictly depend on the recruitment success, mainly including juveniles (age classes from 0 to 2).


FISHERY

Sole exploitation

0

500,000

1,000,000

1,500,000

2,000,000

12 14 16 18 20 22 24 26 28 30 32 34 36 38 40

TL (cm)

Num

ber

of

ind.

Rapido trawl Set nets Otter trawl

Size at first sexual maturity

Date

Vessel

Species by size and kg

DATA SOURCE

Commercial landings (2005-2007)

daily auction documents


samplings of landing

Rapido trawl survey (Fall 2005-2007)

SPRING 2005

FALL 2005

SPRING 2006

FALL 2006

Surface 38.880 km2

FALL 2007

Surface 44.880 km2

Surface 44.880 km2

Surface 44.880 km2

Surface 42.388 km2

0–30 m

30–50 m

50-100 m

Depth strata S’05 F’05 S’06 F’06 F’07

0-30 30 30 20 35 32

30-50 14 12 10 20 19

50-100 24 15 8 8 11

HR islands 0 5 4 4 0

TOTAL 68 62 42 67 62

N° of stations / Stratum

SURVEY DATA

Samplings station


Abundance and Biomass indexes from surveys carried out in 2005 – 2007, were computed using the software AdriaMed Trawl Information System (ATrIS; Gramolini et al., 2005)

GIS Maps of the spatial distribution of the stock, of spawning females and of juveniles.

SURVEY DATA

Data analysis

Length frequency distributions were corrected using the selectivity parameters given by Ferretti and Froglia (1975)

0%

25%

50%

75%

100%

0 5 10 15 20 25 30

Total length [cm]

Pro

po

rtio

n r

eta

ine

d

0

500,000

1,000,000

1,500,000

2,000,000

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40

TL cm

Nu

mb

er

of I

nd

.

0

500,000

1,000,000

1,500,000

2,000,000

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40

TL cm

Nu

mb

er

of I

nd

.

Growth parameters were calculated by indirect methods using the length frequency data from the surveys.

As the demography of males and females did not show relevant differences, Linf, k and t0 were estimated for the whole population.


SURVEY DATA

Growth parameters estimation

Length-frequency

distributions corrected by

selectivity

Age group distributions corrected by

selectivity

SURVEY DATA Demography

Spring 2005

0

50

100

150

200

250

N/k

m2

Fall 2005

0

50

100

150

200

250

N/k

m2

Spring 2006

0

50

100

150

200

250

N/k

m2

Fall 2006

0

50

100

150

200

250

N/k

m2

Fall 2007

0

50

100

150

200

250

0 1 2 3 > 4

Age

N/k

m2

Spring 2005

0

20

40

60

80N

/km

2

Fall 2005

0

20

40

60

80

N/k

m2

Spring 2006

0

20

40

60

80

N/k

m2

Fall 2006

0

20

40

60

80

N/k

m2

Fall 2007

0

20

40

60

80

N/k

m2

5 10 15 20 25 30 35 40TL (cm)

N. ind. km-2

SPRING 2005

FALL 2005

SPRING 2006

FALL 2006 FALL 2007


SURVEY DATA

Spatial distribution of sole

0

10

20

30

40

50

Spring2005

Fall Spring2006

Fall Spring2007

Fall

Total Biomass

Spawning Stock Biomass

Recruits

Spring 2005 Fall 2005 Spring 2006 Fall 2006 Fall 2007

N. ind/km2 (CV %) 176 (12%) 305 (20%) 299 (15%) 327 (22%) 434 (24%)

kg/km2 (CV %) 23 (15%) 27 (15%) 36 (15%) 36 (16%) 40 (18%)


0

10

20

30

40

50

Spring2005

Fall Spring2006

Fall Spring2007

Fall

kg/ k

m2

2005 2006 2007

SURVEY DATA

Abundance and biomass indexes

Data used in VPA flow tuned with trawl survey CPUE

Natural mortality:

The vector of natural mortality by age was calculated from Caddy´s (1991) method, using the PROBIOM Excel spreadsheet (Abella et al., 1997):

Age 0 1 2 3 4 5+

M 0.9 0.3 0.3 0.2 0.2 0.2

Abundance at age (N.ind./km2) from surveys.

Catch at age data considering ages 0 to 5+ (plus group) from commercial fishery in the period 2005-2007 of GSA 17 in number of specimens.

Biological parameter:

Growth: V. Bertalannfy (Linf = 39.6 cm; k = 0.44; t0 = -0.46).

Length-Weight relationship: W=a Lb (a = 0.007; b = 3.0368).

Maturation: logistic, age50% ~ 1.5 years:

Age 0 1 2 3 4 5+

Proportion of Mature 0 0.29 0.75 1 1 1


Time interval = 1 year

Age: 0,1,..., 5+ (plus group)

VPA flow was processed by means of an Excel spreadsheet using catch-at-age from commercial landings, assuming Ma and abundance-at-age (start year) from survey data.

VPA-tuning flow


0

500

1000

1500

2000

2500

3000

0 1 2 3 4 5+Age

To

ns

0.0

0.5

1.0

1.5

2.0

2.5

3.0

F

2005 2006 2007

2005 2006 2007

2005 2006 2007

2005 2006 2007

F vector

Biomass (Tons)

Analytical results and survey indexes showed similar patterns for total biomass, SSB and recruits.

The total biomass and SSB remained practically constant from 2005 to 2007, while a low recruitment occurred in 2006.

Fbar (0+, 1, 2) showed a decreasing trend in the three years reaching the minimum of 0.52 in 2007.

Biomass from VPA Trawl survey


VPA-tuning flow

0

2000

4000

6000

2005 2006 2007

0.0

0.4

0.8

1.2

1.6

Total biomass SSB Recruits (0+) Fbar (0-2)

0

2000

4000

6000

2005 2006 2007

To

ns

0.0

0.4

0.8

1.2

1.6

F

0

10

20

30

40

50

2005 2006 2007

kg

/ km

2

0

20

40

60

80

100

120

140

1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

LP

UE

kg

/da

y/ve

sse

l '

0

500

1000

1500

2000

2500

vess

els

*da

ys

LPUE EFFORT

This Fbar pattern was due to a decrease of the fishing effort by the main Italian rapido trawl fleets in GSA 17 in terms of both number of vessels and fishing time, as well as to the switching of rapido trawlers towards other resources (e.g. gastropods and bivalves).

LPUE and Effort of Ancona rapido trawl fleet Scientific Advisory Committee

Sub-Committee on Stock AssessmentWorking Group on Demersal Species

Fall 2005

Fall 2006Fall 2007

Size class range (TLmode+s.d.):

Fall 2005 TL ≤ 18.5 cm

Fall 2006 TL ≤ 17.5 cm

Fall 2007 TL ≤ 20.0 cm

Recruits of sole


N. ind. km-2

Recruits of sole vs bottom temperature and salinity

Fall 2005 Fall 2006

N. ind. km-2

°C

Fall 2006Fall 2005

S‰PO RIVER INFLOW

0200400600800

10001200140016001800

1 2 3 4 5 6 7 8 9101112 1 2 3 4 5 6 7 8 9101112

2005 2006

m3 /s

S‰

°C


0

0.02

0.04

0.06

0.08

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Fishing mortality

Yie

ld-p

er-R

ecru

it (g

)

Y/R analysis applied with the age at first capture of 0.7 (around 16 cm TL) and using as SRR the Beverton & Holt model (Pilling et al., 2008), showed that the resource is fully exploited / slightly overexploited.

F0.1 = 0.26 Fmax = 0.46 Fbar = 0.52

Yield-per-recruit (Y/R) analysis


FmaxF0.1

Fbar

0

500,000

1,000,000

1,500,000

2,000,000

12 14 16 18 20 22 24 26 28 30 32 34 36 38 40

TL (cm)

Num

ber

of in

d.

Rapido trawl Set nets Otter trawl

Size frequency distributions from commercial fishery

MLS = 20 cm

Fmax (20 cm) = 0.60

0

0.02

0.04

0.06

0.08

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Fishing mortality

Yie

ld-p

er-R

ecru

it (

kg)

More than 40% of soles landed by rapido trawlers had a TL<20 cm (Minimum Landing Size; EC reg. 1967/2006).

The increase of the length at first capture from 16 cm to 20 cm TL produces, according with Y/R analysis, a ≈20% increase of Fmax.

Fmax (16 cm) = 0.46


Management advice


• A further reduction of the rapido trawlers’ fishing pressure of 10% would be recommended, also considering that the exploitation is mainly orientated towards juveniles and the success of recruitment is strictly related to environmental conditions. Hence, in the case of both increasing fishing effort and yearly bad recruitment there could be a high risk of stock depletion.

• A two–months closure for rapido trawling inside 6 nm offshore along the Italian coast, after the biological fishing stop (August), would be advisable to reduce the portion of undersized specimens (mainly YOY).

• For the same reason specific studies on rapido trawl selectivity are necessary. In fact, it is not sure that the adoption of a larger mesh size could correspond to a decrease of illegal catches, considering that the mesh opening used by the Italian rapido trawlers is already larger (48 mm or more) than the legal one.

• SSB remained constant in the 3 years maybe because in late fall - winter the main spawning area is only partially exploited by the Croatian set netters and Italian fleets. Therefore, the safeguard of spawning areas to prevent a possible future exploitation might be crucial for the sustainability of the Adriatic sole stock.

• Finally, a set of specific rules for rapido trawl fishery would be advisable (e.g.: size and number of gears, mesh size, towing speed).

Spatial distribution of spawning females and fishing grounds of the

Italian fleets


Management advice


Management advice

Thank youfor your attention

assessment of sole (solea solea) in gsa 17 (2005-2007) izmir, turkey, 15-19 september 2008 g. fabi...

Documents

stock assessmentworking

age data

survey data abundance

demersal speciesthe

length frequency data

demersal speciesabundance

italian rapido trawl

trammel nets