restoring the boyne estuary mussel population gavin burnell and tony knights aquaculture and...
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Restoring the Boyne Restoring the Boyne Estuary mussel Estuary mussel
populationpopulation
Gavin Burnell and Tony KnightsGavin Burnell and Tony Knights
Aquaculture and Fisheries Aquaculture and Fisheries Development Centre,Development Centre,
University College CorkUniversity College Cork
Imag
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9th International Conference on Shellfish Restoration
Boyne Estuary has a sustainable artisinal fishery
Since at least 1904 the Boyne mussel fishery has provided a partial livelihood of 100 – 450 tonnes per year between 15 – 25 local fishermen. (Dougherty, 1904)
Fishermen use 5.5m (18ft) wooden handled rakes to gather mussels for 2 hours either side of low water in 1.5 – 2.0m depth.
Surveys in 1974 and 1988 estimated the standing stock to be 1000 tonnes (+/- 200 t). “There are regular spatfalls and the fishery is self-perpetuating”.
(Edwards, 1988)
The fishery
Deepening the river to 2.2m below ChartDatum removed over1 million tonnes of spoil.
Drogheda Port Companywere obliged to restore the mussel fishery as part of conditions of the licence.
The river channel was deepened in 1999to allow larger vessels to access Drogheda Port
Mussel spat (20 – 25mm)from the Irish Sea werewere seeded (July – November) from 2000 – 2003.
0
500
1000
1500
2000
1996 1997 1998 1999 2000 2001 2002 2003 2004 2005
Year
Ton
nag
e
17
65
t
83
0t
35
5t
41
5t
Dre
dg
in
g
Changes in mussel biomass during the restoration project
A total of 3365 tonnes mussel spat (20 – 25mm) were seeded over 4 years
Seed was sourcedfrom S. Arklow to N. Dublin banks.
DGPS used to mapaccoustic data (RoxAnn 1998 – 2002; ECHOplus 2003 – 2006) onto a digitised 1: 10,000 marine chart.
Accoustic data ground-truthed by divers using 2 x 0.25m² quadrats at four chosen sites: high; medium; low and zero mussels.
The mussels were sampled each March/April
Wet weight Condition Index calculated for a sub-sample at each site.
2000 2004
2006High density musselsLow density mussels
Sand, gravel, shell
Gravel, stonesMuddy sandMudHard compact sandCompact mud, some rockRocksRocks
Mussel coverage increased from 5.7% to 57%
0
5
10
15
20
25
30
35
40
45
1 6 11 16 21 26 31 36 41 46 51 56 61 66 71 76 81 86
0
5
10
15
20
25
30
35
40
45
1 6 11 16 21 26 31 36 41 46 51 56 61 66 71 76 81 86
0
5
10
15
20
25
30
35
40
45
1 6 11 16 21 26 31 36 41 46 51 56 61 66 71 76 81 86
Shell length (mm)
Fre
qu
ency
1998
2004
2006
Results of mussel surveys
•1998 size structure reflects regular harvesting
• In 2004 some of the seeded cohorts still visible
• By 2006, after 7 years of no fishing, a large proportion of mussels at market size.
• Limited annual recruitment from 2003 onwards.
Before dredging
4 months after last seeding
2 years after last seeding
What factors may have contributed to the partial success of the project?
•New channel was ridged
•4 successive seed imports
•Seed quality was monitored
•Seed was sourced in Irish Sea
•Seed laying was zoned to reduce wash-out
•Moratorium on fishing for duration of project
0
500
1000
1500
2000
1996 1997 1998 1999 2000 2001 2002 2003 2004 2005
Year
Ton
nag
e
17
65
t 83
0t
35
5t
41
5t
Dre
dg
ing
Why did we fail to fully restore the mussel biomass?
•Wash-out: flows up to 6,000 cubic feet /sec and surface flows of up to 6ft/ sec (1.8m/sec).
•Predation
•Lack of suitable substrate
•Poor settlement
•Reduced productivity
•No harvesting – clump formation
• Limited amount of seed after 2000
Is the Boyne mussel fishery self - recruiting?
•Presence of small seed (<25m) from 2004 onwards indicates recent settlement.
•Some evidence for larval retention has been obtained from hydrographic modelling data.
5 0
2 0 0
4 0 0
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8 0 0
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1 2 0 0
1 4 0 0
1 6 0 0
1 8 0 0
2 0 0 0
2 2 0 0
2 4 0 0
2 6 0 0
2 8 0 0
3 0 0 0
3 2 0 0
3 4 0 0
3 6 0 0
3 8 0 0
4 0 0 0
5 0 0 0
7 0 0 0
8 5 0 0
Ireland
U.K.
St. George’s Channel
North Channel
Isle of Man
Boyne River
Dublin Bay
Vertical distribution of Irish Sea mussel larvae was monitored in 2005
1. Site
2. Time period
(May-June, July-August)
3. Phase (Spring/Neap)
4. State (Flood, Ebb, HW, LW)
5. Zone (Top, Middle, Bottom)
0-25m
Knights et al. 2006. Marine Ecology Progress Series (in press)
0
5
10
15
Flood Ebb High Low0
5
10
15
20
Larval density and position varies with tidal state
Mea
n L
arva
l Den
sity
(m
-3 +
SD
)
High nos. during flood tides
Low nos. during ebb tides
Flood Ebb High LowTop
Larvae closer to seabed during high, low and ebb tides
Mixed during flood tides
Mid
dle
Bott
om
Mea
n L
arva
l Den
sity
(m
-3 +
SD
) pe
r de
pth
zone
Dispersal ModelsDispersal ModelsLagrangian Particle Tracking Lagrangian Particle Tracking (Coherens*) (Coherens*)
Lagrangian SPM Module
Initial Particle
Position
River & Open Boundary Input
Advective Transport
Diffusive Transport
Currents Turbulence
Updated Particle Position
* Marine Science and Technology Program
Inputs:
•local hydrodynamics•local meteorology
2 models:
•passive particle• larval behaviour•larval duration (41 days
Larval concentration – Coherens Model
A).
B).
Figure 7
100 hrs 200 hrs 400 hrs 600 hrs 800 hrs 1000 hrs
A).
B).
Figure 7
100 hrs 200 hrs 400 hrs 600 hrs 800 hrs 1000 hrs
Model w
ithout
beh
avio
ur
Model w
ith
behavio
ur
Southern Irish Sea release (50,000particles)
5 0
2 0 0
4 0 0
6 0 0
8 0 0
1 0 0 0
1 2 0 0
1 4 0 0
1 6 0 0
1 8 0 0
2 0 0 0
2 2 0 0
2 4 0 0
2 6 0 0
2 8 0 0
3 0 0 0
3 2 0 0
3 4 0 0
3 6 0 0
3 8 0 0
4 0 0 0
5 0 0 0
7 0 0 0
8 5 0 0
5 0
1 5 0
2 5 0
3 5 0
4 5 0
5 5 0
6 5 0
7 5 0
8 5 0
9 5 0
1 0 5 0
1 1 5 0
1 2 5 0
1 3 5 0
1 4 5 0
1 5 5 0
1 6 5 0
1 7 5 0
1 8 5 0
1 9 5 0
2 0 5 0
2 1 5 0
2 2 5 0
2 3 5 0
2 4 5 0
Boyne Hydrodynamic
Model
5 0
1 0 0
2 0 0
4 0 0
6 0 0
8 0 0
1 0 0 0
1 2 0 0
1 4 0 0
1 6 0 0
1 8 0 0
2 0 0 0
2 2 0 0
2 4 0 0
2 6 0 0
2 8 0 0
3 0 0 0
3 2 0 0
3 4 0 0
3 6 0 0
3 8 0 0
4 0 0 0
4 2 0 0
5 0
1 0 0
2 0 0
4 0 0
6 0 0
8 0 0
1 0 0 0
1 2 0 0
1 4 0 0
1 6 0 0
1 8 0 0
2 0 0 0
2 2 0 0
2 4 0 0
2 6 0 0
2 8 0 0
3 0 0 0
3 2 0 0
3 4 0 0
3 6 0 0
3 8 0 0
4 4 0 0
5 0 0 0
5 6 0 0
5 0
1 0 0
1 5 0
2 0 0
2 5 0
3 0 0
3 5 0
4 0 0
4 5 0
5 0 0
5 5 0
6 0 0
6 5 0
7 0 0
7 5 0
8 0 0
8 5 0
9 0 0
9 5 0
1 0 0 0
1 0 5 0
1 1 0 0
1 1 5 0
1 2 0 0
1 2 5 0
1 3 0 0
5 0
1 5 0
2 5 0
3 5 0
4 5 0
5 5 0
6 5 0
7 5 0
8 5 0
9 5 0
1 0 5 0
1 1 5 0
1 2 5 0
1 3 5 0
1 4 5 0
5 0
1 0 0
1 5 0
2 0 0
2 5 0
3 0 0
3 5 0
4 0 0
4 5 0
5 0 0
5 5 0
6 0 0
6 5 0
7 0 0
7 5 0
8 0 0
8 5 0
9 0 0
9 5 0
Passive particle model shows larval concentration near coast after 4, 8, 16, 25, 33 and 41 days.
If larvae swim up into water column during flood tides, then transport up estuary is possible.
Boyne
Is the Boyne mussel fishery sustainable?
Sustainable development is that
which meets the needs of the present without comprimising the ability of future generations to meet
their own needs.
(The U.N. Brundtland Commission, 1987)
•Fishermen may be compensated and “bought out”.
•If no fishing, then mussel biomass may fluctuate more dramatically than in the past.
•May need to add substrates for seed collection and/or stabalisation of river bed.
Future scenarios for the management of the mussels
Hatcheries may be necessary if Irish Sea seed remains scarce.
AcknowledgementsThanks to
Captain Martin Donnelly,
Drogheda Port Company, for permission to use this data and to my co-author Tony Knights for analysis of data and
preparation of some figures.