Restoring the Boyne Estuary mussel population Gavin Burnell and Tony Knights Aquaculture and Fisheries Development Centre, University College Cork Image:

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

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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.

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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%

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1 6 11 16 21 26 31 36 41 46 51 56 61 66 71 76 81 86

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1 6 11 16 21 26 31 36 41 46 51 56 61 66 71 76 81 86

Shell length (mm)

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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

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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.

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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)

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Larval density and position varies with tidal state

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High nos. during flood tides

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Larvae closer to seabed during high, low and ebb tides

Mixed during flood tides

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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

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B).

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A).

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Southern Irish Sea release (50,000particles)

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Boyne Hydrodynamic

Model

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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.