Restoring the Boyne Estuary mussel population Gavin Burnell and Tony Knights Aquaculture and Fisheries Development Centre, University College Cork Image: GLOBEC/Glynn Gorick 9 th 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 Chart Datum removed over 1 million tonnes of spoil. Drogheda Port Company were obliged to restore the mussel fishery as part of conditions of the licence. The river channel was deepened in 1999 to allow larger vessels to access Drogheda Port Mussel spat (20 – 25mm) from the Irish Sea were were seeded (July – November) from 2000 – 2003.

Year Tonnage 1765t 830t 355t 415t Dredging Changes in mussel biomass during the restoration project A total of 3365 tonnes mussel spat (20 – 25mm) were seeded over 4 years Seed was sourced from S. Arklow to N. Dublin banks.

DGPS used to map accoustic data (RoxAnn 1998 – 2002; ECHOplus 2003 – 2006) onto a digitised 1: 10,000 marine chart. Accoustic data ground- truthed by divers using 2 x 0.25m ² 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.

High density mussels Low density mussels Sand, gravel, shell Gravel, stones Muddy sand Mud Hard compact sand Compact mud, some rock Rocks Mussel coverage increased from 5.7% to 57%

Shell length (mm) Frequency 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 Year Tonnage 1765t 830t 355t 415t Dredgi ng

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. 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 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 Marine Ecology Progress Series (in press)

Larval density and position varies with tidal state Mean Larval Density (m -3 +SD) High nos. during flood tides Low nos. during ebb tides Flood Ebb High Low Top Larvae closer to seabed during high, low and ebb tides Mixed during flood tides Middle Bottom Mean Larval Density (m -3 +SD) per depth zone

Dispersal Models Lagrangian Particle Tracking (Coherens*) Lagrangian SPM Module Initial Particle Position River & Open Boundary Input Advective Transport Diffusive Transport CurrentsTurbulence 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 Model without behaviour Model with behaviour Southern Irish Sea release (50,000particles)

Boyne Hydrodynamic Model 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.

Acknowledgements Thanks 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.