Restoring the Boyne Estuary mussel population Gavin Burnell and Tony Knights

1. Restoring the Boyne Estuary mussel population Gavin Burnell and Tony Knights Aquaculture and Fisheries Development Centre, University College Cork 9th International Conference on Shellfish Restoration Image: GLOBEC/Glynn Gorick

2. 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.

3. The fishery Surveys in 1974 and 1988 estimated the standing stock to be 1000 tonnes (+/- 200 t). “There are regular spatfalls and the fishery isself-perpetuating”. (Edwards, 1988)

4. The river channel was deepened in 1999 to allow larger vessels to access Drogheda Port 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. Mussel spat (20 – 25mm) from the Irish Sea were were seeded (July – November) from 2000 – 2003.

5. 2000 355t 415t 1500 830t Dredging Tonnage 1000 1765t 500 0 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 Year A total of 3365 tonnes mussel spat (20 – 25mm) were seeded over 4 years Seed was sourced from S. Arklow to N. Dublin banks. Changes in mussel biomass during the restoration project

6. The mussels were sampled each March/April 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 0.25m² quadrats at four chosen sites: high; medium; low and zero mussels. Wet weight Condition Index calculated for a sub-sample at each site.

7. Mussel coverage increased from 5.7% to 57% 2000 2004 2006 High density mussels Low density mussels Sand, gravel, shell Gravel, stones Muddysand Mud Hardcompactsand Compactmud, somerock Rocks Rocks

8. 1998 Results of mussel surveys Before dredging • 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. 2004 4 months after last seeding Frequency 2006 2 years after last seeding Shell length (mm)

9. 2000 355t 415t Dredging 1500 830t Tonnage 1000 1765t 500 0 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 Year 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

10. 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

11. Isle of Man North Channel Boyne River U.K. Dublin Bay Ireland St. George’s Channel 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.

12. 0-25m 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) Knights et al. 2006. Marine Ecology Progress Series (in press)

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

14. Lagrangian SPM Module Initial Particle Position River & Open Boundary Input Advective Transport Diffusive Transport Currents Turbulence Updated Particle Position Dispersal ModelsLagrangian Particle Tracking (Coherens*) • Inputs: • local hydrodynamics • local meteorology • 2 models: • passive particle • larval behaviour • larval duration (41 days * Marine Science and Technology Program

15. Larval concentration – Coherens Model Southern Irish Sea release (50,000particles) Model without behaviour Model with behaviour

16. Boyne Hydrodynamic Model Passive particle model shows larval concentration near coast after 4, 8, 16, 25, 33 and 41days. If larvae swim up into water column during flood tides, then transport up estuary is possible. Boyne

17. 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) Is the Boyne mussel fishery sustainable?

18. Future scenarios for the management of the mussels • 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. Hatcheries may be necessary if Irish Sea seed remains scarce.

19. 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. Acknowledgements