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Recent changes in intertidal populations of native oysters ( Ostrea edulis) in Strangford Lough

Recent changes in intertidal populations of native oysters ( Ostrea edulis) in Strangford Lough. David Smyth, Lynne Browne, & Dai Roberts Queen’s University, Belfast. Introduction.

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Recent changes in intertidal populations of native oysters ( Ostrea edulis) in Strangford Lough

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  1. Recent changes in intertidal populations of native oysters (Ostrea edulis) in Strangford Lough David Smyth, Lynne Browne, & Dai Roberts Queen’s University, Belfast

  2. Introduction • Strangford Loughhistorically- supported large numbers of O. edulis with shell middens being found at numerous locations. • Montgomery (1683)– oysters are being dredged in great numbers from deep water as well as being gathered along the shore. • 18th Century- oyster beds were being commercially exploited with a peak in the 19th century when 60 boats were dredging the Lough (Brown 1818). • However- In 1903 Browne reported the oyster fisheries collapse in a government report.

  3. Boretree Islands • Kircubbin Dredging area 19th century

  4. Oyster Aquaculture • Growth trials – In the 1970’s produced favourable results for both Crassostrea gigas and Ostrea edulis. This formed the basis of today’s very successful oyster-culture operations in Strangford Lough.

  5. Restoration • UK Biodiversity Action Plan- lists Ostrea edulis as a species which should be maintained and expanded. • Restoration Project 1997- 99 – development project (EU funded) led byfishermen(SLSF Co-op) to re-establish a sustainable native oyster fishery in the Lough (Restoration Project).

  6. Restoration 1997-99 • Cultch was spread to form the basis for new beds 70-75 tonnes (mainly C.gigas and scallop shells) • Concentratingbroodstock • on new beds • Seeding with O. edulis spat • (250,000)

  7. Current survey • Intertidal- 30 shore sites • Intertidal Island- 8 Island sites • Spat Collection- 13 collectors • Subtidal- 15 dive sites • Plankton sampling- over tidal flux

  8. Castleaverry Ballyreagh Newtownards Sailing Club Island Hill Mount Stewart Chapel Island Grey Abbey Castle Espie Herring Bay Horse Island Paddy’s Point Kircubbin Nendrum Whierock Horse Island Ringhaddy Simmy Island Intertidal sample sites and spat collection stations surveyed from 2002-2004 Marlfield Killyleagh Ballyhenry Nickey’s Point Catsle Island Shore surveys Spat collectors Granagh Bay Catsleward Bay Barn Hall Bay

  9. 19.644 m2 (x106 ) 4.509 m2 (x106) 5.438 m2 (x106) Regional divisions for population estimates 2002-2004. Areas for the survey regions were estimated from scaled maps of the Lough using Global Lab image analysis software.

  10. The total number of intertidal O.edulis in the North, West and mid-East were estimated using the following formula adapted from Gunderson (1993). Where; P= Total population resident in full survey area. Ri= Area of region I in m2 . a= Area sampled within a single sampling unit. F= Correction factor estimating substrate types. _ Ci= Mean number of oysters observed per sampling unit in the region based i on n samples. h= Number of regions composing the survey. h _ P= ∑ ﴾Ri .F ﴿ C i I=1 a

  11. Population estimates pre- and post- restoration • Proportionally weighted correction factors were applied to account for the amount of suitable substratum present.

  12. Population decrease and intertidal harvesting • 2003 intertidal pop/estimate - 1,243,594 • 2004 intertidal pop/estimate - 750,214 • Decrease of 493,380 O. edulis • The average intertidal oyster 88g = 43.4 tonnes • Teams of between 10-20 shellfish gatherers are regularly encountered working the shore. • Ostrea edulis is now absent from previously abundant sites.

  13. Further consequences of harvesting • Decrease in suitable cultch for larval settlement • Anthropogenic disturbance of intertidal habitats within a SAC

  14. Summary • The densities and range of native oysters has increased in Strangford Lough since 1997 • The Restoration Project resulted in the first significant harvests of O. edulis from Strangford Lough for over 100 years • Hand gathering is significant with an estimated 40% decrease in the total intertidal oyster population; many sites have been picked clean • Intertidal habitat disturbance is evident at many sites

  15. Should restoration of Ostrea edulis be continued in Strangford Lough? Yes- because: • the restoration of O. edulis assemblages have the potential to increase biodiversity within Strangford Lough. • the Lough has suitable regions where local circulation will promote larval retention (R Kennedy,1999)

  16. the Lough is fully saline providing protection against the parasite Marteilia refrigens • the Lough has large areas of good intertidal cultch coverage (Mytilus) • the 1997-98 development project (EU funded) led byfishermen (SLSF Co-op) was successful. • Strangford Lough possess one of the few Bonamia-free O. edulis populations in Europe. • there is good settlement and growth • the UK Biodiversity Action Plan- lists Ostrea edulis as a species which should be maintained and expanded

  17. Acknowledgements Malcolm Carter DARD Phil Heath Dick James Richard Kennedy Niall McDonough Jasper Parsons Des Rodgers

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