Stephen Brown, Sean Handley, Keith Michael, David Schiel, Sophie Mormede, Mitch Campbell NIWA

1. Enhancement of the native flat oyster (Ostrea chilensis) in Tasman Bay, New Zealand: Summary of research outcomes Stephen Brown, Sean Handley, Keith Michael, David Schiel, Sophie Mormede, Mitch Campbell NIWA University of Canterbury Challenger Oyster Management Co Ltd

2. Background • Commercial flat oyster fishery in Tasman Bay since 1845 • Catches and biomass have declined in the last three decades (Harvest peaked at 750 tonnes in 1983 down to 20 tonnes in 2008) and fishery is economically marginal. • Anthropogenic impacts from land based activities and dredge and trawl fishing may have modified the seabed, increasing sediment deposition and resuspension and homogenising benthic habitat • Goal is a sustainable oyster fishery

3. Key points about the reproductive biology of Ostrea chilensis • Female incubates or ‘broods’ fertilised larvae to the pediveliger stage, longer than any other oyster (approximately 1 month) • Some larvae may be released throughout brooding period – most released at fully developed stage • Larvae released earlier may be more or less planktonic while late-stage larvae can settle within minutes • Require a hard substrate (such as shell) for settlement and metamorphosis Late stage larvae

4. Hypotheses • Lack of available substratum for oyster settlement and growth limits productivity of the fishery. • Placement of waste scallop and or mussel shell on the seabed (Habitat enhancement) will increase numbers of oysters recruiting to the fishery. • Shell enhancement may improve ecosystem function by restoring seabed habitat heterogeneity • Plentiful supply of waste shell from local scallop fishing and mussel aquaculture industries. Photoquadrat (0.25 m2) showing seabed in Tasman Bay

5. Determine seasonal patterns of reproductive activity Brooding - Monthly sampling of oyster population to determine % brooding larvae Spatfall - Monthly monitoring for 2 years at locations throughout Tasman Bay When to enhance

6. Reproductive seasonality • Commencement of brooding and spatfall detected in September/October • Peak spatfall between November and January • Best time to deploy shell is late October/early November

7. Where to enhance Want to predict spatfall distribution to guide shell placement Utilised oyster biomass survey and spatfall monitoring to investigate potential predictors of spatfall distribution: • background oyster density • water depth • east/west position within the bay

8. Predicting spatfall intensity Spat density correlated most strongly with background oyster density So…. Background oyster density derived from industry biomass surveys is a useful guide for shell placement

9. At low adult densities (< 0.01 m-2), any increase in adult densities is reflected by a large increase in spat settlement density

10. Optimal gains achieved by placing shell where background oyster densities are between 0.005 to 0.01 oysters per m2, ie less than, but close to, commercial density threshold (0.02 oys per m2).

11. Oyster background density 0.01 m-2 Shell Enhancement Experiments • 2 sites in Tasman Bay • 2 shell treatments Piled, Scattered + Control plots (3 reps of each) • Deployed in October 2005 • Monitored: • spat settlement • growth • survival

12. Loading bulk bags of shell onto the barge Lowering shell 23m to the seabed (October 2005) Plots ~4m in diameter Samples analysed in lab Scuba sampling after spat settlement season (March 2006) Surface markers

13. Shell boosts spat density

14. Oyster Growth on Enhanced Habitat Percentage of oysters at legal size (58mm length) and marketable shell depth in each yr class

15. Survival • Variable and Low survival • But enhancement resulted in elevated oyster density after 3 yrs (from 0.01 oys m-2 to between 0.1-0.4 oys m-2 )

16. Disease? (3 yrs old) Pre recruit Mortality (1 yr old) Bimodal length distribution for dead oysters

17. Community level effects of habitat enhancement • Species assemblage on enhanced habitat distinct from that on control plots. • Greater species richness and abundance on enhanced plots • Enhanced plots contained more suspension feeding species, mobile scavenger/predators, and different polychaete assemblage • Control plots had more deposit feeders

18. Conclusions • Habitat enhancement benefits the oyster fishery by increasing oyster production • Habitat enhancement is likely to confer benefits to ecosystem function by increasing benthic habitat heterogeneity and biodiversity (ecological restoration)

19. Further work Fishery Scale Enhancement Trials • 3 X 30 Ha sites in Tasman Bay enhanced with scallop shell at rate of 10 shells per m2 plus 3 X control sites • Determined relative oyster density prior to shell placement • Resample After 4 years (BACI) • More community level experiments in conjunction with fishery trials

20. Acknowledgements • Board and Staff at Challenger Oyster Management Co. Ltd. • All staff at NIWA Nelson office particularly, Dan Cairney, Anna Bradley, Russell Cole, Don Morrisey, Ken Grange, (All from NIWA) Mike Hickford (University of Canterbury) This work was funded by the Challenger Oyster Management Company Ltd and the New Zealand Foundation for Research Science and Technology (Technology for Business Growth Grant CDOC0401, and Enterprise scholarship ENT 1928)