Pacific Oysters Crassostrea gigas in Strangford Lough, Northern Ireland

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Pacific Oysters (Crassostrea gigas) in Strangford
Lough, Northern Ireland

• Claire Guy and Dai Roberts
• The Queens University Belfast

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Outline

• Background information on the oysters in
  Strangford
• Current research
• Future research
• Conclusions

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Historic oyster populations in Strangford

• O. edulis Stock decline in Strangford Lough

• Ostrea edulis

• Human population increase
• Increase in consumption
• Habitat degradation
• Pollution
• Official inquiry into stock levels in 1877
• Cessation of oyster fishery by 1903
• Alternative species for commercial industry was
  sought in the 1970s.

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Pacific oyster (Crassostrea gigas)
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Introduction of C. gigas to Strangford Lough

• Introduced in the 1970s
• Laid out on trestles in the northern lough
• Water temperatures thought to be too low to
  facilitate reproduction

Initial introduction site
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PhD Research

• Ascertain the distribution of C. gigas within
  Strangford Lough
• Settlement substratum preference observation and
  investigation
• Population structure
• Biodiversity supported by the two species
• Initiation of C. gigas removal
• Gametogenesis
• Management options

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1. Distribution

• Aim
• To ascertain if there has been any spread of C.
  gigas outside the licensed aquaculture sites in
  Strangford Lough

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Distribution
Methods - 30 intertidal sites surveyed
covering both the northern and southern
basin - 100m transects placed at 0m, 1m and
2m of tidal elevation parallel to the low
water mark - 0.25m-2 quadrat placed on
alternating sides of the transect every 4m
C. gigas and O. edulis present Only O. edulis
present Only C. gigas present No oysters
present The initial introduction site
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Distribution and Density
The densities in m-2 of C. gigas in the 30 sites
sampled in Strangford Lough 2008 with 95
confidence limits
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Distribution

• Colonisation mostly contained within the northern
  basin
• Highly positive correlation between the
  populations of both species
• Suggesting that the hydrodynamic regime is the
  strongest influencing factor

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Distribution Future Work

• Repeat surveys
• Extend survey area to include more intertidal
  sites
• Extend survey to encompass subtidal sites
• Start work on modelling larval movement within
  the lough

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2. Settlement

• Aim
• To investigate and compare the settlement
  preferences of the two oyster species

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Settlement
The percent of C. gigas (open columns) and O.
edulis (shaded columns) settled on the different
substratum types
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Settlement

• Significant differences between the numbers of C.
  gigas and O. edulis settled on different
  substrata (?2 33.788 d.f. 1 P lt0.0001)
• The initial attachment substratum not
  identifiable with many O. edulis
• Substrate availability not likely to be a
  limiting factor

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Settlement Future Work

• Continue adding to dataset when conducting
  distribution surveys
• Run trials using multi-surface spat collectors

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3. Population Structure

• Aim
• To gain insight into the size frequency of C.
  gigas within Strangford Lough in order to
  ascertain if the species is successfully breeding
  every year.

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Population Structure
The contribution of different sizes of the
oysters in the populations. C. gigas (open
columns) and O. edulis (shaded columns)
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Population Structure

• C. gigas has several peaks with small numbers in
  between
• Suggests irregular recruitment
• Generally higher densities of smaller O. edulis
  suggesting successful reproduction over recent
  years

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Population Structure Future Work

• Continue adding to data set when conducting
  distribution surveys
• Determine age using acetate peel analysis on more
  shells
• Then link the age/length data to temperature data
  to see which years the species successfully
  reproduced

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4. Biodiversity

• Aim
• To ascertain if there are any differences between
  the communities of epibiota found present on the
  different shells

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Biodiversity

• Look at the species living the upper and lower
  valves of both species
• Compare epibiota present on the 2 oyster species
  and between similar sized rocks

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Biodiversity

• Preliminary work has been carried out by an
  honours student in 2007
• Results look interesting
• Want to extend the investigation to include more
  sites
• Relate to age data from acetate peel analysis to
  make the findings more insightful
• Use PRIMER to analyse findings

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5. C. gigas removal funded by DARD

• Aim
• To initiate the first attempt of C. gigas
  management during early phase invasion

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C. gigas removal funded by DARD
How much information is required before
management policy can be initiated? (Simberloff
2003) Take advantage of the lag period when
populations are slowly increasing
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The Allee Effect

• The positive correlation between population
  density and the growth rate of the population
• Initial low Allee effect in first introduction
  site
• High Allee effect in newly colonised areas

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Methodology
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C. gigas removal
A
B
A. Number killed, B. mean oyster density m2 in
2008 (open columns) and 2009 (shaded columns)
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Removal Future Work

• Continue annual cull
• Analyse if differences in population densities
  are statistically significant
• Investigate the effect of the nearest neighbour
  distances-intrinsically linked to the Allee
  effect
• Use O. edulis as a model organism to predict
  impacts on fecundity in C. gigas

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6. Gametogenesis

• Aim
• Ascertain the reproductive cycle of C. gigas in
  Strangford lough and relate this to the observed
  population structure and environmental factors
  such as temperature and nutrients

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Gametogenesis

• Measurements taken
• Total wet weight
• Fresh weight of flesh
• Weight of visceral mass
• Weight of empty shell
• Tissue fixed and preserved
• ready for sectioning

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Gametogenesis

• Embedded in JB4
• Sectioned
• Observed under light microscope
• Graded using the gametogenesis index

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Conclusions

• C. gigas is definitely able to reproduce in
  Strangford Lough
• Distribution not limited by substrate
  availability
• Distribution influenced by hydrology
• Successful reproduction and subsequent settlement
  not achieved every year

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Management options

• Continue annual C. gigas removal
• Use of triploid oysters in any further
  aquaculture activities (Guy Roberts, 2007)
• Removal of unused licensed aquaculture sites
• Do nothing and the problem is likely to keep
  expanding

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Do nothing and the problem is likely to expand.
Taken at the Wadden Sea
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Thanks!
Dai Roberts Dave Smyth Fergal Glynn Rick
Ayre Conor Wilson Matt Jackson
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