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

Claire Guy and Dai RobertsThe Queen’s University Belfast

Outline

• Background information on the oysters in Strangford

• Current research• Future research• Conclusions

Historic oyster populations in Strangford

O. edulis Stock decline in Strangford Lough• 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….

Ostrea edulis

Pacific oyster (Crassostrea gigas)

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

PhD Research

1. Ascertain the distribution of C. gigas within Strangford Lough2. Settlement substratum preference observation and investigation3. Population structure4. Biodiversity supported by the two species5. Initiation of C. gigas removal6. Gametogenesis7. Management options

1. Distribution

• Aim– To ascertain if there has been any spread of C.

gigas outside the licensed aquaculture sites in Strangford Lough

DistributionMethods

- 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 presentOnly O. edulis presentOnly C. gigas presentNo oysters presentThe initial introduction site

Distribution and Density

The densities in m-2 of C. gigas in the 30 sites sampled in Strangford Lough 2008 with 95% confidence limits

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

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

2. Settlement

• Aim– To investigate and compare the settlement

preferences of the two oyster species

Settlement

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

Settlement

• Significant differences between the numbers of C. gigas and O. edulis settled on different substrata (χ2 33.788; d.f. 1; P <0.0001)

• The initial attachment substratum not identifiable with many O. edulis

• Substrate availability not likely to be a limiting factor

Settlement – Future Work

• Continue adding to dataset when conducting distribution surveys

• Run trials using multi-surface spat collectors

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.

Population Structure

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

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

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

4. Biodiversity

• Aim– To ascertain if there are any differences between

the communities of epibiota found present on the different shells

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

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

5. C. gigas removal funded by DARD

• Aim– To initiate the first attempt of C. gigas

management during early phase invasion

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

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

Methodology

C. gigas removal

A. Number killed, B. mean oyster density m2 in 2008 (open columns) and 2009 (shaded columns)

A

B

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

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

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

Gametogenesis

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

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

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

Do nothing and the problem is likely to expand……….

Taken at the Wadden Sea

Thanks!Dai RobertsDave SmythFergal Glynn

Rick AyreConor WilsonMatt Jackson