The reproductive processes of a wild population of the European flat oyster Ostrea edulis in the Solent, UK

Abstract

This thesis presents an investigation into the reproductive processes of a collapsing population of the European flat oyster Ostrea edulis in the Solent, UK. Oyster populations are in decline worldwide. Their ecological and economic importance requires research to address the many uncertainties still existing regarding their reproduction and recruitment, in order to manage or restore populations successfully. The Solent oyster population supported a productive local fishery from 1972 to about 2006, when catch rates started to decline sharply until the fishery became economically unviable in 2011. Recurrent recruitment failures suggested that the collapse might have been caused by a disturbance of the reproductive processes. This study set out to determine if the reproductive processes were indeed inhibited, and if so, at which life history stage. Population fitness was good and gave no reason to assume that the oysters were incapable of successful reproduction due to poor condition. Histological and visual examination of the reproductive status of 362 animals revealed a significant skew towards male phase oysters (chisquare, P 0.001), while gametogenesis as such was normal in each gender. Fecundity was similar to literature values. No relationship was found between the recruitment failures or the sex ratio shift and prevalence of bonamiosis, as only two animals of the 25 examined with a PCR assay were infected. Ostrea edulis larvae were found in the plankton with a peak of 9 larvae 100 l-1 in August 2010. Larval abundance was within the range of previous records for the Solent from 1984-87. Development of Ostrea edulis larvae cultured in the laboratory was found to be normal. No apparent adverse effects of exposure to air with an increased pCO2 of 1500 atm were found either, leading to hope that the long term viability of the species will not be jeopardized by ocean acidification. No spat settlement was observed in this study. Predation pressure from Ocenebra erinacea was found to be low, and hopefully would not jeopardize restoration attempts.

Overall this study showed that while there is a lack of females in the Solent oyster population, the reproductive processes as such function well enough to enable regeneration and maintenance of the population. This study demonstrated the importance of investigating the reproductive processes of a population before planning its regeneration, but it also showed that the basic factors of habitat destruction and unsustainable fishing pressure must not be overlooked.

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ORCID for Antony Jensen: orcid.org/0000-0002-8924-1198

ORCID for Lawrence Hawkins: orcid.org/0000-0002-9236-2396

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