Spatial and temporal differences in gonad development, sex ratios and reproductive output influence the sustainability of exploited populations of the European oyster, Ostrea edulis

1. The European native oyster, Ostrea edulis, has been in severe decline since the early 1900s across Europe with many fisheries now declared commercially extinct. In light of this broad scale population decline, the UK has listed O. edulis as a threatened species, requiring conservation action under the Biodiversity Action Plan (BAP). In addition to this designation, in Scotland O. edulis beds is a search feature (SF) and a priority marine feature (PMF) for MPA site selection. These sites are also listed as a feature of conservation importance and included on the OSPAR list of threatened/declining species and habitats.
2. Recent studies have identified O. edulis populations with heavily male-skewed sex ratios, which may have contributed to fisheries decline due to reduced levels of fertilization. This species is a protandrous alternating hermaphrodite and individuals may change sex in response to local conditions. This study aimed to assess how sex ratios vary temporally and if this is correlated with temperature, by studying two exploited populations in Loch Ryan, Scotland and Chichester Harbour, England.
3. This study suggests that the proportion of male phase oysters is positively correlated with water temperature and that the study population in cooler waters had a more balanced sex ratio overall (the Loch Ryan population was significantly similar to 1:1 for 10 of 13 months, whereas in Chichester only 1 of the 7 months was significantly similar).
4. This study provides evidence to suggest that a critical temperature threshold for sex determination exists in O. edulis and for the Loch Ryan population we suggest that this is 16.5°C. However, further work is required to assess how this threshold may change between sites and how future climate change scenarios might affect the sex ratio of native oyster populations.