The Chichester Harbour Oyster Partnership Initiative (CHOPI) was formed in 2010 to promote stock growth within the Ostrea edulis fishery in the Harbour. An important requirement of this project was to gain an insight into the reproductive success of the Harbours Ostrea edulis population, by monitoring the oyster sex ratio, their gametogenesis and numbers of brooded larvae, some of which had been relaid into high density (minimum 20 m-1) broodstock areas.
Sampling
Samples, each of twenty oysters, were collected every two to three weeks throughout the winter to summer of 2011 from one of the broodstock sites, fixed in formalin, and then the oysters gonads were sectioned and stained to enable identification of each oysters gender and reproductive stage. The reproductive stages ranged from inactive during winter, through various development stages to fully ripe when ready to release eggs/sperm, and spent once the eggs/sperm were released. Environmental factors such as temperature and chlorophyll levels were also recorded for comparison to the changing reproductive stages. The sex ratio was highlighted as an important factor in the likeliness of successful population reproduction, since significantly skewed sex ratios of 6:1 male:female had been observed in the O.edulis population in the Solent (Kamphausen et al., 2011).
Sex Ratio
Oysters in Chichester Harbour showed a significant skewed sex ratio (3:1 male: female) which, although less pronounced than that of the Solent population, suggests a potentially reduced reproductive output than a normal 1:1 population. The occurrence of a male skewed ratio in O.edulis remains largely unexplained, however possible causes may relate to individual stress due to disease or anthropogenic pollutants.
Reproduction
Chichester Harbour oyster reproduction advanced through the year as expected, with ripe individuals being sampled from late May and early June, and an increasing number of spent individuals being recorded as summer progressed; indicating an ongoing release of sperm and eggs. Brooding larvae were collected from a total of six oysters, which represented 5% of the sampled population. This was lower than that recorded in previous research (e.g. Walne, 1974) where 10% of the population were found to be brooding larvae; however this is likely to be a result of the skewed sex ratio found in Chichester Harbour. Larval counts were also lower than previously recorded in O.edulis (e.g. Walne, 1974), which may suggest a lower fecundity for the population than would have been expected. However as this is the first formal description of the reproduction of Chichester Harbour oysters, there are no data from previous years for comparison.
Conclusions
It would appear that although the sex ratio in Chichester Harbour is skewed towards males, the population is maintaining a complete gametogenic cycle and successfully brooding larvae. Following the apparent success of this project in monitoring the oysters reproductive output, future work on this population should include monitoring larval settlement within the harbour and continuation of the sex ratio survey.
January 2013
