Unusually High Proportion of Males in a Collapsing Population of Commercially Fished Oysters (Ostrea edulis) in the Solent, United Kingdom

The Solent, United Kingdom, contains a commercially fished, rapidly declining wild population of the protandric sequential hermaphrodite European flat oyster Ostrea edulis. The population has suffered repeated recruitment failures since 2007, suggesting a disturbance in reproductive processes. As part of an ongoing investigation into the causes of this population collapse, the reproductive status of 362 individuals sampled in 2009 and 2010 was determined by histological and visual examination. In both years, significantly more male-phase oysters than females were found (chi-square, P < 0.001). There was also a significant difference in the size distribution of male- and female-phase oysters (Mann Whitney U-test, P = 0.032), where the median of female-phase animals was 6.5 mm larger than that of males. Monthly analysis of gonad development between June 2008 and September 2009 showed that, despite the skewed sex ratio, the reproductive development in each gender was as expected, and animals were developing and spawning sperm and eggs, peaking toward the end of June.