Two sites in North Wales, UK, were compared for growth, survival, immunocompetence and gonad development of Crassostrea gigas. Juvenile C. gigas spat were purchased as a cohort and placed in bags on trestles at the Menai Strait (MS, two heights on the shore) and the Inland Sea (IS) in the summer of 2003. Performance was examined as a function of several environmental parameters; temperature, coloured dissolved organic material absorption coefficient (cDOM), organic and inorganic nutrient levels, chlorophyll a and phytoplankton community composition. Both sites produced variations in growth, as assessed by dry weight indices and condition index, which were found to be lower at IS, where the animals were permanently immersed, than at MS, where they were emersed for at least 1 h/tide (at the low shore height) and 4 h/tide (at the high shore height) during spring tides. Condition index was also significantly higher at MS high shore than at MS low shore. Mortality was low at all sites, though an increase at IS at the end of the trial led to a significantly greater final percentage mortality of 15.6%. Gonad development was limited, particularly at IS where most animals remained undifferentiated. Gonad development was significantly higher at MS high shore than at MS low shore. cDOM, organic and inorganic nutrients were similar at the two areas. At IS the temperature and chlorophyll a levels were higher than MS. The phytoplankton communities were significantly different, with a bloom of Prorocentrum micans dominating at the Inland Sea for much of the experimental period. Differences in the hydrodynamics, as well as Phytoplankton community appeared to be the only parameters that might cause a lower growth rate in the Inland Sea. The possibilities of P. micans having a sub-lethal effect on C. gigas are discussed.
