ON THE FACTORS INFLUENCING JUVENILE FLATFISH ABUNDANCE IN THE LOWER SEVERN ESTUARY, ENGLAND

Bridgwater Bay within the Bristol Channel, Somerset, England is a nurs ery ground for sole, Solea solea, and dab, Limanda limanda, during the autumn and winter. Flounder, Platichthys flesus, both juveniles and a dults, are common during the summer. Using a 13-year data set of fish in the bay, correlations were studied between climatic, predatory and competitive factors and juvenile flatfish abundance. The major factor was found to be seawater temperature. For sore, abundance was positive ly correlated with the temperature in the spawning period (April and M ay), For flounder, abundance was negatively correlated with average te mperature during the previous year. For dab, average winter temperatur e over the spawning period was negatively correlated with juvenile abu ndance and with mean length observed during the following autumn. Thes e climatic changes were also found to influence the abundance of a lar ge number of other fish and crustacean species which were potential pr edators or competitors and which in some cases were significantly corr elated with flatfish abundance. The data set was analysed using multip le correlation analysis, Multi-factorial models of population change w hich included interspecific and climatic factors were examined. Using first-order partial correlations it was possible to distinguish betwee n different causal models. In every case it was found that interspecif ic correlations were attributable to both species independently changi ng in abundance with temperature. No significant correlations between the abundance of potential predators or competitors and juvenile flatf ish were detected.