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

Bridgwater Bay within the Bristol Channel, Somerset, England is a nursery ground for sole, Solea solea, and dab, Limanda limanda, during the autumn and winter. Flounder, Platichthys flesus, both juveniles and adults, 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 positively correlated with the temperature in the spawning period (April and May), For flounder, abundance was negatively correlated with average temperature during the previous year. For dab, average winter temperature over the spawning period was negatively correlated with juvenile abundance and with mean length observed during the following autumn. These climatic changes were also found to influence the abundance of a large number of other fish and crustacean species which were potential predators or competitors and which in some cases were significantly correlated with flatfish abundance. The data set was analysed using multiple correlation analysis, Multi-factorial models of population change which included interspecific and climatic factors were examined. Using first-order partial correlations it was possible to distinguish between different causal models. In every case it was found that interspecific correlations were attributable to both species independently changing in abundance with temperature. No significant correlations between the abundance of potential predators or competitors and juvenile flatfish were detected.