The Biology of the Flounder, Platichthys flesus L. (Pisces: Pleuronectidae) in the Thames, a Polluted Estuary.

PhDFlounders (Platichthys flesus L. ) were sampled from the cooling water intake screens of West Thurrock power station, Essex, from September 1987 to October 1990. Changes in population structure are determined from age and morphometric data. Adult flounders congregate in the mid-estuary in late spring, prior to their spawning migration. New recruits arrive in the estuary in summer and disperse upstream after a few weeks. Dispersal appears to involve density-dependant population regulating mechanisms. Growth appears seasonal to an extent, being reduced during winter months. The gut contents were analysed and a trophic spectrum constructed. P. flesus is an opportunist predator, feeding predominantly on crustaceans in the mid estuary and molluscs in the outer estuary. There is some evidence of resource partitioning between adult and juvenile flounders. The significance of changes in the lipid reserves of 0-group flounders is discussed. The seasonal dynamics of external disease symptoms and parasite burden of these fish was evaluated, and related to population structure, dietary condition and water quality variables. 2 The prevalence of all external disease in flounders from Kest Thurrock is closely related to population structure. Younger fish generally display lower disease prevalences. -Increased prevalence of lymphocystis may be related to increases in the prevalence of ulcerations or heavy infections by the copepod Lepeophtheirus pectoralis. A correlation between the minimum monthly dissolved oxygen (D. O. ) level and the condition factor of flounders is demonstrated. Fewer epidermal ulcers are observed on flounders with a high condition factor. The prevalence of epidermal ulcerations, and to an extent lymphocystis, are thus indirectly associated with estuarine pollution. The use of external disease prevalence as a bioindicator of estuarine pollution is considered. Live flounders were observed in various lighting regimes using time-lapse video techniques to determine the nature and extent of intrinsic rhythmic behaviour. Observations were made on the behaviour modifying effects of contaminated sediments and hypoxic conditions. The findings are discussed in relation to the ecology of the flounder in the Thames estuary