The age, growth and diet of a freshwater population of the flounder, Platichthys flesus (L.), in southern England

Otoliths from 640 flounders from the River Frome, a chalk river in southern England, were used for age and growth studies together with length-frequency data. Mean lengths at ages I–V were 89, 151, 225, 304 and 323 mm respectively; these are higher than have been reported for marine populations. All flounders caught were immature and, on reaching the sea, both sexes probably became sexually mature a year older than did the marine populations. Flounder diets in freshwater comprised largely aquatic insects, Mollusca and Crustacea but there were differences between large and small fish; seasonal changes in diet largely reflected prey availability. It is suggested that competition for food and space in estuaries between 0 group flounders have given rise to the migration into fresh water.