Sex-specific distribution and diet of Platichthys flesus at the end of spawning in the northern Baltic Sea

This study examined the relationship of seascape structure, prey availability and sex on the post-spawning distribution and diet of European flounder Platichthys flesus in the northern Baltic Sea. The objectives were to determine whether: (1) wave exposure and substratum affect abundance and distribution of P. flesus, (2) diet reflects the benthic prey composition and (3) sex affects the distribution or diet of P. flesus. The results showed that P. flesus was evenly spread in the archipelago with no correlation to wave exposure. The distribution was, however, sex specific; reproductive males dominated the exposed zone and mainly post-reproductive females dominated the intermediate and sheltered zones. Platichthys flesus fed mainly on two bivalve prey species: blue mussels Mytilus edulis and Baltic tellins Macoma balthica. Hard substratum invertebrates dominated the diet in all habitats and apart from some typical soft substratum species, there was no clear link between fish feeding and the dominance structure of benthic prey. Diet was further sex specific, with females showing a broader range of diet than males. Results suggest that P. flesus is a specialist molluscivore found commonly and equally in soft- and hard-substratum habitats throughout the archipelago area. Previous studies on P. flesus in the Baltic Sea have yielded inconsistent results regarding diet and it has commonly been believed that the distribution of Baltic Sea P. flesus is linked to sand and soft substrata. The present findings emphasize the importance of including the entire range of habitats when diet and regional species distributions are assessed.