Food choice of different size classes of flounder (Platichthys flesus) in the Baltic Sea

Abstract: Flounder, Platichthys flesus, is considered to be an important predator on blue mussels, Mytilus edulis, in the Baltic Sea. The purpose of this study was to investigate if flounder diet, i.e. blue mussels, could have an impact on the relationship between weight and length of flounders, i.e. their condition. Flounders and blue mussels were captured in August 2013 outside Herrvik, on eastern Gotland. Experiments on feeding preference and weight changes of flounders feeding on either small or larger mussels took place at Ar Research Station, Uppsala University. Blue mussels were kept in aquaria. The fish were kept in basins (2 × 2 m), with sea water (about 19°C and 6.3 ‰). The fish were divided into different size classes and some of them tagged individually. Analyses of substance in flounder stomachs as well as calculations of mussel weight/length relation were accomplished with data from 2012. Results from the experiments showed that all flounders, independent of size, preferred small mussels before larger ones. They all lost weight during the experiments. The weight/length ratio (shellfree dry weight/shell length) of blue mussels was proved to be positively correlated with shell length. Because of this it was concluded that the size of ingested blue mussels does not have any impact on the differences in condition between larger and smaller flounders.