An experimental investigation of salinity effects on growth, development and condition in the European flounder (Platichthys flesus. L.)

European flounder (Platichthys flesus. L.) is a euryhaline flatfish species which can actively migrate towards and cope with low salinity environments. A laboratory experiment was undertaken to analyse the effect of salinity on condition and growth of metamorphosing European flounder. The working hypothesis was that flounder, which preferentially settle in low salinity habitats, would display accelerated development and/or enhanced growth and condition at lower salinities. The fish used in the experiment were in the late stages of metamorphosis. At the end of the 21. day laboratory rearing period no significant difference in ontogenetic development was found between exposures (salinity of 0, 10, 20 and 30). No significant differences in somatic growth rate, somatic condition or standard length were observed between treatments. There was no correlation between RNA:DNA ratio and somatic condition. Contrary to expectations, mean RNA:DNA ratios (measure of short-term well being) tended to increase with salinity and were significantly higher in the 30 salinity exposure compared to the 0 salinity exposure. The working hypothesis was, therefore, rejected. The results demonstrate that laboratory observations can fail to capture the complex ecological interactions at play in field environments. The preference for low salinity environments may be driven by other environmental factors such as predator/competition avoidance and food supply.