Food quality influencing growth and survival rates of juvenile freshwater pearl mussels (Margaritifera margaritifera)-Compensation for growth limitation due to low water temperatures?

Human impacts have led to dramatic declines of the endangered freshwater pearl mussel Margaritifera margaritifera (FPM) and to a shift of its main distribution areas to more undisturbed but colder headwaters. In this study, we investigated whether feeding on high-quality food (i.e. food rich in n-3 polyunsaturated fatty acids) can compensate for growth limitations due to low water temperatures and whether food quality influences the survival rate of juvenile FPM. Therefore, we conducted feeding experiments with juvenile FPM at three different temperatures (11, 15 and 19°C) and with three different food sources. Afterwards, mussels were reared in cages in a stream to analyse survival during the first winter. Our results showed that food quality can compensate to some extent for growth limitations due to low water temperatures, but sufficiently high growth rates of juvenile FPM can only be achieved by the availability of high-quality food in warmer (> 15°C) environments. The availability of high-quality food during the main growing season also improves the survival of FPM over the first winter. Consequently, restoration efforts must aim to restore habitats in the historical main distribution areas of FPM and to create habitat conditions that allow the development of high-quality food.