Diet Analysis of Piscivorous Predators Based on DNA Metabarcoding: Application to Grey Seals and Cormorants in the Baltic Sea

The number of grey seals and cormorants in the Baltic Sea has increased considerably during the last decades. As a consequence, information about the ecological and economic impacts, relying on adequate dietary estimations, of these piscivores is requested. Conventional methods to analyse the diet are based on morphological identification of prey remains, mainly hard parts, found in/from the digestive tracts. Such methods are however limited by biases. In this study, DNA metabarcoding was used to assess the diet of 155 grey seals. The results were compared to the diet assessed from conventional analysis. About the same number of species was found by the two methods, but DNA barcoding increased the prey detection rate considerably. Prey was found in more seals and the frequency of occurrence of most prey species was higher when using DNA barcoding. DNA metabarcoding is well suited for dietary monitoring and we suggest the method to be used in combination with other dietary methods to give a more detailed picture of the diet. Results will also be presented from a minor study using DNA barcoding to assess the diet of cormorants in the northern Baltic Sea.