Harbour seals (Phoca vitulina) in Dutch inland waters: an overview of reported sightings and some first data on diet
In the Netherlands, harbour seals (Phoca vitulina) inhabit all marine waters, including estuaries and the lower tidal parts of rivers. However, by damming most of the inland waters, these inland habitats became less accessible. Yet seals still venture inland, negotiating a range of man-made barriers. The seals move through devices that discharge water into the sea, or use shipping locks to reach inland waters. Today, considerable numbers of seals are found in two relatively open barred estuaries in the southwest of the country, i.e. Lake Grevelingen and the Eastern Scheldt. Smaller numbers are found in the more closed freshwater bodies Lake IJssel and Lauwersmeer in the north of the country and only few seals have been sighted at any one time in these lakes. Other individuals have swam up-river through the brackish ports of Rotterdam and beyond. Little is known on the feeding habits of these inland seals, despite the large numbers of animals involved. An overview of reported sightings of inland harbour seals has been compiled. Furthermore, for the first time the diet of a harbour seal, found dead in Lake IJssel, was studied. Prey species and sizes found in its stomach are described. A river lamprey (Lampetra fluviatilis) and 28 European flounders (Platichthys flesus) had constituted its last meal, while 12 European smelts (Osmerus eperlanus) had been taken shortly before that. All three prey species can be found in fresh, brackish and marine waters, but had in all likelihood been consumed in the freshwater Lake IJssel.