First evidence of grey seal predation on marine mammals in the German Baltic Sea

Abstract

Grey seal predation on marine mammals has been reported throughout the northern hemisphere except for the Baltic Sea. After the Baltic subspecies (Halichoerus grypus grypus) was on the brink of extinction in the early 1990s, numbers increased steadily during the past three decades from a few thousands to >40,000 in 2020. In recent years, grey seals were observed more frequently in the southern and western Baltic Sea, areas where the subspecies was a rare guest for many decades. Nevertheless, <10% of the Baltic grey seals are counted in the southern Baltic Sea. A standing stranding monitoring programme investigates carcasses of marine mammals washed up in Mecklenburg- Western Pomerania, Germany. In August 2021 a harbour seal (Phoca vitulina) carcass was found with lesions typical for grey seal predation: amongst others the typical “cork-screw”, helical and cut-like lesions with smooth wound margins. The juvenile harbour seal showed 9 of 10 parameters indicating grey seal predation according to van Neer et al., 2021. Earlier suspicious cases in harbour porpoise (Phocoena phocoena) suggest that the presented case may not be an isolated one. Rather, it is a so far rare observation of a behaviour that may become more common with the ongoing recovery of grey seals in the southern Baltic Sea in an area of overlapping habitats with harbour porpoise and harbour seals. In this region harbour seals and harbour porpoise are more abundant than in the Northern Baltic Sea where, as of yet, grey seals are more numerous.