Potential limits to anthropogenic mortality for harbour porpoises in the Baltic region Original Research Article

We estimated potential limits to anthropogenic mortality for harbour porpoises in the Baltic region (the Skagerrak, Kattegat, Great Belt and Little Belt Seas, the Kiel and Mecklenburg Bights, and the Baltic Sea) using conservation objectives set by the Agreement on the Conservation of Small Cetaceans in the Baltic and North Seas (ASCOBANS). Mortality limits (ML) were calculated as the product of: a minimum estimate of abundance, one-half the maximum rate of increase and an uncertainty factor. Previous models show that if anthropogenic mortality is less than ML, a depleted population should recover to more than 80% of carrying capacity, meeting the conservation objectives of ASCOBANS. Minimum estimates of by-catches exceed ML for the population structure hypothesis tested, indicating that these catches will impede recovery. The same result was also evident for other hypothetical population structures. We conclude that immediate management actions are necessary to reduce the magnitude of by-catches to meet the conservation objectives of ASCOBANS.