Concern has been raised about the long-term viability of the harbour porpoise population in the Baltic Sea. Bycatch at possibly unsustainable levels, contaminants, overfishing of prey species and disturbance have been identified as possible threats. This literature study summarises the current knowledge about harbour porpoises in the Baltic Sea and, on the basis of reviewed material, tries to identify critical remaining uncertainties and suggests management needs. Information on porpoise demography like distribution, abundance, migration, population structure, and factors affecting survival are presented.
The historic range of the harbour porpoise extended into the north-eastern parts of the Baltic Sea. During the second half of the 20th century, numbers of harbour porpoises have declined and the distribution range narrowed. Currently there is a considerable difference in abundance in the Kattegat and Belt Sea (0.73 – 0.99 animals km2) as opposed to the Baltic Proper (<0.01 animals km2). Although recent morphometric, genetic and contamination studies of harbour porpoises in the Baltic Sea are somewhat inconsistent with respect to population structure, the existence of a distinct Baltic subpopulation appears to be a valid concept. Migrational patterns of Baltic Sea animals are still ambiguous. In historic times large numbers of harbour porpoises were hunted in the Danish straits during winter and spring. Therefore it was often concluded that porpoises escaped from ice cover in the eastern Baltic Sea in the winter and re-colonised the Baltic Proper in spring. Recent observations indicate that migration behaviour is much more complex and diffuse. There seems to be a tendency of animals from the Kattegat to migrate into the North Sea during winter. But also animals remaining in the western Baltic or Baltic Proper have been described. Available nutritional studies suggest that harbour porpoises take a variety of different prey. Herring, sprat and cod are their most important prey items. Sexual maturity is attained at an age of 3–4 years. A larger proportion of females give birth to one calf every year (pregnancy rates were reported between 0.61 and 0.84). The average life span of harbour porpoises in the Baltic Sea is unknown. From existing data, a maximum age of 22 to 23 years seems to be a realistic assumption. However, a high mortality in the first years of age and a proportion of less than 5% of the animals living beyond 12 years have a significant impact on the potential for increase of the stocks. It is assumed that shallow areas play an important role for this species with respect to calving and nursing. A variety of studies report heavy attacks from parasites such as Anisakis simplex, Tonyurus convolutus, Stenurus minor, Halocercus invaginatus or Pseudalisus inflexus. However, when compared to samples from Greenland these can be regarded as normal infestations. Environmental contaminants most likely affect the long-term viability of Baltic Sea harbour porpoise stocks and might have been a major cause for the decline of Baltic Sea harbour porpoise stocks between the 1940s and the 1970s. Since then concentrations of PCBs and other organochlorine contaminants have declined. To date, the most important threat to Baltic Sea harbour porpoises is by-catch. Noise pollution has the potential to increasingly become a major threat due to the development of new activities in the Baltic Sea (offshore-windpower plants, fast ferries, etc.). This study lists a number of life history variables for which data is urgently needed. On the basis of the currently available knowledge, an effective management strategy must include political and technical means of mitigating threatening activities such as by-catch, disturbance to critical habitat, disposal of contaminants and over-fishing. In this respect it is important to establish marine protected areas and time- and area closures for certain fisheries which are likely to be unsustainable, to establish mandatory fishery observer programmes and to compile appropriate fisheries statistics.