Harbour porpoisesPhocoena phocoena in the North Sea: Numerical responses to changes in environmental conditions

Changes during the last 40 years of abundance and occurrence of harbour porpoises in the North Sea are illustrated as consequences of changes in environmental factors. The major changes in environmental conditions which influenced the presence of harbour porpoises in the North Sea, in particular the southern North sea, are postulated to be changes in prey availability and changes in accidental catches. Limitations of prey made them move out of the coastal areas, but accidental catch has reduced their overall abundance. Prey availability was affected by limitation of herring and mackerel as the major food source, caused initially by overfishing, followed by a shift in spawning and feeding areas towards the North. Without ignoring the possible complementary impact of pollution and disturbance, it is believed that accidental catch is the only other major threat. The present size of the harbour porpoise population(s) in the northeast Atlantic, including the stock in the Danish inner waters, is estimated to average 160,000 (range 70,000–225,000) specimens. However, this should not divert the attention from the fact that bycatch and directed catch have significantly reduced the size of the North Sea harbour porpoise populations (including the Baltic), by at least between 53,000–89,000 animals, and more likely the higher end of the range. It is emphasized that management measures should be taken in order to reduce the bycatch of porpoises as was already agreed at the latest Ministerial North Sea Conference. The only short-term, pragmatic and efficient way to assess whether the population will benefit from increasing prey availability is to organise international sighting surveys. Then it may be possible to obtain a clear perception of whether the porpoise has only temporarily moved out of the southern North Sea or whether we are dealing with a decreased population, unable to recover because it can not sustain the existing accidental catches.