Strip transect aerial surveys were conducted in the Skagerrak and Kattegat Seas for 8 days in June-August, and 2 days in October 1991. Nine sightings of harbour porpoises were recorded in the Skagerrak during 2,161km of transects and 16 sightings in the Kattegat during 1,995km surveyed. The summer densities were 0.021 porpoises/km2 in the Kattegat Sea and 0.013 porpoises/km 2 in the Skagerrak Sea. The densities in October were a magnitude lower suggesting a seasonal change in distribution. Uncorrected minimum abundance estimates were calculated for the Skagerrak and Kattegat survey areas during June-August and compared to minimum bycatch levels of harbour porpoises in Swedish fisheries during the same months and areas, as reflected by carcasses submitted by fishermen. The results indicate that bycatch levels are not likely to be sustainable. A voluntary programme for fishermen, coastguards and ferry operators reported sightings of harbour porpoises year round during a five year period; 1988-1992, in the Skagerrak, Kattegat and Baltic Seas. A total of 332 sightings (949 porpoises) were reported but only one sighting was from the Baltic. The sighting rate per effort (SPUE) for the Skagerrak was about twice that for the Kattegat. The only trend in SPUE over the five year study period was a decline in the Skagerrak Sea in 1992, compared to 1988-91. Harbour porpoises were sighted throughout the year in the Skagerrak and Kattegat Seas, with a peak occurrence during the spring and summer months. The low densities recorded in the Skagerrak and Kattegat Seas during the aerial surveys, compared with documented bycatch in these areas and the lack of trends in the sighting rates reported,
indicate that the status of harbour porpoises in these areas is uncertain. The virtual lack of sightings in the Baltic Sea and the threat posed by bycatches in the area indicate that harbour porpoises in the Baltic Sea are endangered.