Abstract
The distribution and seasonal density of the finless porpoise were examined in the coastal waters of western Kyushu, Japan in 1987-1992 by ship sighting surveys and interview surveys with fishermen and ferry crews. Interview surveys showed that the porpoise may occur rarely in Sumo Nada and the waters of northern Nagasaki Prefecture and that it was found frequently in Ariake Sound, Tachibana Bay, and Omura Bay. In ship surveys by the line transect method, mean densities in the middle and mouth of Ariake Sound were estimated as 1.31-1.60 and 0.341 individuals/km, 2 respectively. The porpoise density in the middle of Ariake Sound increased from September to April and showed a minimum in August. In the mouths of Ariake Sound and Tachibana Bay, the sightings became higher in summer. The porpoise distribution was considered to shift seasonally between the two seas. Water depth and distance from the coast seem to influence porpoise occurrence. Porpoises may favor depths of less than 50m and distances of less than 2-3 nautical miles. The season of high density in Ariake Sound coincided with the reported calving season but not with the increase in the mean school size. The mean school size was 1.70 (SE 0.05).
