To improve knowledge of the life history of Dall’s porpoise, Phocoenoides dalli (True, 1885) that are incidentally caught by the Japanese salmon mothership fishery in the northwestern North Pacific, a 34-day cruise was conducted during August-September, 1982, after the fishing season. A total of 710 Dall’s porpoise were sighted and 80 were caught using hand-held harpoons. The species’ southern limit was at 41°N latitude, at a surface water temperature of 18°-19°C, and overlapped the range of Lissodelphis and Lagenorhynchus. Distribution of the truei-type was limited to the Japanese coastal waters and the number sighted was negligible east of 155°E. Density of Dall’s porpoise was higher in the western and northern parts of the research area, and somewhat lower in the southeast. More Dall’s porpoise rode the ship’s bow wave during mid-day than in the morning or evening, and in areas where the surface water temperature was 11°C or above. In areas with a surface water temperature of 11 °C or below, most of the Dall’s porpoise avoided the ship. The latter areas had a high number of mother-calf pairs and calves possibly weaned, none of which rode the bow wave. Adults not accompanying calves were sighted in both areas. School size was not different between the two areas. Most of the specimens were taken in the southern part of the study area. Males predominated and most were 2 to 6 years old, around the age of attainment of sexual maturity. Lactating individuals were significantly scarce in the sample. The dalli-type reached sexual maturity at a smaller body length than the truei-type.
Based upon comparisons of results from this cruise with those obtained from the salmon mothership fishery since 1978, we conclude that, during the season from June through September, (1) females in the state of late pregnancy, lactation, or pregnancy and simultaneous lactation are mainly distributed in the northern area, and (2) the southern area is mainly occupied by males and some females not accompanied by calves.