BEHAVIOR AND SEGREGATION OF THE DALL ' S PORPOISE IN THE NORTHWESTERN NORTH PACIFIC OCEAN
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 l8°-l9°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, (I) 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. 1 Present address: Far Seas Fisheries Research Laboratory, 5-7-1 Orido, Shirnizu-shi, Shizuoka-ken 424, Japan. Sci. Rep. Whales Res. Inst., No. 35, 1984, 107-128 108 KASUYA AND JONES
GROWTH OF DALL ' S PORPOISE IN THE WESTERN NORTH PACIFIC AND SUGGESTED GEOGRAPHICAL GROWTH DIFFERENTIATION
Some growth parameters of the western North Pacific dalli-type Dall's porpoise were estimated from 167 specimens taken with hand-held harpoons on two cruises and compared with data obtained previously from porpoises killed incidentally in the salmon gill net fishery. These parameters were compared with previously published figures for the Japanese coastal population. Individuals from the Pacific coast of Japan (96% were truei-type) attained sexual maturity at body lengths 12 to l 7cm larger than dalli-type individuals in the offshore waters. This is an indication of genetic differentiation between the two populations in addition to the previously reported external pigmentation. The possible usefulness of the growth parameters for the detection of genetic isolation between two major color types off the Pacific coast of Japan is indicated.
Food of Porpoises in Waters off Japan
Relatively little information on the food of the smaller cetaceans has appeared in the literature. This is true for most species because scientists usually lack the opportunity and the techniques for taking more than an occasional cetacean. When advantage can be taken of some commercial operation, specimens often become available in large enough numbers to provide important information. The skilled marine mammal hunters of the northern prefectures of Japan, for example, may take thousands of porpoises and dolphins annually.

Porpoises and dolphins are widely distributed and in total comprise a very large population. It is, therefore, of interest to know how they fit into the ecology of their marine habitat and also what economic influence, if any, they may have. Generally, few accusations of damage have been leveled at the small cetaceans. This is at least partly true because fishermen know very little about the animals and have no reason …