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Fishery Bulletin 91(3), 1993 



must be analogous to that of the outer Estuary and 

 inner Gulf of the St. Lawrence in eastern Canada 

 (Laurin, 1976) and the Baltic Sea (M0hl-Hansen, 1954), 

 where harbor porpoises leave ahead of the ice forma- 

 tion. The situation is likely to be different in the Sea 

 of Japan, which in general has a much less severe 

 marine climate than the Sea of Okhotsk (Hirano, 1957). 



The records from Wakkanai show that harbor por- 

 poises occur off northern Hokkaido during early win- 

 ter in some years. In December 1986, a small zone of 

 inshore surface water off Wakkanai was still at 9°C 

 (Japan Meterological Agency, 1986). Some specimens 

 have been taken incidentally by the gill-net fishery 

 around the Shakotan Peninsula in January and Feb- 

 ruary 7 where sea surface temperatures are about 

 4-5°C. The seasonal isotherm distributions suggest that 

 in mid-winter P. phocoena might be expected to be 

 found sometimes as far south as eastern Shikoku, to 

 the southern tip of western Honshu and also along the 

 east coast of the Korean peninsula (Fig. 3). Neverthe- 

 less, it has yet to be confirmed further south than 

 Nishiyama (about 39°N) on the Japan Sea coast of 

 Honshu (Institute of Cetacean Research, 1989). On the 

 east coast of Honshu, the known southern limit of range 

 at Taiji also coincides with the intrusion of deep water 

 associated with the Fossa Magna geological disconti- 

 nuity, which is a feature of the northern extremity of 

 the Philippine tectonic plate margin (Pinet, 1992). The 

 west coast manifestation of this structure occurs in 

 Toyama Bay, at about 37°N. 



In the summer months, harbor porpoises probably 

 do not occur much further south on the west coast of 

 Japan than the Tsugaru Strait and the western ex- 

 tremity of Hokkaido (Kawamura et al. 1983; Kawamura 

 1986) (Fig. 4), where surface temperatures of 12 to 

 16°C often persist through summer (Miyashita and 

 Kasuya, 1988). The progression of seasonal reoccu- 

 pation of northern waters in the western North Pacific 

 is not well understood. The changes in distribution 

 predicted here are based on the limits for the occur- 

 rence of about 90% of the population in the 6.5°-17.0°C 

 range in the western North Atlantic (Winn 6 ). Further- 

 more, Yurick (1977) noted that the greatest concentra- 

 tion of harbor porpoises in the lower Bay of Fundy 

 was associated with the 8°-15°C surface isotherm zone. 



Sea ice leaves the northern coast of Hokkaido usu- 

 ally in late April-early May and warmer surface wa- 

 ters of up to 8°C intrude into this region in May. By 

 June surface temperatures of 5.0°-7.0°C occur as far 

 north as the Komandorski Islands in average years, 

 and by mid-July the same temperatures can be found 

 off Cape Navarin at 63°N (Nasu, 1963, 1966). There is 



Hiroshi Nitto, Otaru Aquarium. Otaru, Shakotan, Hokkaido. Japan, 

 pets, commun. 



much year-to-year variation; in seasons with late de- 

 velopment of summer conditions, the same regions can 

 have surface temperatures as low as 2° to 4°C (Nasu, 

 1966). During August, surface waters reach 10°C both 

 in this region and off Unalaska Island in the Aleutians 

 (Nasu, 1966). Surface temperatures in the Sea of 

 Okhotsk during June are often only 4°-5°C; but, by 

 August-September 12°-15°C (Japan Meterological 

 Agency, 1986). Although cooler intermediate water up- 

 welling in summer around the central Kuriles is still 

 only at 4°-6°C, highly productive boundary conditions 

 for marine life are developed here (Hikosaka and 

 Watanabe, 1957). The summer (August and early Sep- 

 tember) sightings of harbor porpoises in the Sea of 

 Okhotsk during the extensive cruises of 1989 and 1990 

 (Miyashita and Doroshenko, 1990; Miyashita and 

 Berzin, 1991) were almost all made over shelf regions; 

 in contrast the pelagic central region was dominated 

 by Phocoenoides dalli. 



On the basis of distributional and energetic data 

 from the western North Atlantic (Yasui and Gaskin, 

 1986), P. phocoena should tolerate a 6.5°-12°C surface 

 regime with relative ease. Nevertheless, recent results 

 of mitochondrial DNA comparisons of harbor porpoises 

 in eastern Canadian waters (Wang et al., 1991) sup- 

 port the concept of distinct genetic demes coexisting 

 within the regional range of the western North Atlan- 

 tic population. We should not discount the possibility 

 that some of these demes, either through differing di- 

 etary requirements or variation in thermoregulatory 

 capacity, may be adapted to somewhat different sur- 

 face temperature regimes. A specific area could be oc- 

 cupied by porpoises from one deme in summer and 

 another in the winter, giving the appearance of con- 

 tinuous occupation by a single population. Migrations 

 may exist but would be difficult to define without ex- 

 tensive tagging experiments. 



In other areas of the northern hemisphere where 

 the diet of harbor porpoises has been investigated (sum- 

 marized by Gaskin, 1985), the main prey are schooling 

 fishes of the coastal shelf, particularly herring Clupea 

 sp., mackerel Scomber spp., squid, and small gadoids. 

 Too little is known of the harbor porpoise in the west- 

 ern North Pacific to assess its dietary spectrum; in 

 other areas, prey switches occur when herring avail- 

 ability is low (Recchia and Read, 1989). It seems, from 

 the few data currently available, that harbor porpoises 

 in Japanese waters eat sardines, anchovy, small hake, 

 squid, and herring when encountered. 



The intensity of inshore fishing activity in Japanese 

 coastal waters is such that the harbor porpoise cer- 

 tainly has been subjected to some level of incidental 

 catch historically, especially by set or drift nets of vari- 

 ous kinds. Entrapments still occur; total annual in- 

 cidental catches around the Shakotan Peninsula, 



