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Fishery Bulletin 91(3), 1993 



the case of small marine mammals there is energetic 

 advantage in evolving a limited mating period, together 

 with a reduction in testis size out of the breeding sea- 

 son (Gaskin et al., 1984). Field observations of Phocoena 

 phocoena populations suggest that there is little co- 

 herent school structure (eg., Amundin and Amundin, 

 1971; Watson, 1975; Watts and Gaskin, 1989); there is 

 a mean group size of only two or three individuals, 

 often mother, calf and yearling. Larger groups are gen- 

 erally temporary feeding aggregations. Similar results 

 were obtained in studies of Neophocoena phocoenoides 

 by Kasuya and Kureha ( 1979). 



Distributional limits 



The most southerly record of harbor porpoise from 

 Japanese waters (Taiji, Wakayama, 34°15'N) quite 

 closely parallels that for the oceanographically similar 

 western North Atlantic (Cape Fear, southern N. Caro- 

 lina) (Gaskin, 1984). The confirmed northernmost limit 

 for the western North Pacific in recent years is repre- 

 sented by a number of sightings on the eastern side of 

 the entrance of Shelikhova Bay, between 57°-58°N and 

 157°-159°E on 10-11 August 1989 (Miyashita and 

 Doroshenko, 1990). On the eastern side of the North 

 Pacific there are verified records in the Beaufort Sea 

 as far as the MacKenzie Delta at about 70°N (Van 

 Bree et al., 1977). Tomilin (1957), citing Sleptsov (un- 

 published data in manuscript report) who made obser- 

 vations in 1947-48, noted that the harbor porpoise 

 was known in the USSR as far south as Peter the 

 Great Bay, through the Sea of Okhotsk, around 

 Kamchatka (where it was sometimes trapped in fishing 

 nets), and northwards at least to Olyutorskii Bay at 

 60°N. A similar distribution was indicated by Klumov 

 (1959). Tomilin (1957) thought its range might extend 

 into the western part of the Chukchi Sea but gave no 

 records. Sleptsov had also recorded porpoises in sum- 

 mer months around the Komandorski Islands and on 

 both the east and west sides of the Kurile Islands. 

 Miyashita and Berzin (1991) observed harbour por- 

 poises just west of the southwestern Kuriles in early 

 August 1990 and to the northwest of these islands and 

 close to southeastern Sakhalin in mid-late August of 

 the same year. Recent records from Alaska and the 

 eastern Aleutian Islands are summarized by Gaskin 

 (1984); and from the Attu island group in the western 

 Aleutians, by Jones (1984). 



Postulated seasonal changes in distribution 

 in the N.W. Pacific 



Given the similarities of life history parameters of har- 

 bor porpoise populations in both major oceans of the 

 northern hemisphere, one way to estimate the likely 



range of the species in the western North Pacific and 

 Bering Sea is to plot surface isotherms for values which 

 generally appear to be limiting in regions where dis- 

 tribution is better known (Figs. 3 and 4). Winn 6 , work- 

 ing off the eastern United States, found the species 

 were confined almost entirely to coastal shelf waters 

 and 90% of sightings were concentrated over water 

 depths of 18-224 m off the east coast, in sea surface 

 temperatures of 6.5-17.0°C. Barlow (1988) found that 

 sightings off the west coast of the United States were 

 most abundant over depths of 18-37 m and there were 

 no sightings at depths greater than 110 m. Surface 

 temperatures were not specified in this study. The ar- 

 eas of the North Pacific and Bering Sea which ap- 

 proximate these average conditions are shown for win- 

 ter and summer in Figures 3 and 4. There is no doubt, 

 however, that this species must sometimes follow 

 productive convergence zones away from the coastal 

 shelf, or it could not have attained its present distri- 

 bution (Gaskin, 1992). Following the important off- 

 shore North Pacific record of Jones (1984) (see Intro- 

 duction), Stenson and Reddin (1990) reported harbor 

 porpoises over deep water in the Labrador Sea. Given 

 the timing of seasonal occurrence in this region (Gaskin, 

 1984), these animals may have been travelling between 

 the coastal shelves of Canada and West Greenland. 



The surface temperature regime in the western North 

 Atlantic indicated by Winn 6 more or less defines the 

 productive, well-mixed boundary interaction zone be- 

 tween the cold, southward-flowing Labrador Current 

 and its lesser branches and the warm northeastward- 

 flowing North Atlantic Drift; but the temperature re- 

 gime also defines the general range of the harbor por- 

 poise. The range of the species in the northwestern 

 Pacific, therefore, may be similarly defined by the in- 

 teractions of the cold Oyashio water masses coming 

 out of the western Bering Sea and Sea of Okhotsk 

 with the northward and northeastward flow of the 

 Kuroshio current and its subsidiaries around Japan 

 (Hikosaka and Watanabe, 1957; Fukuoka, 1962). The 

 Sea of Okhotsk is significantly colder in winter than 

 the surrounding ocean regions, and the northern and 

 western zones of it are characteristically ice-covered 

 from late December to April, as is the Tartary Strait 

 between Sakhalin Island and Siberia from early De- 

 cember to May (eg., Japan Meteorological Agency, 

 1986). While a small number of harbour porpoises ei- 

 ther stay in or visit the western Bay of Fundy in win- 

 ter (November-April) when water temperatures range 

 from 1° to 4°C (Gaskin, 1984), any P. phocoena that 

 stay around northern Japan during the winter months 

 can be no further north in coastal waters than the 

 extreme southeastern Sea of Okhotsk adjacent to the 

 central and northern Kuriles and southwest of Sak- 

 halin. The situation in most of the Sea of Okhotsk 



