Gaskin et al.: Phocoena phocoena in the coastal waters of northern Japan 



447 



Atlantic pattern of births in late spring or early sum- 

 mer and to mating between early and late summer. 

 These populations are distributed in the same latitu- 

 dinal zone and hence the same photoperiod regime. 

 Sea surface temperature regimes are also similar, eg., 



Winn 6 and the Japan Meterological Agency 

 (1986). 



Harbor porpoises have testes that are large 

 in proportion to body size (Fisher and Harrison, 

 1970; Gaskin et al., 1984). Miyazaki et al. 

 (1987), comparing their material to data of 

 Fisher and Harrison (1970), noted that adult 

 harbor porpoises from Japanese waters had 

 smaller testes than those from the North At- 

 lantic and had weight ranges that did not over- 

 lap. They concluded that the difference in sam- 

 pling periods in the two areas was not a factor 

 (April-May off Japan v. May-September in the 

 eastern North Atlantic, respectively). However, 

 Gaskin et al. (1984) demonstrated a seasonal 

 cycle of spermatogenesis, including a dramatic 

 change in testis volume/size in adult harbor 

 porpoises during June-August in the Bay of 

 Fundy region. Read (1990a) concluded that the 

 active mating period extended only from late 

 June into July or occasionally early August. The 

 specimens examined by Miyazaki et al. (1987) 

 in April-May appear to have been collected be- 

 fore adult males had begun to exhibit seasonal 

 spermatogenesis and testis enlargement. 



P. phocoena appear to be a relatively asocial 

 odontocete which has also undergone selection 

 for sperm competition and for consequent de- 

 velopment of proportionately large testes like 

 some other cetaceans (Brownell and Ralls, 1986). In 



fi Winn, H. E. 1982. A characterization of marine mammals and turtles 

 in the Mid- and North Atlantic areas of the U.S. outer continental shelf. 

 U.S. Dep. Interior, Bureau of Land Management, Washington D.C., 

 Final Rep. of the Cetacean and Turtle Assessment Program, 437 p. 



