Shimada and Kimura: Seasonal movements of Gadus macrocephalus 



815 



netically distinct stocks were detected: a western 

 North Pacific Ocean (Asian) group, and an eastern 

 North Pacific group which included the Bering Sea, 

 Aleutians Islands, and Gulf of Alaska regions. There 

 were virtually no regional genetic differences among 

 any of the North American samples. These authors 

 were unable to identify where the effective northern 

 boundary between the western and eastern groups 

 lies, though the western Bering Sea was considered 

 most likely. Grant et al. (1987) attributed this lack 

 of genetic differentiation to gene flow between vari- 

 ous subareas and regions on either side of the Pa- 

 cific. Grant et al. (1987) were puzzled that most of 

 the literature on Pacific cod pointed toward locally 

 isolated stocks (Moiseev, 1953; Svetovidov, 1948; 

 Ketchen, 1961; Wilimovsky et al., 1967). Our tagging 

 study confirms sufficient migration to explain Grant 

 et al.'s findings of genetic homogeneity in Pacific cod 

 over broad areas of the North Pacific. 



We have confirmed from tagging that Pacific cod 

 migration occurs between the Bering Sea and Aleu- 

 tian Islands. Because of the experimental design, the 

 majority of tag returns demonstrate emigration from 

 the Bering Sea but have not shown conclusively that 

 immigration to the Bering Sea takes place. Even so, 

 this study shows that significant exchange may oc- 

 cur within the open ocean populations of Pacific cod 

 off Alaska. Whether Bering Sea Pacific cod have a 

 reciprocal exchange to the wider Gulf of Alaska be- 

 yond the nearby waters of major Aleutian passes re- 

 mains an open question. Lack of data precludes any 

 further statement or quantification of exchanges be- 

 tween these regions. Further elucidation must await 

 additional tagging results, particularly from the east- 

 ern Gulf of Alaska and western Aleutian Islands. 



Acknowledgments 



We thank R. G. Bakkala, J. T. Fujioka, D. A. Som- 

 erton, G. G. Thompson, V. G. Wespestad, and H. H. 

 Zenger for reviewing our manuscript. Numerous in- 

 dividuals in the domestic and foreign commercial 

 fisheries, the Alaska Department of Fish and Game, 

 and the National Marine Fisheries Service Fishery 

 Observer Program contributed by returning 

 tag-recapture information to the Alaska Fisheries 

 Science Center. We thank Jack Turnock for develop- 

 ing the coastal mapping program in the S Language 

 which greatly facilitated our analysis. 



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