Fishery Bulletin 101(1) 



1977), which would preclude mixing in the Aleutians and 

 the Gulf of Alaska. Zoogeographic analysis with patterns 

 of prevalence showed that Bering Sea parasites are rarely 

 found outside the Bering Sea (Blaylock et al., 1998b). 



The patterns identified in our analysis agree only in part 

 with zoogeographic analyses (Blaylock et al., 1998b). The 

 southern boundaries in both studies are in the vicinity of 

 the Queen Charlotte Islands, providing additional support 

 for the existence of a southern group of halibut. However, 

 this analysis, unlike the zoogeographic analyses, indicated 

 no sign of a division in the vicinity of Kodiak Island, sug- 

 gesting that the division near Kodiak Island depends on 

 short-lived species not included in this analysis. The evi- 

 dence for the existence of a northern Bering Sea group is 

 equivocal; it was supported by the clustering of localities 

 by using prevalences and, to some degree, the clustering of 

 individuals, but was not supported by any other analyses 

 (Blaylock etal., 1998b). 



With respect to juveniles, Skud's (1977) analysis clearly 

 indicates compensatory movement from the Gulf of Alaska 

 and southern Bering Sea to southern areas, and, as such, 

 predicts that juveniles should have more similar parasite 

 faunas among areas. Our data show this similarity, but 

 there are significant caveats. First, our samples of juve- 

 niles came from areas that form a single group in the clas- 

 sification of adults. The sample from the northern Queen 

 Charlottes is near the southern boundary of that group, 

 and the sample from Nunivak Island is near the northern 

 boundary. Samples of juveniles from other areas, particu- 

 larly the southern area, should be examined to help clarify 

 this issue. Second, and maybe more important, in these 

 smaller fish, prevalences and intensities are low and per- 

 haps hinder separation. However, because halibut at this 

 stage are susceptible to bycatch in other fisheries (IPHC, 

 1996), management should probably consider juveniles a 

 mixed stock to prevent impacts on future halibut popula- 

 tions in distant localities. 



Overall, our analysis provides a less clear picture than 

 that of Arthur and Albert (1993) for Greenland halibut in 

 the northwest Atlantic. Part of the lack of clarity may be 

 due to our use of the training and test set method rather 

 than the bootstrapping method used by Arthur and Albert, 

 which would increase the likelihood of correctly classify- 

 ing similar fish. Also, Arthur and Albert were dealing with 

 a very different system. Geological and oceanographic 

 conditions around the Gulf of St. Lawrence are quite com- 

 plex and create great potential for the isolation of stocks. 

 The northeast Pacific is more open and has fewer isolating 

 mechanisms than the northwest Atlantic. Further, the 

 system is clinal (Blaylock et al., 1998b) and Pacific hali- 

 but are quite capable of migrating along the entire Pacific 

 coast; therefore, less clear cut divisions are expected. Nev- 

 ertheless, wc successfully identified groups of fish, some 

 with a high degree of accuracy. 



Skud (1977) suggested that juveniles will, as adults, 

 homo to the areas in which they were spawned, making the 

 existence of reproductive stocks at least possible. Modern 

 molecular methods could address the issue. For example, 

 molecular methods could potentially address the existence 

 of separate stocks in the south and in the northern Bering 



Sea. The limited molecular studies done to date, however, 

 have not elucidated any indentifiable stock structure be- 

 cause of limited sampling localities, the limited number of 

 loci examined, and the use of juveniles only. Tsuyuki et al. 

 (1969) examined a single serum hemoglobin transferrin lo- 

 cus in halibut from ten sites from Vancouver Island to the 

 Bering Sea and found that only one southeast Alaska local- 

 ity was different. Grant et al. (1984) found no differences 

 between Gulf of Alaska and Bering Sea halibut at five loci 

 but were able to distinguish northeast Pacific halibut from 

 Japanese halibut. However, it is important to note that 

 biochemical and genetic information measures differentia- 

 tion at a different time scale than that reflected in parasite 

 data (Lester et al.. 1988). According to Grant (1984), move- 

 ment of only a few Atlantic herring (Clupea harengus) may 

 be sufficient to obscure true differences between different 

 breeding stocks. Thus, even limited gene flow could obscure 

 any differences in the loci examined. 



Parasite or tagging information alone, however, can not 

 determine whether or not the groups we identified are 

 reproductive stocks. Therefore, all potential factors that 

 might refine the halibut stock concept should be consid- 

 ered. The parasite data suggest a conservative approach 

 to management that recognizes a mixed stock of juveniles 

 and three potential stocks of adults — one in the south, an- 

 other in the northern Bering Sea, and a third and largest 

 centered in the Gulf of Alaska. 



Acknowledgments 



We thank the International Pacific Halibut Commission, 

 Seattle, WA, for coordinating sampling and for financial 

 support. Mark Higgins and John Quintero provided 

 invaluable assistance in the laboratory. Tom McDonald 

 and Dave Whitaker provided technical assistance. We also 

 thank Al Shostak and Jeff Lotz for advice and comments. 



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