FISHERY BULLETIN: VOL. 81, NO. 4 



0.07% was due to differences between the Gulf of St 

 Lawrence and the shelf populations, and to popula- 

 tion differences on the continental shelf. No signifi- 

 cant allele-frequency differences were detected 

 among these samples. When the Bering Sea sample is 

 included in the analysis, 6.9% of the gene diversity 

 was due to subspecific differences between the two 

 areas. 



Also in eastern Canada, the gene products of 15 loci 

 (2 polymorphic loci) were examined in samples of 

 witch flounder, Glyptocephalus cynoglossus, (Fair- 

 bairn 1981b). The gene- diversity analysis of these 

 data also shows low levels of genetic differentiation 

 among witch flounder populations; 99.2% of the total 

 gene diversity was contained on average within pop- 

 ulations and 0.8% was due to all population differ- 

 ences combined. In spite of the low level of genetic 

 differentiation among populations, significant allele- 

 frequency differences were detected between some 

 of the areas. Tag- and- recovery and distribution data 

 show that adults tend to be sedentary and that pop- 

 ulations tend to be separated by weak barriers to 

 migration such as the cold shallow waters of the 

 Grand Bank (Bowering 1976; Bowering and Misra 

 1982). However, the potential gene flow at the egg 

 and larval stages is great (Evseenko and Nevinsky 9 ) 

 and no doubt counters genetic divergence of the par- 

 tially isolated adult populations. 



For plaice, Pleuronectes platessa, in the eastern 

 North Atlantic Ocean, there are data for the five loci 

 in common in the studies of Ward and Beardmore 

 (1977) and Purdom et al. (1976). Allele-frequency 

 differences were not detected either within the Irish 

 Sea, within the North Sea, or between these seas for 

 these loci. The results of the diversity analysis re- 

 flected this degree of homogeneity; 99.6% of the total 

 gene diversity was contained within populations and 

 only 0.47c was due to differences between popu- 

 lations. 



Grant et al. (in press) examined the gene products of 

 17 polymorphic loci in three widely separated samples 

 of Pacific halibut, Hippoglossus stenolepis, in the 

 North Pacific Ocean and Bering Sea. The gene- 

 diversity analysis showed that 98.7% of the gene 

 diversity was contained within populations, that 

 0.4% was due to differences between the Bering Sea 

 and the Gulf of Alaska, and that 0.9% was due to 

 trans-Pacific Ocean differences. Tsuyuki et al. 

 (1969) examined genetic variation at one locus in 

 samples from 10 locations in the Bering Sea and in 



'Evseenko, S. A., and M. M. Nevinsky. 1973. Breeding and 

 development of witch flounder (Glyptocephalus cynoglossus) in the 

 Northwest Atlantic Ocean. Int. Comm. Northwest Atl. Fish. Res. 

 Doc. 73/49, Ser. No. 2990, 23 p. 



the eastern North Pacific Ocean. The gene- diversity 

 analysis of these results estimated the within- 

 population diversity to be 99.6%, that due to dif- 

 ferences between regions to be 0.09%, and that due 

 to differences among populations within regions to 

 be 0.04%. This high degree of genetic homogeneity 

 which was detected in both studies reflects the long 

 distance migrations that Pacific halibut are known to 

 make. For instance, tagging studies have demon- 

 strated migrations of at least 3,200 km (Skud 

 1977). 



The results of these studies show that there is very 

 little genetic differentiation among populations of 

 flatfishes located over areas of about 1,000 km. For 

 Pacific halibut, areas of genetic homogeneity appear 

 to be even larger because of its ability to migrate long 

 distances. These areas of genetic homogeneity prob- 

 ably cannot be considered randomly mating pop- 

 ulations in the strict sense, because it is unlikely, for 

 example, that fish located on one edge of a genetic 

 unit have an equal chance of mating with fish on the 

 other side of the genetic unit. Rather, these units 

 reflect long-term processes that influence population 

 size and migration over several generations. The divi- 

 sion of yellowfin sole into two genetic groups by coast- 

 al glaciation in the Pleistocene is an excellent 

 example of the importance of interpreting present- 

 day allele-frequency distributions in terms of past 

 population events. 



ACKNOWLEDGMENTS 



We thank G. Smith of the Northwest and Alaska 

 Fisheries Center, D. Fairbairn, and an anonymous 

 reviewer for constructively reviewing the manu- 

 script. 



LITERATURE CITED 



Allendorf, F. W., and F. M. Utter. 



1979. Population genetics. In W. S. Hoar, D. S. Randall, and 

 J. R. Brett (editors), Fish physiology, Vol. 8, p. 407-454. 

 Acad. Press, N.Y. 

 Bowering, W. R. 



1976. Distribution, age and growth, and sexual maturity of 

 witch flounder (Glyptocephalus cynoglossus) in Newfound- 

 land waters. J. Fish. Res. Board Can. 33:1574-1584. 

 Bowering, W. R., and R. K. Misra. 



1982. Comparisons of witch flounder (Glyptocephalus cyno- 

 glossus) stocks of the Newfoundland-Labrador area, 

 based upon a new multivariate analysis method for meris- 

 tic characters. Can. J. Fish. Aquat. Sci. 39:564-570. 

 Chikuni, S. 



1971. Report on the biological research of groundfish in the 

 Bering Sea and the northeastern Pacific by Inase Maru 

 No. 3 in 1970. Jpn. Fish. Agency, Far Seas Fish. Res. 

 Lab., Shimizu, Jpn., Data Rep., 175 p. 



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