Quinn et al Origin and genetic structure of Oncorhynchus tshawytscha 



519 



Interpopulation differences in NZ chinook 



The reduced divergence among the three NZ popu- 

 lations indicated by the allozyme data, relative to 

 the Sacramento River populations, may reflect the 

 recent origins of the NZ groups. The similar magni- 

 tude of divergence evident among the NZ populations 

 and between years within two Sacramento River fall- 

 run hatchery populations (Fig. 3B) suggested no dif- 

 ferentiation beyond year-class variation of a single 

 population. The frequency of mtDNA types among 

 the NZ stocks also did not indicate isolation between 

 most populations. However, Clutha River fish (which 

 expressed only two of the five haplotypes found in 

 NZ samples) tended to be the most different, espe- 

 cially from Rakaia River fish. The Clutha River popu- 

 lation, for which we had no allozyme data, was 

 planted from the Hakataramea River in 1917 

 ( McDowall, 1994 ) but has been landlocked since 1956 

 by Roxburgh Dam and may have undergone a popu- 

 lation bottleneck after completion of the dam. The 

 Clutha River population is thus less susceptible to 

 genetic exchange than the three anadromous popu- 

 lations sampled, likely to be less polymorphic, and 

 hence is not typical of NZ chinook. 



The gene-frequency differences among the anadro- 

 mous populations are consistent with panmixia 

 caused by straying or successful transfers of salmon 

 among the rivers. Unwin and Quinn (1993) docu- 

 mented straying rates of about 12% by hatchery-re- 

 leased chinook salmon from the Rakaia River to other 

 NZ rivers. In addition, during the 1980s several mil- 

 lion juvenile salmon were transferred from the 

 Rakaia, Rangitata, and Waimakariri Rivers to vir- 

 tually all major east coast rivers except the Rakaia 

 River. Nevertheless, the populations now differ in 

 many life history traits, including fecundity (Quinn 

 and Bloomberg, 1992), freshwater and marine age, 

 length at age, weight at length, date of return to 

 freshwater, and spawning date (Quinn and Unwin, 

 1993), and egg size and body shape. 9 These pheno- 

 typic differences are consistent with some level of 

 genetic isolation among the populations but do not 

 demonstrate it (Waples, 1995). 



Our results provide a perspective on the manage- 

 ment and conservation of salmon populations. 

 Founder effects or strong selection against certain 

 genotypes can reduce the genetic diversity of popu- 

 lations established by colonization, relative to the 



9 Unwin, M. J., M. Kinnison, N. Boustead, and T. P. Quinn. In 

 prep. Variation in body size and morphology of adult chinook 

 salmon (Oncorhynchus tshawytscha) from two New Zealand 

 populations and their ancestral Sacramento River, California, 

 population, ninety years later. Natl. Inst, of Water and Atmo- 

 sphere, P.O. Box 8602, Christchurch, New Zealand. 



source population. However, the NZ experience indi- 

 cates that this may not prevent the transplant from 

 being successful. The life history differences among 

 NZ populations indicate that salmon populations may 

 evolve quite quickly, although the genetic basis of 

 the life history differences seen in NZ has yet to be 

 demonstrated. The implications of such rapid evolu- 

 tion are open to interpretation. One might argue that 

 it demonstrates the importance of the stock concept 

 in salmon management because reduced gene flow 

 and population adaptation are such fundamental 

 aspects of salmon biology. One might be encouraged 

 to attempt to restore salmon populations to areas 

 from which they have been extirpated, given a suit- 

 able source population. However, skeptics of the ap- 

 plication of the species concept (including the U.S. 

 Endangered Species Act) to salmon populations 

 might argue that the rapid diversification of salmon 

 populations indicates that they are more plastic than 

 has been assumed and that only a diverse gene pool 

 needs be preserved, not every spatially and geneti- 

 cally discrete population. The concept of "evolution- 

 arily significant unit" (Waples, 1995) provides a com- 

 promise by focusing on critical genetic and ecologi- 

 cal variables involved in grouping and managing 

 populations. In any case, it is important to remem- 

 ber that the success of the NZ chinook transplant was 

 the exception and that the vast majority of other trans- 

 plants have failed. Presumably, local adaptation was 

 not sufficiently rapid or flexible to prevent extinction. 



Acknowledgments 



We thank Nelson Boustead, Ceri Hopkins, Lindsay 

 Hawke, Michael Weekes, Cliff Halford, and Dick 

 Marquand for assistance in collecting the NZ 

 samples, Greg Kelly for drafting the maps, and Paul 

 Bentzen and Robin Waples for comments on the 

 manuscript. Funding for this study was provided by 

 the New Zealand Foundation for Research, Science and 

 Technology, the H. Mason Keeler Endowment, the 

 Puget Power and Light Company, and the U.S. Forest 

 Service and California Department of Fish and Game. 



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Aebersold, P. B„ G. A. Winans, D. J. Teel, G. B. Milner, 

 and F. M. Utter. 



1987. Manual for starch gel electrophoresis: a method for 

 the detection of genetic variation. U.S. Dep. Commer., 

 NOAATech. Rep. NMFS 61, 19 p. 

 Bartley, D. M„ and G. A. E. Gall. 



1990. Genetic structure and gene flow in chinook salmon 

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