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Fishery Bulletin 1 10(2) 
in stages. For the odd-year broodline, applications in 
southern British Columbia would appear to be possible 
if fishery management objectives are to separate pink 
salmon of Washington, Fraser River, and southern Brit- 
ish Columbia origin. Finer subdivision of stock composi- 
tion estimation, particularly in the Washington region, 
may be possible, as separation of Hood Canal and Strait 
of Juan de Fuca populations from those in Puget Sound 
may be practical. 
Studies of population structure in pink salmon 
have revealed some consistent patterns. The great- 
est differentiation observed in population structure 
has been consistently reported to occur between the 
two broodlines, whether in Asia or North America 
(Beacham et al., 1988; Kartavtsev, 1991; Varnavs- 
kaya and Beacham, 1992; Zhivotovsky et ah, 1994; 
Salinenkova et ah, 2006; Golovanov et ah, 2009). In 
Asia, studies have indicated that genetic differen- 
tiation among populations is greater in the even-year 
broodline than in the odd-year broodline (Hawkins et 
ah, 2002; Golovanov et ah, 2009). In North America, 
the reverse situation occurs, with population differ- 
entiation among populations greater in the odd-year 
broodline than in the even-year broodline (Beacham et 
ah, 1988; Gharrett et ah, 1988; current study). These 
findings support the concept of two main refugia oc- 
cupied by pink salmon during the most recent Pleisto- 
cene Era glaciation some 10,000 years ago (Aspinwall, 
1974). The even-year broodline may have survived the 
glaciation in a northern refugium (Aspinwall, 1974). 
Once the glaciation ended, the even-year broodline 
dispersed from the northern refugium, colonizing 
southern regions more recently than northern ones. 
Conversely, the odd-year broodline may have occupied 
a southern refugium during the Pleistocene Era gla- 
ciation (McPhail and Lindsey, 1970), and dispersed 
northward, with northern populations derived more 
recently than southern ones. As populations closer to 
the refugium have had greater time to accumulate 
genetic mutations and thus display greater population 
differentiation, the current pattern of broodline and 
population differentiation is consistent with dispersal 
from a northern refugium for the even-year broodline 
(greater population genetic differentiation in even-year 
broodline) and dispersal from a southern origin for the 
odd-year broodline (greater population differentiation 
in odd-year broodline). Additionally, embryonic sur- 
vival of the even-year broodline has been reported to 
be higher than that of the odd-year broodline in a cold 
(4°C) incubation environment, with higher alevin and 
fry growth of the even-year broodline also observed 
in the cold incubation environment (Beacham and 
Murray, 1988). Greater suitability of the even-year 
broodline to a colder environment is also illustrated 
by the spawning distributions of the broodlines in 
North America, with the even-year broodline in very 
low abundance from the southern portion of the range 
(Fraser River, Washington) and the odd-year broodline 
in low abundance in western Alaska. Alternatively, 
Krkosek al. (2011) suggested that the distribution of 
even-year and odd-year populations result from densi- 
ty-dependent mortality caused by interactions between 
the broodlines. However, it seems difficult to account 
for genetic population structure observed in pink salm- 
on as a result of broodline interactions. 
Conclusion 
The level of differentiation observed among the pink 
salmon populations within broodlines surveyed in the 
current study was considerably less than in other species 
of Pacific salmon. Sockeye salmon (O. nerka) typically 
display high levels of genetic differentiation ( F ST =0.097, 
14 loci, average 30 alleles per locus, 299 populations) 
(Beacham et al., 2006), with the other species displaying 
levels of genetic differentiation ranging between sockeye 
salmon and pink salmon. The low level of differentia- 
tion observed in pink salmon may be a result of a more 
recent colonization history (Hawkins et al., 2002), but 
may also be a result of straying among local populations 
within regions. As pink salmon juveniles spend little 
time in fresh water after fry emergence, imprinting on 
natal streams may not be as strong as in other species, 
and as a result may stray more upon returning spawn- 
ing migrations (Quinn, 1993). Chum salmon ( O . keta ) 
juveniles spend similar amounts of time in fresh water 
as pink salmon, and population differentiation in the 
species is higher only than pink salmon ( F ST =0.033, 
14 loci, average 57 alleles per locus, 380 populations) 
(Beacham et al., 2009). The low level of genetic differ- 
entiation observed in pink salmon population structure 
likely reflects higher levels of straying among popula- 
tions during spawning than those observed for other 
Pacific salmon species. 
Acknowledgments 
A very substantial effort was undertaken to obtain the 
pink salmon samples used in this study. We thank vari- 
ous staff of Fisheries and Oceans Canada (DFO), the 
Pacific Salmon Commission (PSC), and the Washington 
Department of Fish and Wildlife (WDFW) for sample 
collection, as well as First Nations staff. We acknowl- 
edge those within the Kitasoo Fisheries Program who 
sampled pink salmon, as well as the Gitxsan Watershed 
Authority, the Kitselas and Kitsumkalum field staff, 
the Skeena Fisheries Commission, the Nisga’a First 
Nation, the Haida Fisheries Program, and the crew 
of the Canadian Coast Guard Vessel Arrow Post. L. 
Fitzpatrick drafted the map. C. Wallace assisted in the 
analysis. Funding for the study was provided by DFO 
and the PSC. 
Literature cited 
Aro, K. V., and M. P. Shepard. 
1967. Salmon of the North Pacific Ocean, part V. Spawn- 
