Quinn et al Origin and genetic structure of Oncorhynchus tshawytscha 



517 



diversity (Table 2). Greater mtDNAhaplotype diver- 

 sity also occurred within the Sacramento subgroup 

 where haplotype 6, detected in winter-run fish, was 

 absent from the NZ fish. These observations com- 

 port with the introduction of Sacramento River 

 chinook from a single source to NZ a century ago. 



In the NZ collections, the common clustering ( Figs. 

 3 and 4) and generally lower heterozygosities and 

 percentages of polymorphic loci (Table 2) support the 

 occurrence of a limited effective population size (i.e. 

 a "bottleneck") during the founding and perhaps dur- 

 ing the early generations of these populations. En- 

 hanced genetic drift resulting from a bottleneck fol- 

 lowed by a larger and more stable population size 

 would explain the common clustering of these col- 

 lections and the lower levels of genetic variation, 



Table 6 



Four variable base-pair sites and nucleotide changes found 

 in five chinook salmon mtDNA types from four wild popu- 

 lations in New Zealand and from fish collected in the Sac- 

 ramento River, California, 1991-93. 



' The symbol "81i" represents an 81-bp exact repeat. An asterisk 

 (*) represents a nucleotide deletion. The complete sequence am- 

 plified in chinook salmon by the primers S-phe and P2 is given 

 in Nielsen et al. 1 1994al. 



compared with those observed in the Sacramento 

 River collections. There are no records of population 

 sizes in the first few generations, but natural mor- 

 tality and the general tendency of salmon to home 

 would have reduced the number of adults colonizing 

 the rivers. Current populations (catch plus escape- 

 ment) are on the order of 10,000 adults in the four major 

 rivers, including the three sampled in this study. 



Several factors might have contributed to the ini- 

 tial bottleneck. The founding population probably 

 represented more females than males, as hatchery 

 staff commonly spawn females with a few "choice" 

 males. The founding individuals experienced a very 

 different environment and presumably a different 

 selection regime from the Sacramento River. The NZ 

 chinook salmon rivers lack the estuary used by fall 

 chinook salmon in the Sacramento River system 

 (Kjelson et al., 1982), and smolts unable to make the 

 abrupt transition to seawater may have died. The 

 surviving smolts of the first generation experienced 

 ocean currents, temperature, and other factors in- 

 fluencing migratory patterns and orientation that 

 differed from those experienced off the coast of Cali- 

 fornia. There may have been strong selection for 

 coastal distribution or other behavior patterns fa- 

 cilitating homeward orientation. Upon return, the 

 mature adults found short, steep, gravel-rich, un- 

 stable, recently deglaciated rivers, whose channels 

 lacked woody debris. In addition to these habitat dif- 

 ferences that may have exerted selection on the 

 founding generations, the nature of the salmonid 

 mating system (semelparous life cycle and intra- 

 sexual competition for nesting sites or mates) also 

 generates considerable variation in reproductive suc- 

 cess, especially among males (e.g. Fleming and Gross, 



