824 
Fishery Bulletin 95(4), 1 997 
ers (the Clutha and Waitaki) following hydroelectric 
development (McDowall, 1990) may be partly linked 
to a reduction in the magnitude and frequency of 
spring floods. A positive association between salmon 
production and large offshore plumes is also consis- 
tent with the general distribution of salmon in east 
coast rivers (McDowall, 1990), with the largest popu- 
lations confined to major rivers draining the main 
divide. Traditionally this distribution has been at- 
tributed to the presence of stable headwater spawn- 
ing tributaries such as Glenariffe Stream, but this 
explanation is not fully convincing. Many minor east 
coast rivers support self-sustaining stocks of brown 
trout (Jowett, 1990; McDowall, 1990), and spawning 
requirements for chinook are not dissimilar. 
From an evolutionary standpoint, the present re- 
sults help to shed further light on the processes by 
which chinook salmon have been able to succeed in 
New Zealand waters. In addition to the emergence 
of stream-type fish as a significant component of 
modern New Zealand stocks, several other recent 
studies have noted differences between present-day 
New Zealand and Sacramento chinook at both phe- 
notypic and genotypic levels (Quinn and Unwin, 
1993; Quinn et al., 1996), suggesting that the pro- 
cess of adaptation may be ongoing. The somewhat 
unusual mechanisms which have apparently enabled 
New Zealand chinook salmon to establish the only 
self-perpetuating stocks outside their native range 
(Harache, 1992) underscore the great phenotypic 
plasticity of the species, and the value of the New 
Zealand populations as a laboratory for studying this 
plasticity. 
Acknowledgments 
I thank all staff associated with the Glenariffe 
programme for technical assistance; Greg Kelly and 
Mark Weatherhead for help with drafting the fig- 
ures; Bob McDowall, Charles Pearson, Ian Jowett, 
Tom Quinn, and three anonymous referees for com- 
ments on earlier drafts of the manuscript. Kathy 
Walter and Graeme Davenport extracted Rakaia 
River flow data from the National Water Resources 
Archive, and the study was funded by the Founda- 
tion for Research, Science and Technology through 
Contracts C01417 and CO1501. 
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