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Adaptive diversification of juvenile life histories in the chinook salmon, Oncorhynchus tshawytscha (Walbaum)

University of British Columbia

Over its native range in the north Pacific, the chinook salmon (Oncorhynchus tshawytscha (Walbaum)) exhibits two life history patterns distinguished by the age at which juveniles migrate from their natal streams to the sea. Those that migrate seaward during their first 3 months of stream residence are "ocean-type" chinook, while those that migrate after 1 or more years in freshwater are "stream-type" salmon. Both types may be found within populations but they also distinguish populations dominated by one type or the other. Stream-type salmon were found to be more aggressive, had a stronger seasonal rheotactic response, grew more slowly, and were less tolerant of saline water than ocean-type chinook. These differences were present in both wild- and laboratory-reared juveniles and were concluded to be, at least in part, genetically determined. Consistent differences in behavioural and physiological traits were found in comparisons among chinook populations from different river systems, between populations within a river system, and among families with different freshwater life histories from a single population. These phenotypic differences were in directions appropriate for different durations of freshwater residence which is consistent with the idea that they represent adaptive diversification and components of functionally important "strategies" for alternative juvenile life histories. A geographic survey indicated that stream-type chinook predominate in interior streams south of 56° N, and in both coastal and interior streams north of this latitude. Although ocean-type salmon predominate in coastal streams south of 56° N, there are also many rivers with both stream- and ocean-type populations. Ocean-type populations are characterized by earlier emergence in areas of warmer air temperature and longer growing season, in streams close to the sea. In laboratory stream environments, juvenile coho salmon socially dominated juvenile chinook salmon from a number of populations. In nature, sympatric coho and chinook used different stream microhabitats, but allopatric chinook used microhabitats more similar to those of coho. Manipulative field experiments demonstrated that coho and chinook preferred different habitat types ("pools" and "riffles" respectively), but that coho may be able to displace chinook from "pool" habitats. While genetic differences in habitat preference between coho and chinook probably minimize the influence of coho on freshwater residence behaviour of chinook, aggressive interaction with coho may promote early downstream migration of chinook when alternative stream habitats preferred by chinook are lacking. It is concluded that: (i) stream- and ocean-type juvenile life histories are, at least in part, genetically based, (ii) stream- and ocean-type chinook represent functional diversification of juvenile phenotype resulting in alternative life history patterns, (iii) selection for size at migration and time of migration are major factors driving divergence in juvenile life history among chinook populations; environmental control via "growth opportunity" or behavioural exclusion from stream habitats by coho salmon is of secondary importance, and (iv) the sympatric occurrence of stream- and ocean-type chinook in many river systems is probably the result of repeated, independent episodes of divergence. Thus, stream- and ocean-type salmon probably do not represent distinct evolutionary lineages.

Text

2010-10-18

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http://hdl.handle.net/2429/29305

Zoology

University of British Columbia

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