Effects of freshwater and marine growth rates on early maturity in male coho and Chinook salmon

In many populations of salmonid fishes, a fraction of the males mature at a younger age than the females; these males are termed jacks if they have migrated to sea and precocious parr if they matured in freshwater. We examined detailed data on the University of Washington hatchery populations of coho salmon Oncorhynchus kisutch and Chinook salmon O. tshawytscha for more than 30 years to test whether rapid growth in freshwater or at sea increases the probability of early maturity in males. The average size of Chinook salmon smolts (2.6-20.9 g) increased over the years in question. The proportion of males maturing as jacks increased with smolt size but not with the potential for growth at sea. The size of the smolts (but not growth potential at sea) was positively correlated with the proportion of the jacks maturing after only one summer at sea (so-called minijacks) rather than two summer,.;. In coho salmon, average smolt size (6.1-22.4 g) did not vary consistently with time. There was a slight tendency for cohorts with larger smolts to produce more jacks, but marine growth potential was negatively related to the proportion of jacks. For neither species did marine environmental variables influence the proportion of jacks. Taken together. these results support the hypothesis that size prior to seawater entry has the predominate effect on the probability of early maturation and that the growth potential at sea has a neutral or even inhibitory effect on early maturity.