• Key words: ;
  • behavioural ecology;
  • foraging strategies;
  • growth hormone;
  • Salmonidae;
  • sex determination;
  • sexual dimorphism

Examination of historical records for coho salmon in Big Beef Creek, in western Washington, U.S.A., indicated that more adult males than females returned to spawn, and that the mean length of the females exceeded that of males. Sex-biased survival and faster growth among females are unusual among salmonids but precedented in some other coho salmon populations. To help determine the stage of life at which sex-biased mortality might occur, the sex-linked GH-ψH pseudogene was used to determine the sex of smolts emigrating from Big Beef Creek in 1995–1997. In each of the 3 years the sex ratio was indistinguishable from 50:50, indicating similar survival rates in fresh water, and implying that the male and female coho salmon follow different foraging strategies when they are at sea. The female strategy apparently results in greater mortality, but benefits survivors with greater size. The male strategy appears to allow greater survival at the cost of reduced size.