SEX-BIASED MARINE SURVIVAL AND GROWTH IN A POPULATION OF COHO SALMON

Examination of historical records for coho salmon in Big Beef Creek, i n western Washington, U.S.A., indicated that more adult males than fem ales returned to spawn, and that the mean length of the Females exceed ed 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 m ortality might occur, the sex-linked GH-Psi pseudogene was used to det ermine the sex of smells emigrating From Big Beef Creek in 1995-1997. In each of the 3 years the sex ratio was indistinguishable from 50 : 5 0, 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 greate r mortality, but benefits survivors with greater size. The male strate gy appears to allow greater survival at the cost of reduced size. (C) 1998 The Fisheries Society of the British Isles.