Genetic variation in mortality of chinook salmon during a bloom of the marine alga Heterosigma akaskiwo

Mortality in a netpen-reared population of 3-year-old chinook salmon Oncorh ynchus tshawytscha during an extensive 1997 bloom of the alga Heterosigma a kashiwo in Puget Sound, Washington, was low (7.2%), and corresponded to a r eduction in variance effective population size of 9.4%. Under a liability t hreshold model, the heritability of mortality ( +/- S.E.), based on paterna l half-sibs, was estimated at 0.15 +/- 0.04. No significant genetic variati on was detected for date of death. Despite the low overall mortality, the c onsequences for variation in family size underscore the importance of maxim izing genetic variation in cultured fish populations later released to the wild as a precaution against mortality and losses of genetic variation over the life cycle. The pattern of family variation in response to this algal bloom provides evidence for potentially selective mortality of anadromous s almonids in the marine environment during natural perturbations.