SPATIAL AND TEMPORAL FACTORS AFFECTING SURVIVAL IN COHO AND FALL CHINOOK SALMON IN THE PACIFIC-NORTHWEST

Abundance and survival of Pacific salmon have been declining. Of sever al hypotheses advanced to explain the declines, the major ones attribu te declines to genetic deterioration, disease accumulation, variations in oceanic conditions, and density-dependent mortality. To test some of these hypotheses, we estimated the survival rate of hatchery releas es of coho and fall chinook. The data consisted of mark and recapture rates of coded-wire-tagged fish from the northeastern Pacific Ocean. T he mean survival by hatchery and location was analyzed by means of gra phs, generalized linear models, and cluster analysis. The results indi cate that salmonid populations continue to decline in most of the geog raphical range. The declines are particularly marked from the late 197 0s to the early 1980s and from the late 1980s to the most recent years available. Age of the hatchery is of little or no importance, so dise ase accumulation and genetic changes are not the causes of the observe d survival declines. Some hatcheries continue to have good survival ra tes for long periods. The overall results indicate that changes in oce an conditions could be at least partially responsible for the survival declines of coho and chinook in the Pacific Northwest.