High incidence of a male-specific genetic marker in phenotypic female Chinook salmon from the Columbia River

Numerous populations of anadromous salmonids in the northwestern United Sta tes have been declining for many years, resulting in Endangered Species Act listings and in some cases extinction. The degradation of river ecosystems has been proposed as one of the major reasons for the inability of salmon to maintain their populations. However, the specific factors interfering wi th the reproduction and survival of salmon during the freshwater phase of t heir life cycle have not been fully described. This study was initiated to determine the incidence of phenotypic sex reversal in wild, fall chinook sa lmon (Oncorhynchus tshawytcha) that returned to spawn in the Columbia River . Fish were sampled at different locations within this watershed to determi ne whether they were faithfully expressing their genotype. We report a high incidence (84%) of a genetic marker for the Y chromosome in phenotypic fem ales sampled from the wild, which was not observed in female fish raised in hatcheries. It appears likely that female salmon with a male genotype have been sex reversed, creating the potential for an abnormal YY genotype in t he wild that would produce all-male offspring and alter sex ratios signific antly.