Contrasting sex ratios in juvenile and adult chinook salmon Oncorhynchus tshawytscha (Walbaum) from south-west Alaska: sex reversal or differential survival?

A sex-specific genetic marker (growth hormone pseudogene) and age-at-maturation data were used to infer the cause of male-biased phenotypic sex ratios in adult chinook salmon Oncorhynchus tshawytscha from the Kwethluk and Tuluksak Rivers, south-west Alaska. The results showed that: (1) adult genetic and phenotypic sex ratios were similar and were male biased, (2) genetic sex ratios of pre-smolts were not male biased and (3) the average age-at-maturation for males was significantly less than for females. These results suggest that the sex ratio distortion is due to gender bias in marine survival probably related to gender differences in marine life-history strategies.