Evidence for temperature-dependent sex determination in sockeye salmon (Oncorhynchus nerka)

Temperature-dependent sex determination has been demonstrated in several animals, but in salmonids sex is generally believed to be under strict genetic control. We observed distorted sex ratios, attributable to a temperature manipulation during embryonic development, in experiments conducted during 1991 with the anadromous and nonanadromous (kokanee) forms of sockeye salmon (Oncorhynchus nerka) from Takla Lake, British Columbia. Sex ratios ranged from 62 to 84% female, and the biased ratio could not be accounted for by differential mortality by sex. The effect was observed independently in sockeye and kokanee crosses, as well as in the reciprocal hybrids. Similar crosses from Takla Lake in a previous year (1989) in which incubation temperature was not manipulated resulted in normal (1:1) sex ratios. Other populations of sockeye and kokanee from the same year (1991) but undergoing no temperature manipulation maintained normal sex ratios, as did populations from several disparate locations and years (1986-1994). The parsimonious conclusion is that the temperature manipulation during development was responsible for the biased sex ratio through a direct influence on sex differentiation. Hence, the possibility that temperature-dependent sex determination occurs in O. nerka, and perhaps other salmonids, deserves rigorous testing.