Precocial male maturation in laboratory-reared populations of chinook salmon, Oncorhynchus tshawytscha

The incidence of precocial male maturation in yearling chinook salmon, Oncorhynchus tshawytscha, was examined in four laboratory-reared populations. Slim Creek and Bowron River chinook salmon were about 4 weeks older than Harrison and Nanaimo river chinook salmon when sampled (14 vs. 13 months of age), but were also 20–40 g smaller. Approximately 29, 12, 0, and 0% of all males were precocious in Bowron River, Slim Creek, Harrison River, and Nanaimo River chinook salmon, respectively. Precocial male chinook salmon had gonadosomatic indices of about 5–6%, whereas immature salmon from all populations had indices under 1%. Precocial male chinook salmon were more robust bodied than immature salmon; precocial males had deeper bodies, deeper heads, and larger adipose fins. Variation among the study populations in the incidence of precocial male maturation may be related to differences among the populations in migration distance to the sea or in juvenile freshwater rearing life history. The chinook salmon would probably be a productive species with which to study the evolutionary ecology of precocial maturity in Pacific salmonids.