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Age and size at maturation affect an individual's reproductive success and reflect both evolutionary adaptations and phenotypic responses to environmental conditions. We evaluated age and size at maturity of sockeye salmon Oncorhynchus nerka from 2 populations within the Columbia River drainage: the Okanagan River, British Columbia (Canada), population and the endangered population from Redfish Lake, Idaho, USA. We compared available data on wild fish to data on fish reared under controlled laboratory conditions. Data obtained from the wild revealed that Okanagan River fish smolted at younger ages and larger sizes and then matured at younger ages than Redfish Lake fish. In captivity, fish from both populations were grown at the same rate and all smolted at Age 1. Despite their similar lengths as smolts, a greater proportion of Redfish Lake (>90%) than Okanagan River sockeye salmon (68%) matured 1 yr after smolt transformation. Calculated proba- bilistic maturation reaction norm midpoints for the captive fish indicated that Redfish Lake sockeye salmon in this study may have lower maturation thresholds than Okanagan River fish. Expression of different life history traits in the 2 populations under controlled conditions is consistent with a genetic basis for these traits and countergradient variation in maturation thresholds. Understanding life history differences and local adaptation among populations is important for their conservation, including anticipating responses to captive breeding regimes and supplementation, changing environmental conditions, and selective harvest.

Comparative maturation schedules of two Columbia River sockeye salmon Oncorhynchus nerka populations