Maturation responses of salmonids to changing developmental opportunities

Maturation is the allocation of energy to growth and differentiation of germinal tissue to the ultimate production of gametes. In Atlantic salmon Salmo salar, maturation begins in the egg soon after fertilisation and continues intermittently until the individual is capable of spawning. Completion of the process depends on exceeding genetically determined biochemical thresholds (lipid status) in critical seasons (through responsiveness to photoperiod cues). Hence, maturation is regulated by inhibition, and age and size at maturity depend on physiological efficiency (genetic endowment) and developmental opportunity (environmental context). This interaction of genetic diversity and developmental flexibility leads to multiple maturation trajectories (up to 32 in steelhead trout Oncorhychus mykiss) and wide variation in age and size at spawning. Severe depletion of a Kamchatka sockeye salmon Oncorhychus nerka population through 50 yr of oceanic fishing resulted in increasing proportions maturing rapidly at small size before emigration from freshwater. In the absence of such a fishery, genetic evidence suggests that stabilising selection would ultimately restore the anadromous, slower-maturing pattern as the predominant life style to such a stock.