Understanding the maturation process for field investigations of fisheries-induced evolution

The probabilistic maturation reaction norm approach has been widely heralded as an empirical approach to distinguish between the effects of genetic selection and phenotypic plasticity on maturation probability. However, applications of this approach have considered maturation state in relation to fish size long after the period when fish make the 'decision' to mature. Evidence, mostly from salmonids, indicates that maturation is controlled by successive hormonal inhibition linked to energy state during critical periods of the year. Thus, any genetic selection cannot be acting upon the size of fish late in the reproductive cycle, but rather during the timing of these critical periods. Indeed, experimental studies demonstrate that size attained by late stages of gametogenesis need not necessarily be a good predictor of the probability of maturing. Therefore, changes in energy status around the time of maturation decisions represent an unknown and possibly significant source of variability in the reaction norm midpoints. Clearly, there is a need to apply physiologically realistic models of maturation probability - as have been developed for Atlantic salmon - to other fish species. Future investigation of fisheries -induced evolution may also benefit from examining historical changes in fecundity and by comparing current reproductive investment in fish from heavily and lightly exploited populations that are held under common-garden conditions.