Why Should We Closely Monitor Fecundity in Marine Fish Populations

Reproductive success, best defined as lifetime total offspring to reach maturity, is the product of the number of eggs produced (i.e., fecundity) per season, adult reproductive life span and offspring survival to maturity. Life history traits at the population level can be seen as a representation of the fate of individuals through their life cycle. For practical purposes, reproductive success is often separated in two components: reproductive potential and offspring survival to maturity. In studies of exploited marine fish populations, spawning stock biomass (SSB) is commonly used as a proxy of reproductive potential instead of direct measures of the egg production of the stock. This technique assumes, however, that egg production per unit of biomass is time-invariant. This assumption is unsupported by a review of the literature. Seasonal fecundity varies in relation to parental quality (e.g., size, condition), resource availability (e.g., food abundance and quality), environmental (e.g., temperature) and evolutionary factors (e.g. stock biomass, fishing pressure). Recent studies and use of generalized linear models to hindcast fecundity variations demonstrate that stock reproductive potential estimated by the total egg production can lead to different perceptions of the state and productivity of the stock. The recent development of cost-effective methods to count egg numbers of fish now makes it practical to routinely determine potential fecundity. Adding measures of fecundity to other demographic parameters that are already commonly measured for exploited marine fish stocks allows a more precise measurement of the reproductive potential. One possible outcome of measuring reproductive potential as demonstrated using northern Gulf of St. Lawrence cod as a case study is the calculation of the intrinsic rate of population increase (r), an essential parameter in population dynamics and evolutionary ecology, which can be used in determining sustainable harvesting, resilience, and potential rates of recovery of populations.