Assessing stock reproductive potential in species with indeterminate fecundity: Effects of age truncation and size-dependent reproductive timing

Annual fecundity has been increasingly advanced as a preferred measure of stock reproductive potential (SRP), yet estimates are methodologically challenging, particularly for species with indeterminate fecundity. Inherent variability due to energetic and environmental constraints increases the uncertainty of annual fecundity estimates, and this uncertainty may have important consequences for management guidance. In this study, we assessed how size-dependent reproductive timing interacts with age truncation to impact estimates of SRP in spotted seatrout (Cynoscion nebulosus), using a combination of modeling techniques. Positive size dependence in spawning frequency, or the number of spawning events in a season, had a substantial impact on both the SRP and estimates of stock status relative to management targets. The loss of the oldest age classes, even under relatively low fishing pressures, resulted in a significant decline in SRP due to the disproportionately large contribution by these age classes. Not surprisingly, the effect increased with the strength of the size-dependent relationship, which also resulted in reducing the proportionality between SRP and spawning stock biomass (SSB). The potential for erroneously classifying the health of a stock was amplified when the population approached the management target because of the uncertainty and inherent variability in SRP. For management, characterizing all of the size-dependent relationships with SRP (batch fecundity, reproductive timing, egg/larval quality) may be more critical than obtaining precise estimates of SRP when management reference points are relative to unexploited conditions, as in spawning potential ratios (SPRs).

► We model stock reproductive potential (SRP) for fish with indeterminate fecundity.
► Older age classes contribute a disproportionately large amount of the SRP.
► Spawning biomass is not proportional to SRP when size dependent spawning occurs.
► Uncertainty in size dependent spawning impacts perceptions of stock health.