Minimum size limits and the reproductive value of numerous, young, mature female fish

Fisheries management relies on various catch and effort controls to preserve spawning stock biomass and maximize sustainable yields while limiting fishery impacts on marine ecosystems. These include species-specific minimum or maximum size limits to protect either small non-reproductive subadults, a portion of reproductively mature adults, or large highly fecund individuals. Protecting size classes of mature fish is expected to yield a viable source of larvae for replenishing populations and reduce the risk of recruitment overfishing, yet size-specific recruitment contributions have rarely been assessed empirically. Here, we apply genetic parentage analysis to measure the reproductive success of a size-structured population of a commercially important species of coral grouper (Plectropomus maculatus-Serranidae) in no-take marine reserves (NTMRs) in the Great Barrier Reef Marine Park, Australia. Although the per capita reproductive success of individual fish increases rapidly with body length, the numerous young mature female fish, below the minimum size limit (MSL) (38 cm total length), were responsible for generating disproportionately large contributions (36%) towards larval replenishment of both fished and reserve reefs. Our findings indicate that MSLs are an effective harvest control measure to safeguard a portion of the spawning stock biomass for coral grouper and supplement recruitment subsidies assured from NTMRs.