Protection of functionally important parrotfishes increases their biomass but fails to deliver enhanced recruitment

Abstract

Burgeoning threats to coral reefs have prompted calls for management actions that can enhance ecosystem resilience, such as restoring herbivore populations whose grazing is critical to maintaining ecological function. However, lack of longitudinal datasets has hindered objective assessment of strategies aimed at recovering herbivory. Addressing this gap, we investigated the response of the Bermuda fish assemblage to a trapping ban that amounted to de facto protection of herbivorous parrotfishes (Scaridae). Hook-and-line fishing for piscivores continued during the ban, creating a natural experiment that freed scarids from both fishing mortality and adult-stage predation. Over the 9 yr study period, biomass of piscivores remained low because of the hook-and-line fishery, with the exception of trumpetfish Aulostomus maculatus whose biomass increased more than 6-fold. Although scarid post-recruit biomass increased by a factor of 3.7, there was no increase in recruits (<5 cm), contrary to our expectation of observing a stock-recruitment relationship (SRR) in a demographically closed system such as Bermuda. Although the unavoidable lack of a before-after-control-impact design in our study precludes making strong mechanistic inferences, we hypothesize that the observed increase in scarid biomass may indeed have driven a commensurate increase in larval settlement within this closed system, but density of settlers was subsequently regulated by A. maculatus, a predator of small fish that was free to respond to prey enrichment owing to the absence of large predators. Our results provide compelling evidence that scarid populations can rapidly recover from overfishing once protected, even if any SRR is decoupled.