Experimental evidence of resilience to size-selective harvesting in a protogynous hermaphroditic reef fish, Rhinogobiops nicholsii

Abstract Most fishing methods are inherently size-selective, and many fisheries have minimum size limits, resulting in removal of the largest, most fecund size-classes of harvested species. Some modeling and correlative studies suggest protogynous (female-to-male sex changing) fishes may be particularly strongly impacted by removal of large individuals, but controlled experiments are necessary to test this expectation. By manipulating the density and size structure of populations of the blackeye goby (Rhinogobiops nicholsii) in the field via size-selective removals, this experiment aimed to identify the effects of size-selective harvesting on the (1) growth, (2) maturation, (3) sex change, and (4) reproductive output of a protogynous hermaphroditic reef fish that uses exogenous cues for sex change. Removals of ~ 25% of fish established in populations on standardized artificial reefs simulated a pulse of fishing mortality with minimum size limits, slot size limits, and maximum size limits, which are used in some fisheries. Size-selective removals had no statistically significant effects on the growth, maturation, sex change, or reproduction in blackeye gobies, indicating that this protogynous species is resilient to moderately intense size-selective removals. Some trends consistent with predicted effects of size-selective removals were present, but we had low power to detect some effects. Specifically, the rate of maturation was highest in the slot size limit treatment, and the rate of sex change was highest where large fish had been removed. Regardless of simulated fishing type, populations in all treatments exhibited similar size- and sex-structure by the end of the experiment, nearly two months after population structure had been manipulated. These results suggest that sequentially hermaphroditic fishes with exogenous cues for sex change may be able to quickly replace individuals with high reproductive value after experiencing moderately intense size-selective harvesting. However, experiments incorporating additional sex-changing species, particularly with different modes of reproduction and cues for sex change, as well as higher fishing intensities or longer durations of fishing are still necessary to reconcile differences in results reported herein and those from modeling and correlative studies.