An applied framework to assess exploitation and guide management of coral-reef fisheries

Evolutionary management of coral-reef fisheries using phylogenies to predict density dependence

Harvesting models are based upon the ideology that removing large, old individuals provides space for young, fast-growing counterparts that can maximize (fisheries) yields while maintaining population stability and ecosystem function. Yet, this compensatory density dependent response has rarely been examined in multispecies systems. We combined extensive data sets from coral-reef fisheries across a suite of Pacific islands and provided unique context to the universal assumptions of compensatory density dependence. We reported that size-and-age truncation only existed for 49% of target coral-reef fishes exposed to growing fishing pressure across a suite of Pacific islands. In contrast, most of the remaining species slowly disappeared from landings and reefs with limited change to their size structure (i.e., little to no compensation), often becoming replaced by smaller-bodied sister species. To understand these remarkable and disparate differences, we constructed phylogenies for dominant fish families and discovered that large patristic distances between sister species, or greater phylogenetic isolation, predicted size-and-age truncation. Isolated species appeared to have greater niche dominance or breadth, supported by their faster growth rates compared to species with similar sizes and within similar guilds, and many also have group foraging behavior. In contrast, closely related species may have more restricted, realized niches that led to their disappearance and replacement. We conclude that phylogenetic attributes offered novel guidance to proactively manage multispecies fisheries and improve our understanding of ecological niches and ecosystem stability.

Standing Out in a Big Crowd: High Cultural and Economic Value of Naso unicornis in the Insular Pacific

Hundreds of fish species are harvested in coral-reef fisheries in the Indo-Pacific region using a variety of fishing gears. This diversity makes the economics, ecology, and management of reef-associated fisheries inherently complex. However, across insular fisheries spanning the tropical Pacific, one species consistently dominates contemporary fisheries catch: the bluespine unicornfish (Naso unicornis, Acanthuridae). I evaluated the relative contribution of N. unicornis to commercial fisheries in the insular Pacific region from long-term (>1 year) fishery surveys across various jurisdictions and provide evidence of the contemporary cultural value of this species. Overall, evidence suggests that N. unicornis is the most commercially-valuable reef-associated fish species across the insular Pacific. This notion, coupled with a diverse representation of N. unicornis across Pacific cultures, suggests that the species is presently underappreciated in its role in coastal fisheries across Pacific Island nations.

Variable density dependence and the restructuring of coral-reef fisheries across 25 years of exploitation

Variable density dependence within multispecies fisheries results in species restructuring as exploitation intensifies that is poorly understood. We examined unique species-based records across 25 years of exploitation to evaluate patterns, consequences, and predictions of species replacements within three coral-reef fisheries. Body-size was an expected determinant of species replacements, as larger fishes were consistently replaced by smaller, faster-growing counterparts. However, many species with similar sizes and growth rates responded differently. Naso unicornis, a primary component of coral-reef fisheries across the Pacific, was one of the most resilient species to exploitation despite having a similar maximum size and growth as many large parrotfishes that slowly disappeared from landings. Assessments conducted for all primary target species revealed clear distinctions in compensatory responses: 31% had diminishing size structures, 18% had diminishing proportional contribution, but only 5% showed both. Standard approaches to fisheries management assume constant rates of size-and-age restructuring and rely upon metrics such as fishing-versus-natural mortality. Instead, a deeper appreciation for varying recruitment rates may help to (re)define fisheries management units and reduce complexity in multispecies fisheries. We last consider our results alongside traditional knowledge and management in the Pacific that clearly appreciated species responses, but have been lost over the years.