Abstract
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.
Highlights
Commercial coral-reef fisheries comprise hundreds of species, but a small subset contribute disproportionally to landings even on diverse Pacific reefs[1,2,3]
Species-specific responses are depicted in fewer novel datasets derived from sources such as restaurant menus, photographs of historical fishing competitions, and rare governmental statistics reports[11,20,21]
Species with significant responses to exploitation in more than one fishery had similar management categories in all but one instance (n = 6 species with significant responses in more than one fishery, only the grouper Variola louti differed between bottom and SCUBA (Tables 2–4). Both common and unique species replacement trends were revealed within three coral-reef fisheries across two decades
Summary
Commercial coral-reef fisheries comprise hundreds of species, but a small subset contribute disproportionally to landings even on diverse Pacific reefs[1,2,3]. While density-dependent responses are fundamental principles of population ecology, the variation in their magnitude may select for species that can become dominant through time as fishing pressure increases, and select against those that diminish or even disappear from landings[9] This situation causes shifting baselines, or reference points, that are not new to fisheries or coral reefs[10,11]; nor are the difficulties of predicting the outcomes of complex trophic interactions when species replacements occur[12]. Sample sizes for target species near human populations are usually low in visual assessments compared to catch landings, leading to management benchmarks that compare observed biomass to expected, or ‘unfished’, biomass[27,28,29] These beneficial benchmarks can readily be monitored through time to assess the status of fisheries, but they do not necessarily link with species-specific management criteria for policy development. We last build a framework to evaluate target species that comprised 70% of the landings and discuss species responses with respect to modern and traditional forms of management
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