Climate impacts alter fisheries productivity and turnover on coral reefs

Abstract

Alteration of benthic reef habitat after coral bleaching and mortality induces changes in fish assemblages, with implications for fisheries. Our understanding of climate impacts to coral reef fisheries is largely based on fish abundance and biomass. The rates at which biomass is produced and replenished (productivity and turnover) are also important to sustaining fisheries, yet the responses of these metrics following bleaching are largely unknown. Here, we examine changes in fish productivity and turnover after mass coral bleaching events in Seychelles, on reefs that were recovering to coral-dominated habitats and those that shifted to macroalgae-dominated regimes. Productivity of fish assemblages increased on all recovering reefs, particularly on fished reefs resulting in levels similar to protected reefs 19 years after bleaching. Herbivore-detritivores, such as scraping and excavating parrotfish, appeared to drive biomass production through increased abundance on recovering reefs. Productivity on regime-shifted reefs remained stable at 1994 levels in fished areas, with increases observed on protected reefs. Large increases in browser productivity (particularly on protected reefs), combined with increases for invertivores, maintained post-bleaching productivity on macroalgal reefs. For all diet groups, net turnover was generally higher on fished regime-shifted reefs than on recovering reefs, suggesting fish biomass is more readily replenished on macroalgal reefs. Reef structural complexity was a positive predictor of productivity for all diet groups. These findings indicate that post-bleaching reef fish productivity is strongly influenced by benthic recovery trajectories, and demonstrates the importance of herbivore and invertivore species in sustaining small-scale inshore fisheries following climatic disturbances.

Towards process-oriented management of tropical reefs in the anthropocene

“…Process-based metrics such as biomass production provide important information on fisheries yields and biomass build-up and consequently need to be incorporated into decisions about where and how to manage tropical reefs 5 . Several studies show the decoupling of biomass turnover, a dynamic ecosystem rate, from standing stock biomass, which presents new opportunities to improve and adapt conservation strategies 29,30,41 . We propose a conceptual framework accounting for both fish standing biomass and biomass turnover to guide the incorporation of ecological processes into the management of tropical reefs.…”

“…2). High biomass turnover has been identified as a compensatory ecological mechanism whereby fish populations characterized by high growth rates (such as herbivores) positively respond to size-selective fishing gears and biomass collapse through increased biomass turnover 29,41 . However, this compensatory mechanism may be only short term, as intensive degradation may eventually also lead to a decrease in herbivore biomass 49 .…”

“…From this information, we computed a species-specific growth coefficient, K max , a standardized growth parameter under a von Bertalanffy growth model. This coefficient represents the von Bertalanffy growth coefficient (K) scaled to apply to a theoretical population where L inf , the species-specific asymptotic maximum length, is equal to the species L max , the maximum observed length, and has previously been used to estimate growth trajectories 4,25,29,41 . Biomass production, expressed as the accumulated biomass of an assemblage in a day, was obtained by subtracting the modelled weight gained by a fish after one day from the observed fish weight, calculated using the length-weight coefficients.…”

Multi‐decadal stability of fish productivity despite increasing coral reef degradation

“…Such variation in productivity has been reported in specific groups of fishes across various reef systems, including studies on the GBR (e.g. Morais et al, 2020; Tebbett et al, 2021) and the Seychelles (Hamilton et al, 2022). Interestingly, despite this relatively high species turnover, the functioning of the benthic fish communities after the first mass bleaching event has remained remarkably stable, which was consistent in both process‐based and static proxies of productivity.…”

“…Specific fish groups tend to react negatively to the loss of corals; for example, increases in the sediment load on reefs coincided with a decrease in herbivorous fish biomass production (Tebbett et al, 2021) and a loss of brightly coloured fishes (Hemingson et al, 2022). Conversely, when the entire communities are sampled across several years, fish biomass production can increase following both mass coral cover decline (Morais et al, 2020) and coral cover recovery (Hamilton et al, 2022). This discrepancy in functional responses raises the question: how will the energy fluxes of entire communities be affected over longer time‐scales?…”