How do fisher responses to macroalgal overgrowth influence the resilience of coral reefs?

Abstract

Many coral reefs have shifted from coral-to macroalgae-dominated community states, heightening the need to understand resilience of coral communities. Fishing on herbivores often reduces resilience of the coral state, as lower herbivory fosters macroalgal establishment. Despite the acknowledged importance of fishing, relatively little attention has been paid to how fishers change their behavior as macroalgae overgrow reefs, or how the resulting dynamic feedbacks might affect resilience. We address these questions in Moorea, French Polynesia, where local fishers target herbivorous fishes and where shifts to algal dominance have occurred on some lagoon reefs. We quantified fisher preferences for reef habitats where they target various taxa. For the two most ecologically important taxa of herbivores targeted in the fishery, parrotfish (Scaridae) and unicornfish (Naso), fishers preferred to harvest from locations with less macroalgae. We incorporated these habitat preferences into a spatially explicit socialecological model of reef dynamics to explore consequences of changes in fishing behavior for resilience of the coral state, particularly following disturbance. Fishing that targets low-macroalgae locations typically generates resilience by facilitating local recovery of herbivores and thus of coral in the less-targeted macroalgae-dominated patches. However, the resulting movement of fishers across the seascape can sometimes create fragility; if coral loss is widespread, avoidance of macroalgae concentrates fishing in patches having the highest coral cover, resulting in loss of coral via reduced herbivory. Our results emphasize that resilience and coral-macroalgae regime shifts cannot be understood without considering humans as a dynamic part of the system.

Modeling the effects of selectively fishing key functional groups of herbivores on coral resilience

“…Fishing often generates a cascade of indirect effects that can alter community structure, ecosystem function, and resilience. Lagged responses between trophic levels (e.g., herbivores and primary producers) can create nonlinear community dynamics by promoting thresholds and tipping points (Salomon et al, 2009), potentially triggering an abrupt regime shift to a degraded state (Frank et al, 2005; Österblom et al, 2007; Rassweiler et al, 2022). Our model reveals that resilience of corals on coral reefs is not only affected by the intensity at which fishers harvest herbivores but also by the type of herbivore that is targeted.…”

“…Our main objective is to explore how fisher selectivity coupled with changes in catch alters the precariousness and reversibility of a coral-to-macroalgae regime shift. Our model is broadly grounded by the coral reef system of Moorea, French Polynesia, where there is detailed knowledge of the dynamics of the reef community (Adam et al, 2011(Adam et al, , 2014(Adam et al, , 2022Han et al, 2016;Holbrook et al, 2016Holbrook et al, , 2018Holbrook et al, , 2022Schmitt et al, 2019Schmitt et al, , 2022 and the small-scale reef fishery and fisher behavior (Holbrook et al, 2022;Lauer et al, 2022;Leenhardt et al, 2016;Nassiri et al, 2021;Rassweiler et al, 2020Rassweiler et al, , 2022.…”

Evaluating the precariousness of coral recovery when coral and macroalgae are alternative basins of attraction

“…In addition, browser biomass appears to be particularly susceptible to fishing on many coral reefs globally (Edwards et al 2014), a pattern that could be exacerbated in Moorea and elsewhere where local small‐scale fisheries disproportionately target species of browsers (Rassweiler et al 2020). Furthermore, movement patterns of fishers can further weaken resilience of the coral state after a large‐scale coral mortality event if fishers avoid macroalgae‐dominated areas and focus their fishing effort for herbivores on reefs that support the greatest remaining cover of coral (Rassweiler et al In press).…”

Long‐term ecological research and the COVID‐19 anthropause: A window to understanding social–ecological disturbance

“…With decreased commercial fishing due to COVID‐19, we expect that there will be greater top‐down control this year compared to years when fishing pressures are higher. On the coral reefs of Moorea, seaweeds are controlled by herbivorous fishes (Adam et al, 2011 ) that are targeted by the local small‐scale fishery (Leenhardt et al, 2016 ; Rassweiler et al, 2020 ), and research has shown that intensification of fishing (Holbrook et al, 2016 ) and alteration of fisher behavior (Rassweiler et al, 2021 ) can reduce resilience of the coral state and promote a switch to seaweed dominated reefs that can represent a difficult‐to‐reverse, alternative basin of attraction (Schmitt et al, 2019 , 2021 ). Relaxation of fishing would strengthen resilience of coral communities to other disturbances such as storms and heat waves (Holbrook et al, 2016 ).…”