Outlasting the Heat: Collapse of Herbivorous Fish Control of Invasive Algae During Marine Heatwaves

Marine heatwaves (MHWs), coral bleaching, and chronic local stressors such as eutrophication are accelerating regime shifts from coral- to algae-dominated reefs, increasingly favoring the proliferation of invasive, fast-growing, and often more grazing-resistant turf and macroalgae. A central tenet of global reef management strategies is that herbivorous fishes can sustain critical top-down control of algal proliferation as oceans warm. Here, we challenge this tenet by experimentally evaluating, under controlled laboratory conditions, whether herbivorous coral reef fishes across three key functional groups—browser (Naso lituratus), grazer (Acanthurus triostegus), and scraper (Chlorurus spilurus)—can maintain effective algal control across present-day (24.0°C–27.5°C) temperatures and into projected MHWs (31°C). We assessed (1) whether individuals evacuated thermally stressed conditions, effectively abandoning algal control, and (2) for those that remained, whether they could meet elevated energetic demands by foraging ad libitum on a mixture of Caulerpa spp.—a rapidly spreading and archetypal group of invasive algae in the Indo-Pacific. All species gained body mass while foraging exclusively on these algae during winter and summer (~0.18%–0.62% per day). However, despite remaining in thermally stressed conditions and maintaining stable foraging rates, all species experienced consistent body mass declines (~0.41%–1.62% per day) under MHW exposure. This precipitous decline in body mass was driven by ~54%–60% increases in basal energetic demands without corresponding increases in food intake. Survival estimates based on body mass loss ranged from ~20–81 days, which is substantially shorter than the projected ~126–152-day average duration of future MHWs. Our findings reveal that while short-term algal control may persist during thermal stress, prolonged exposure appears to erode herbivore physiological condition, effectively undermining top-down control of some algal types. Consequently, as ocean warming intensifies, herbivore protection strategies may become increasingly less effective at staving off algae proliferation and dominance in threatened reef ecosystems.