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.