UC Santa Barbara

With marine heat waves increasing in intensity and frequency due to climate change, it is important to understand how thermal disturbances will alter coral reef ecosystems, which are highly susceptible to thermally-induced, mass bleaching events. In Moorea, French Polynesia, we evaluated the response and fate of stony coral following a major thermal stress event in 2019 that caused a substantial amount of branching coral (dominantly Pocillopora) to bleach and die. We investigated whether Pocillopora colonies that occurred within territorial gardens protected by the farmerfish Stegastes nigricans were less susceptible to or survived bleaching better than Pocillopora on adjacent, undefended substrate. Bleaching prevalence (proportion of the sampled colonies affected) and severity (proportion of a colony’s tissue that bleached), which were quantified for >1,100 colonies shortly after they bleached, did not differ between colonies within or outside of defended gardens. By contrast, the fates of 399 focal colonies followed for a year revealed that a bleached coral within a garden was a third less likely to suffer complete colony death and, for survivors, about twice as likely to recover to its pre-bleaching cover of living tissue compared to Pocillopora outside of a farmerfish garden. Our findings indicate that while residing in a farmerfish garden may not reduce the bleaching susceptibility of a coral during thermal stress, it does help buffer a bleached coral against severe outcomes. This oasis effect of farmerfish gardens, where survival and recovery of thermally-damaged corals are enhanced, is another mechanism that helps explain why large Pocillopora colonies are far more abundant in farmerfish territories than elsewhere in the lagoons of Moorea, despite gardens being much less common. As such, farmerfish may have a growing role in maintaining the resilience of branching corals as the frequency and intensity of marine heat waves continue to increase.