Marine heatwaves imperil emblematic reef fishes by altering the energetic landscape of coral reefs.

Marine heatwaves are increasingly common due to human-induced climate change. Under prolonged thermal stress on coral reefs, corals can undergo bleaching, leading to mass coral mortality and large-scale changes in benthic community composition. While coral mortality has clear, negative impacts on the body condition and populations of coral-dependent fish species, the mechanisms that drive these changes remain poorly resolved. Specifically, little is known about the effects of coral bleaching on (1) the nutritional quality of corals, (2) nutrient acquisition in coral-feeding butterflyfishes and (3) fish dietary selectivity and potential supplementary consumption of non-coral prey. Here, we evaluate the response of obligate coral-feeding butterflyfishes to a mass coral bleaching event in French Polynesia, which resulted in high coral mortality and a 50% decline in obligate corallivore density. We examine benthic and butterflyfish community composition over two decades, including a mass bleaching event in 2019 and multiple prior disturbances. We couple these data with surveys of butterflyfish feeding selectivity, high-resolution molecular assays of gut contents and nutrient acquisition before, during and after the bleaching event. Contrary to previous studies, obligate corallivores did not strongly alter their feeding preferences for different coral genera in response to bleaching. They did not increase their consumption of non-corals in response to coral mortality, and hard corals continued to dominate their diets (>90%). Instead, butterflyfishes targeted partially bleached corals that were likely releasing nutrient-rich mucus, and they avoided fully bleached and dead corals that were likely nutrient-depleted. Moreover, after bleaching, butterflyfishes exhibit reduced nitrogen assimilation, indicating that coral stress may adversely impact butterflyfish nutrient acquisition. Coupled with the increasing frequency of recurrent bleaching events, severe, long-term nutritional impacts of coral bleaching on butterflyfish populations may jeopardize the persistence of coral-feeding fishes in the future.