Rapid onsets of warming events trigger mass mortality of coral reef fish

Significance Our study reveals a hitherto overlooked effect of warming on coral reefs. Traditionally, ecological studies of warming events focused on maximum temperatures and duration, rather than the rate of warming at the onset. Here, we show that onsets can trigger widespread mortality of reef fish. Hundreds of thermally stressed fish, belonging to dozens of species, became fatally infected with a common pathogen in the Red Sea. Differential susceptibility led to selective mortality, with disproportional death among predators and benthic feeders. A reassessment of past reports suggests that extreme onset might have been an overlooked trigger of fish kills elsewhere. Warming tropical and subtropical reefs may face an increasing frequency of extreme onsets, eliciting calamities far beyond coral bleaching.