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

Our study reveals a hitherto overlooked ecological threat of climate change. Studies of warming events in the ocean have typically focused on the events' maximum temperature and duration as the cause of devastating disturbances in coral reefs, kelp forests, and rocky shores. In this study, however, we found that the rate of onset (Ronset), rather than the peak, was the likely trigger of mass mortality of coral reef fishes in the Red Sea. Following a steep rise in water temperature (4.2 °C in 2.5 d), thermally stressed fish belonging to dozens of species became fatally infected by Streptococcus iniae Piscivores and benthivores were disproportionately impacted whereas zooplanktivores were spared. Mortality rates peaked 2 wk later, coinciding with a second warming event with extreme Ronset The epizootic lasted ∼2 mo, extending beyond the warming events through the consumption of pathogen-laden carcasses by uninfected fish. The warming was widespread, with an evident decline in wind speed, barometric pressure, and latent heat flux. A reassessment of past reports suggests that steep Ronset was also the probable trigger of mass mortalities of wild fish elsewhere. If the ongoing increase in the frequency and intensity of marine heat waves is associated with a corresponding increase in the frequency of extreme Ronset, calamities inflicted on coral reefs by the warming oceans may extend far beyond coral bleaching.

Hydrodynamic regime determines the feeding success of larval fish through the modulation of strike kinematics.

Larval fishes experience extreme mortality rates, with 99% of a cohort perishing within days after starting to actively feed. While recent evidence suggests that hydrodynamic factors contribute to constraining larval feeding during early ontogeny, feeding is a complex process that involves numerous interacting behavioural and biomechanical components. How these components change throughout ontogeny and how they contribute to feeding remain unclear. Using 339 observations of larval feeding attempts, we quantified the effects of morphological and behavioural traits on feeding success of Sparus aurata larvae during early ontogeny. Feeding success was determined using high-speed videography, under both natural and increased water viscosity treatments. Successful strikes were characterized by Reynolds numbers that were an order of magnitude higher than those of failed strikes. The pattern of increasing strike success with increasing age was driven by the ontogeny of traits that facilitate the transition to higher Reynolds numbers. Hence, the physical growth of a larva plays an important role in its transition to a hydrodynamic regime of higher Reynolds numbers, in which suction feeding is more effective.