Extreme marine heatwave linked to mass fish kill in the Red Sea.

Anthropogenic climate change has precipitated an increase in marine heatwaves (MHWs) that have significant and multifaceted impacts on marine ecosystems. In late August 2023, an intense heatwave coincided with a mass fish kill event on the Saudi Arabian coast of the central Red Sea. Here, we compile MHW metrics from satellite data to illustrate the mortality event was linked with the most intense period of rapid heating in the central Red Sea in recent history. Using field surveys, we quantified the impact of the event on the fish community and found that nearly 1000 fish washed ashore along a 60 km stretch of coastline. Representatives of 54 species were detected, which illustrates the impact of the MHW event on a broad range of fishes. The exact cause of mortality during the event is unknown, but likely related to temperature-induced physiological stress and associated factors. Sparse coastal monitoring limited our ability to rapidly respond to the event and document the proximate cause of mortality. This study not only sheds light on the immediate impacts of a MHW on components of a coral reef ecosystem, but also emphasizes the broader ecological consequences. Mass fish kills may have cascading impacts on ecosystem functioning by causing shifts in community structure and a decrease in biodiversity, which can undermine both the ecological functioning and economic stability of marine-dependent regions. This may be especially true for reefs already occupying a thermal niche that approaches the upper limits of many species, such as those in the Red Sea. Our study highlights the critical need for enhanced reporting mechanisms and forecasting tools to effectively document and help mitigate further impacts linked to MHW-induced mass marine die-offs.