Mass fish deaths linked to extreme marine heatwave in Red Sea

In August 2023, an intense marine heatwave along the Saudi Arabian coast of the central Red Sea coincided with the mass beaching of dead fish, prompting a team of KAUST researchers to investigate^[1].

Temperatures in oceans and seas around the globe are rising due to anthropogenic climate change, increasing stress on marine ecosystems and wildlife. Marine heatwaves occur when localized water temperatures exceed historical average sea temperatures; extreme events occur when temperatures reach 2ºC or more above average for a prolonged period. This can have multifaceted and catastrophic effects on marine life.

“Coastguard officials began reporting high numbers of dead fish and invertebrates, such as cuttlefish, washing ashore,” says Matthew Tietbohl, postdoctoral fellow, who led the project under the supervision of KAUST’s Maggie Johnson. “By the time we got to the beaches, about a week later, the fishes had degraded enough that we couldn’t collect useful samples to determine the cause of death, such as possible bacterial infection.”

The team found almost 1,000 fish washed ashore along a 60-kilometer stretch of coastline north of Thuwal. The mortality event included at least 54 species of fish, highlighting a broad impact across the reef community. This is a highly conservative estimate, notes Tietbohl: It was impossible to survey the entire 60+ kilometers of impacted coastline, and only a quarter of dead fish typically wash ashore.

In collaboration with Ibrahim Hoteit’s team, the marine scientists turned to satellite data analyses to understand the environmental conditions in the weeks prior to the mass beaching.

Satellite data of sea surface temperatures confirmed that an intense marine heatwave was spread across the whole region. However, the area just north of where the fish were found had the highest water temperatures and the strongest daily temperature changes, which the researchers believe had placed physiological stress on the fish. The weakened and dying fish were likely pushed southwards by prevailing winds and currents.

“Just like humans, all creatures struggle when they get too hot: Once a certain thermal threshold is reached, it becomes very difficult for their bodies to function correctly,” explains Tietbohl. “Bodies also burn more energy when stressed and require more oxygen, but higher temperatures reduce the amount of oxygen that water can hold.”

Extreme heat can also lead to stratification, where water settles into layers with limited mixing. This reduces oxygen exchange and stops the water from cooling down. Suffocating algal blooms are common in this scenario, although the researchers did not find evidence of this phenomenon during the 2023 heatwave. Extensive coral bleaching also occurred in the region that summer, which is also an indication that temperatures were high.

“Satellite instruments tend to underestimate water temperatures in the Red Sea, so it is likely the region experienced temperatures well above 2ºC of warming,” adds Tietbohl.

“These results, together with data from similar fish kills in summer 2024, suggest that the Red Sea maximum temperatures may be getting too hot for coral reefs and many marine creatures,” says Johnson. “Their chances of recovery would be improved by implementing fishery regulations that limit fishing during this stressful time.”

“We’ve presented our findings to the Saudi National Center for Wildlife and are in discussions to incorporate mass fish mortality monitoring into their wider marine animal stranding network,” says Tietbohl. “Improved monitoring would feed back into temperature models to help predict marine heatwaves, thus boosting preparation windows and supporting possible mitigation strategies or damage control.”

Reference

1. Tietbohl, M.D., Genevier, L.G.G., Krieger, E.C., Kattan, A., Wang, Y., Gokul, E.A., Rodriguez Bravo, L.M., Palm, L., Mele, G., Hoteit, I., & Johnson, M.D. Extreme marine heatwave linked to mass fish kill in the Red Sea. Science of the Total Environment 975, 179073 (2025).| article.