We tend to focus on coral bleaching when considering ocean warming, but a recently published case study of a 2017 marine heatwave in the Red Sea urges scientists to consider reef fish health, too. When temperatures rapidly increased on a reef in the northern Red Sea, scientists started seeing fish carcasses on the seabed – an unusual occurrence in a healthy reef community. The scientists immediately teamed up with community scientists to survey reefs. Over the 10 week heatwave, the team found over 400 fish carcasses. 

A closer inspection of some carcasses revealed bacterial infections as the likely cause of death. But the responsible bacteria are commonly found in the area and usually cause only minor, asymptomatic infections in fishes. 

Scientists concluded that the rapid change in temperature stressed fish, leading to a suppressed immune response to the bacteria. As bottom-feeding fish, like parrotfishes, succumbed to infections, carcasses accumulated on the seabed. When other fish, like groupers, ate the infected carcasses, the infection spread through the food chain, exacerbating the fish die-off. Only fish that fed predominately on zooplankton were spared. The scientists worry that the selective die-off of some species could alter food-web dynamics and overall ecosystem functioning. 

After comparing this warming to previous events, the scientists concluded that the magnitude of a heatwave was less important than the rate of onset of the warming event and that fish may be particularly susceptible to warming events with rapid onsets. Scientists predict marine heatwaves will increase in both frequency and severity due to climate change. If this coincides with a faster onset of warming events, we will have more to worry about than just coral bleaching.