Ocean Warming Is Driving Sharks Out of Coral Reef Habitats, Study Finds

As the oceans heat up, sharks are leaving their coral reef habitats, putting both sharks and entire ecosystems at risk, scientists have found.

In a new study, published in the journal Communications Biology, scientists used satellites and underwater acoustic receivers to monitor more than 120 grey reef sharks (Carcharhinus amblyrhynchos) in coral reefs of the Chagos Archipelago in the Indian Ocean from 2013 to 2020. 

Over this time period, the researchers were able to collect 714,000 acoustic detections to complement satellite data. This allowed them to trach the sharks’ movements and behaviors along with coral reef environmental stressors, such as sea surface temperatures, ocean currents and wind.

The study authors found that after periods of stress on the corals, such as the El Niño event of 2016 that led to massive coral bleaching in the study site, sharks would abandon their coral reef habitats for several months. Sharks abandoned stressed corals for as many as 16 months in the study period.

“Sharks are ectotherms — cold-blooded animals whose body temperature is regulated by their external environment,” explained Michael Williamson, lead author of the study and post-doctoral research associate at the Zoological Society of London’s Institute of Zoology. “Reef sharks in other regions exhibit behavioral thermoregulation to avoid physiological damage from adverse water temperatures, and this is one of the potential drivers of the findings in this study.”

As the UN Environment Programme reported, failure to curb greenhouse gas emissions and climate change could lead to annual coral bleaching events, and 99% of the world’s coral reefs could undergo severe bleaching every year by the end of this century. Further, it can take five years or more for the coral to recover.

In response, the sharks may continue to seek cooler waters for longer periods of time. Already, the absence of the grey reef sharks is concerning scientists.

“As large predators, grey reef sharks play a very important role in coral reef ecosystems,” Williamson said. “They maintain a delicately balanced food web on the reef and they also cycle nutrients onto coral reefs from deeper waters where they often feed. A loss of sharks, and the nutrients they bring, could affect the resilience of reefs during periods of high environmental stress.”

However, the report did include some more positive findings. Some of the monitored locations experienced an increase in shark residency, but the study authors noted that more research is needed to determine what was leading to such increases. Some corals could be more resilient to stressors, or there could be other factors influencing the sharks’ behaviors. As The Guardian reported, invasive rat removal and increased bird populations could help improve coral resiliency, therefore leading to an increase in sharks that live in the reefs.

“Recent research in the Chagos Archipelago, where we conducted our study, has shown that those reefs that have greater nutrient flows from seabirds have significantly enhanced fish biomass and therefore a higher likelihood to be resilient to multiple stressors,” Williamson explained. “Some of our receivers that were seeing a greater number of sharks residing were also near islands with seabird populations.”