Rising Ocean Temperatures Force Sharks to Abandon Coral Reefs, Study Finds

The escalating climate crisis is driving crucial marine predators away from their coral reef habitats, posing a significant threat to already fragile ecosystems, researchers have revealed.

As ocean temperatures continue to rise, sharks are increasingly abandoning their coral reef homes, jeopardizing their survival and destabilizing reef ecosystems. These reef sharks play a vital role in maintaining the balance of these biodiverse environments, and their departure could lead to severe ecological imbalances.

From 2013 to 2020, scientists tracked over 120 grey reef sharks in the remote coral reefs of the Chagos Archipelago in the central Indian Ocean. The study found that during periods of heightened ocean warming, particularly the intense El Niño event of 2015-16, the sharks significantly reduced their presence on the reefs. In some cases, sharks did not return to their usual habitats for up to 16 months following these stress events.

Interestingly, the sharks tended to remain longer on certain healthier and more resilient reefs. These robust reefs benefited from the removal of invasive rats and had higher populations of birds, which contribute to reef fertility. This finding suggests that enhancing protection measures against human-induced damage could help retain shark populations on their native reefs.

Sharks, being ectothermic, rely on the surrounding water temperature to regulate their body heat. Dr. David Jacoby, a zoology lecturer at Lancaster University and the study's lead researcher, stated, "If the water becomes too warm, sharks must relocate. Many are likely moving to deeper, cooler offshore waters, which is alarming as some sharks have been absent from reefs for extended periods."

Currently, reef sharks are missing from nearly 20% of coral reefs worldwide, primarily due to overfishing. This new trend of climate-induced displacement could worsen the decline of these essential predators.

Coral reefs, known for their stunning biodiversity, support a quarter of all marine fish species. However, they are under severe threat from rising sea temperatures, overfishing, and pollution. Without immediate action, global warming could lead to the loss of most coral reefs if temperatures increase by 2°C, with frequent severe bleaching events expected by mid-century—even if the Paris Agreement targets are met. Approximately 500 million people depend on coral reefs for their food and livelihoods.

The grey reef sharks observed in this study spend their days socializing, seeking shelter from larger predators, and feeding on reef fish. Their nocturnal feeding habits off the reefs contribute essential nutrients back to the ecosystem through their waste. The research, published in Communications Biology, involved over 700,000 data points collected via tagged sharks and satellite measurements tracking environmental stressors like sea surface temperature and ocean currents.

Dr. Michael Williamson, a post-doctoral researcher at the ZSL Institute of Zoology and a lead author of the study, suggested that elevated temperatures likely caused the immediate departure of sharks. Additionally, the detrimental effects of heat on reef health may have prevented their return, although further research is needed to confirm these conclusions.

Dr. Anna Sturrock, a senior lecturer at the University of Essex and an external expert, emphasized the critical role of sharks in reef ecosystems. "Sharks help maintain the balance of reef systems by preying on both herbivorous and smaller predatory fish, preventing coral overgrazing and algal overgrowth. The absence of sharks during the reefs' most vulnerable periods is deeply concerning," she stated. However, she noted a positive aspect: not all sharks exhibited the same migratory behavior.

This study highlights the intricate connections between climate change and marine life, underscoring the urgent need for comprehensive strategies to protect both predators and their habitats. Ensuring the survival of reef sharks is crucial for the health and resilience of coral reef ecosystems worldwide.