Sharks are vacating struggling reefs as oceans heat up

As ocean conditions continue to worsen, particularly during extreme events such as the 2015-16 El Niño, sharks are spending less time around reefs. Often, they completely disappear for extended periods.

The absence of sharks is concerning not just because their overall populations are dwindling, but also because their departure could destabilise already struggling ecosystems.

Coral reefs are known for being rich in biodiversity, hosting a quarter of all marine fish species. Reef sharks play a crucial role within this system, ensuring that coral reefs do not become overgrown with algae, a state that accelerates the process of coral bleaching and fuels further degradation.

In that case, the news that sharks are ‘functionally extinct’ from 1 in 5 reefs globally should be a major wake-up call.

The study used satellite tracking data to monitor environmental conditions, focusing on factors like sea surface temperature and wind patterns.

These measurements helped scientists correlate the departure of sharks with the stress levels on the reefs. The primary driver, according to the researchers, was rising ocean temperatures.

‘Sharks are cold-blooded, meaning their body temperature is directly tied to the water temperature,’ said Dr. David Jacoby, the study’s lead researcher and a lecturer at Lancaster University. ‘If it gets too hot, they will need to move.’

The study suggests that many sharks are seeking refuge in deeper, cooler waters – a dangerous zone for reef sharks which are a declining species. It has been noted that sharks are already missing from nearly 20 percent of coral reefs globally, largely due to overfishing.

While this discovery should be raising alarm bells, the study noted that not all reefs are equally affected.

A small group of healthy reefs saw sharks spending more time within their borders. These reefs, bolstered by higher bird populations, were better able to withstand environmental stress.

Birds play a key role in fertilising the reefs, diversifying coral species and building a more resilient ecosystem.

‘The hopeful side of it is that not all sharks responded in the same way,’ said Dr. Anna Sturrock, a senior lecturer at the University of Essex.

‘It suggests that with the right conservation strategies, we might be able to mitigate some of the negative impacts of climate change on these critical ecosystems,’ she concluded.

Even with this silver lining, we’ll need to act fast.

Coral reefs are facing the possibility of widespread collapse if global temperatures rise more than 2 degrees C, with severe bleaching events projected to become annual occurrences by 2050.

If this occurs, the effects of this new reality will be felt by more than just marine life. More than 500 million people depend on coral reefs for food and income, particularly in tropical regions, meaning people’s livelihoods are at stake.

The researchers said the study should spark immediate action to protect both sharks and coral reefs. Conservation efforts, such as the removal of invasive species and better protection of reef habitats from human activities could help prevent these precious ecosystems from collapsing.

That said, without global efforts to curb the climate crisis – especially reducing greenhouse gas emissions – these efforts may prove insufficient.