Risk of overfishing rises as climate change gives silky sharks the squeeze

As our climate warms, silky sharks are avoiding higher summer temperatures in the Red Sea by keeping to a narrow band of cooler water and aggregating at a depth of 70–100 metres (230–328 feet). It is a behaviour that has not previously been seen in this threatened species and it raises concerns about an increased risk of overfishing.

Our warming world is changing, and shark scientist Dr Collin Williams wants to know how sharks are going to respond to sustained higher temperatures across the oceans. Can they adapt? And if so, how and where will they move as temperatures rise above what their bodies have spent millions of years evolving to tolerate?

Collin’s latest study shows that silky sharks – typically a species that prefers waters shallower than 50 metres (164 feet) – are responding to soaring sea surface temperatures by moving into a narrow band of cooler, deeper water to beat extended heat waves.

‘We were not expecting our data to reveal this behaviour,’ he says. ‘Pelagic sharks are known to undertake more frequent and deeper dives when surface temperatures rise, but we did not anticipate that Red Sea silky sharks would shift their behaviour entirely and remain in deeper, cooler waters for extended periods of time.

‘As we continue studying sharks in extreme environments, I suspect that we’ll encounter other unexpected behaviours that offer new insights into shark responses to climate change.’

The silky shark is already a species under pressure

Dr James Lea started his career as a marine biologist in the sultry seas where silky sharks once abounded, helping to pioneer the field of silky shark movement ecology and population estimates in the Red Sea. Today, the waters where he started his work harbour a shadow of their former marvels. ‘The silky shark is considered vulnerable to extinction, having once been one of the most abundant shark species in the world,’ he explains. ‘To date its decline has been caused almost entirely by overfishing, as demand for its fins and meat drives an extensive global trade.’

The silky shark is a large predator found across the world’s tropical and subtropical waters. It is highly migratory, capable of extraordinary journeys. Genie, the jet-setting silky shark, broke all records for her species when she travelled 4,755 kilometres (2,955 miles) from her tagging location in the Galápagos Marine Reserve, completing 27,666 kilometres (17,190 miles) of ocean travel in less than two years. These sharks are also able to dive as deep as 1,000 metres (3,280 feet), but it’s rare for them to do so; they typically spend 99% of their time in waters shallower than 100 metres.

‘Learning that shifts in movement behaviour in warmer waters may cause them to overlap with fisheries emphasises even more the need for urgent conservation action for this species,’ says James, who is a co-author on Collin’s scientific paper. On the International Union for Conservation of Nature’s (IUCN) Red List – the barometer that scientists update to track the extinction risk and conservation status of species around the world – the silky shark is listed as Vulnerable, with a decreasing trend. ‘Climate change could exacerbate the impacts of fisheries and accelerate these sharks towards extinction,’ he adds.

How do sharks behave when it’s warmer?

The Red Sea gives us a real-time opportunity to see what might be happening across our oceans as the world continues to warm. As a semi-enclosed basin, it already experiences increasingly warmer waters and frequently has periods of sustained marine heat waves. Collin tagged sharks to follow where they moved and when they dived, and monitored their preferred temperature range.

It was thought that silky sharks moved away from the Red Sea’s central reefs during the warmest months of the year. That is because scientists had been using diver surveys and acoustic telemetry, which was limited to listening stations that could detect sharks on shallower reefs. But Collin’s study, which used satellite tags, showed that these sharks are not all disappearing for part of the year. Instead, they’re finding refuge from the heat at depths that were previously not monitored.

For most of the year, silky sharks were regionally resident, moving less than 50 kilometres (31 miles) from the reefs where they were tagged.

How does this information help us steer the Red Sea’s silky sharks away from crisis?

Up to now, the silky shark has been managed as a migratory species. But there could be better conservation impact with this new information; Saudi Arabia’s authorities could manage the silky shark as regionally resident, giving it local protection and enforcement.

But Collin’s study points to something urgent; we need to figure out how to protect sharks at different depths in the ocean rather than simply closing off areas based on surface distances. ‘Sharks are more vulnerable to overfishing when they are concentrated, which is why aggregation sites are typically prioritised in spatial management frameworks,’ he explains. A good example of this is the Important Shark and Ray Areas (ISRAs) process, where information about where sharks gather to breed, feed and grow is consolidated to advise where marine protected areas could be created to protect sharks where they are most at risk.

‘Integrating vertical concentrations of sharks into conservation strategies is much less straightforward,’ says Collin. ‘Shark scientists have already been sounding the alarm for depth-integrated fishery management, particularly as climate-driven ocean de-oxygenation forces pelagic sharks into shallower waters. Expanding oxygen minimum zones, combined with the thermal habitat compression we document, may squeeze sharks into an increasingly narrow depth range. Our findings are especially concerning because silky sharks become concentrated at depths that directly overlap with commercial longline sets in the Indian Ocean (at about 100 metres), which could substantially increase the likelihood they will be caught.’

It’s not only the way we protect and manage silky sharks that might need consideration. ‘Vertical habitat compression also complicates shark population monitoring,’ says Collin. ‘Fisheries catch data are often used as indicators of abundance, but if sharks are becoming concentrated at depths targeted by fisheries, catch rates may rise even as populations decline. Fishery-independent monitoring using tools like pelagic BRUVs surveys (baited underwater cameras that hang and drift in the water column) may become essential to tracking pelagic shark population trends alongside vertical habitat compression.’

What does the (warmer) future hold?

This study has confirmed that silky sharks change their behaviour to cope with rising temperatures, driving them into compressed depth ranges.

For James, the worry is that our warming world adds pressure to a species that already has odds stacked against it. ‘That silky sharks may overlap more with fisheries in warmer waters is important to know, as it highlights how climate change can accelerate declines from overfishing and jeopardise recoveries. For me, this is especially concerning to see in the Red Sea, where my personal journey with sharks shifted from awe to conservation action after witnessing severe declines first hand. There have been some recent signs of hope as sightings appear to have increased in the region, but based on what we’ve learned here, climate change may just make that hope evaporate.’

Collin is using this as cause to think of the bigger picture; only if we know what we’re dealing with, can we respond appropriately. ‘To inform broader management strategies, we now need to better understand how sustained surface temperatures above 30°C influence shark depth-use in other regions with different oceanographic regimes.’

Reference

Williams C, McIvor A, Richardson E, Lea J, Clarke C, Cochran J, Kattan A, Ormond R, Berumen M. 2025. Elevated sea surface temperatures drive greater seasonal depth use in a baited aggregation of silky sharks Carcharhinus falciformis. Marine Ecology Progress Series 773: 115–128