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Whale sharks 15,000 times more likely to be hit by ships in warming oceans
The fate of the world’s largest fish is increasingly at risk as global temperatures rise.
Whale sharks look set to lose much of their food and habitat as climate change drives the fish into new areas and pushes them into the paths of shipping.
Climate change is making the life of the biggest fish in the world more dangerous.
While whale sharks currently live in warm waters around the equator, they are expected to move to cooler regions as the oceans become hotter.
New research predicts that this shift will increasingly bring them into international shipping lanes. As the fish often filter feed near the ocean surface, this will put them at risk of being hit by ships.
The scientists predict that in a high emissions future this will make whale sharks up to 15,000 times more likely to be injured or killed by ship strikes. The western coasts of the USA and Sierra Leone are set to become particular collision hotspots.
Professor David Sims, a co-author of the research from the University of Southampton, adds that the predicted rise in ship strikes on whale sharks will fall sharply if urgent action is taken to address climate change.
“The shifts we predict are likely to be less extreme if we are able to slow warming and mitigate climate change, suggesting that even complex, multi-factor impacts of climate change can be somewhat alleviated by our actions,” David says.
The findings of the research were published in the journal Nature Climate Change.
Ship strikes can see animals hit by ships, caught in propellers or harmed in numerous other ways. © Gonzalo Araujo
Tracking elusive whale sharks
Whale sharks live in all the world’s tropical waters, but their populations are divided in two. Around three quarters of the fish live in the Indian and Pacific Oceans, with the rest living in the Atlantic Ocean.
Though you might think the size of these fish makes them easy to study, surprisingly little is known about their lives. For example, even basic details about their reproduction, breeding grounds and population structure are thin on the ground.
To try and change this scientists have been tracking the animals using satellite tags and combining this with reports from boats. This has helped to identify around 25 ‘aggregation sites’ where whale sharks come together in large numbers. These include areas off the coast of Australia, Mexico and the Atlantic island of St Helena.
In the current study, the researchers worked out the kind of habitats the fish most often frequent by combining 15 years of satellite tracking data from almost 350 whale sharks with details of ocean conditions. This was then used to model where whale sharks might live in three future climate scenarios featuring low, medium and high emissions.
In the worst emissions scenario, almost 60% of countries will lose more than half of their whale shark habitat by 2100. The largest changes will come in the eastern Pacific, where it’s estimated that as much as five million square kilometres of whale shark habitat, an area larger than the EU, will become unsuitable for the fish to live in.
In the Atlantic Ocean, a swathe of ocean stretching from the northern tip of South America across to Angola is expected to become more suitable for the sharks. Dr Freya Wormersley, the lead author of the study, said the degree of the change was ‘unexpected’.
“We expected to see some shifts in whale shark habitat as warmer surface waters expand into to higher latitudes, but the degree of difference between the climate scenarios that we revealed was somewhat larger than anticipated,” Freya says.
“When we saw the maps we quickly realised that the new habitat regions were home to some of the world’s busiest sea ports and shipping highways.”
On the USA’s north Pacific coast, for example, the risk of ships hitting whale sharks will increase by 95 times. While these will be partly offset by declines elsewhere, such as in the Gulf of Mexico, the overall risk of ship strikes are set to rise sharply in a high emissions future.
Even if global temperature rises are limited to a low-emissions scenario, the risk of ships hitting whale sharks will still increase by 20 times. In light of other pressures facing the animals, it’s important to take action now to ensure the survival of this species.
Ship strikes are not just a hazard to whale sharks, and also pose a risk to other large marine animals like whales and dolphins. © Aerial_View /Shutterstock.
Protecting whale sharks
Whale sharks are currently considered Endangered by the IUCN Red List because of severe population declines over the past 75 years. It’s estimated that their numbers have fallen by around 63% in the Indian and Pacific Oceans, and over 30% in the Atlantic. This is due to a combination of fishing bycatch, unsustainable tourism and pollution.
Though these threats are severe, it had been thought that whale sharks were relatively well-placed to bounce back from population declines. Some estimates suggest that the population could fully recover in the next 100 years if the impacts from fishing are brought under control.
The new research calls that into question. As climate change itself is not expected to immediately pose a direct risk to the whale sharks, the increasing levels of ship strikes associated with it are a much more pressing concern.
Freya has called for action to limit the impact of these collisions by putting new mitigation measures in place. This could include reducing the speed of ships passing through whale shark hotspots by up to 75%, which previous research shows would only increase a ship’s journey time by about 5%.
“Making ships travel more slowly in high collision risk areas or designating key aggregation areas as no go zones for vessels at certain times of year will make an important contribution to protecting whale sharks,” Freya says. “Many suggestions on how best to protect whale sharks have already been put forward by the Convention on Migratory Species.”
“Now, nations which are home to this enigmatic species need to implement these suggestions as the issue will not resolve on its own. Our study predicts things will get much worse without action, so the time to act is now.”
