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Whale shark ship-strike risk spots identified

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Whale shark (Derek Keats)
Whale shark (Derek Keats)
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Some of the world’s busiest shipping lanes pass through whale shark feeding grounds, posing a major threat to the Endangered species – but now the worst hazard spots have been mapped out by a team of researchers led by the UK's University of Southampton, hoping that it will help in tackling the problem.

Whale sharks are mainly solitary, but when they assemble to feed in high densities at what are known as “constellation” sites they are particularly vulnerable to ship-strikes. 

When the researchers overlaid the locations of constellations with shipping traffic data, they found that the whale sharks were most vulnerable to ship-strikes off the Ecuador coast; Isla Mujeres and La Paz in Mexico; Ewing Bank in the northern Gulf of Mexico; Kota Kinabalu and Redang Island in Malaysia; Pintuyan in the Philippines; Musandam in Oman and around Seychelles and Taiwan.

At 39 sites, peaks in shipping activity coincided with peak seasonal occurrences of whale sharks, sometimes lasting for several months.

“Our findings highlight the need for targeted measures within these areas to reduce the risk of collision and improve the conservation status of endangered whale sharks,” said lead author Dr Freya Womersley, who works at the university and with the Marine Research & Conservation Foundation (MARECO) and Marine Biological Association (MBA). These bodies and the Marine Megafauna Foundation (MMF) were all involved in the new study.

Negatively buoyant

The size of the world’s merchant fleet has doubled over the past 16 years, with more than 100,000 ships now transporting goods around the globe. This figure is expected to grow by as much as 1,200% over the next 30 years.

More than 75 marine species are considered at high risk from ship-strikes, and especially the whale sharks that spend almost half their time in surface waters, often in busy coastal areas.

“Collisions with large ships are likely to be fatal for whale sharks, but evidence is scarce,” said MARECO director Dr Gonzalo Araujo. “That’s because whale sharks are slightly negatively buoyant, so their bodies sink. To inform conservation efforts, it’s important to quantify collision-related threats, even when direct evidence is lacking.”

The researchers surveyed more than 75 specialists to identify those “core habitats” in which they had encountered the most whale sharks, as well as “buffer zones” in which they had been spotted.

This process identified 107 areas in 26 countries, covering observations of more than 13,000 individual whale sharks – more than half of all those identified to date. The data was overlaid with that on large ship positions provided by Global Fishing Watch.

“Many of these sites had more than one vessel per square kilometre in core habitats,” said MMF principal scientist Dr Chris Rohner. “For example, the constellation in Isla Mujeres in Mexico has an average of 56 ships passing through the core habitat monthly. These sites require urgent action to reduce the threats posed by shipping.”

According to the study, some of the experts had under-estimated the threat posed by large-ship collisions in constellations because of the lack of evidence of injuries, or the sort of witness accounts available in relation to tourism interactions or small vessel collisions.

Speed limits

When the researchers simulated vessel movements in the constellation at Ewing Bank, they found that reducing vessels’ speed by 75% reduced the risk for the whale sharks at what was regarded as a small cost to shipping and an increase of about 5% in transit time

“One of the benefits of speed reductions is that they can be temporarily introduced during whale shark peak seasons,” said Womersley. “These speed limits can also be applied to smaller vessels, which are less deadly but can still damage the sharks.”

Rerouting ships around core habitats had even less impact – about 0.5% additional transit time (2.4 hours per vessel) and a 1.1% average increase in distance travelled.

“Rerouting is the most direct way to reduce the risk of collision and our results suggest that this will often be more cost-effective than speed reduction, mainly because whale shark core sites are small,” said Araujo.

“Movements of as little as 12 nautical miles away from a core whale shark habitat could mean that fast-transiting ships avoid the site entirely.”

The open-access study is published in Science Of The Total Environment.

Also read: Increase in Whale Shark Injuries, Galapagos Whale Shark Research

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