The world’s largest fish roams less than previously thought, new research has found. Using a ‘biological passport’, results show that whale sharks in the western Indian Ocean and the Arabian Gulf rarely swim more than a few hundred kilometers from their feeding grounds, making local action vital to their conservation.
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‘Whale sharks are amazing swimmers, often moving over 10,000km each year, and they can dive to around 2,000m in depth. Biochemical studies tell us more about where they go and what they do when they’re out of our sight,’ said Dr Clare Prebble, who led the research project by the Marine Megafauna Foundation, University of Southampton, and Sharkwatch Arabia.
Photograph: Simon Pierce/Marine Megafauna Foundation and University of Southampton
Previous research indicated that whale sharks mixed within distinct populations in the Indo-Pacific and Atlantic Ocean. This new study used stable isotope analysis to demonstrate that whale sharks feeding at three disparate sites in the Western Indian Ocean (Mozambique and Tanzania) and the Arabian Gulf (Qatar) rarely swim more than a few hundred kilometres north or south from these areas.
Photograph: Clare Prebble/Marine Megafauna Foundation and University of Southampton
The researchers used nitrogen and carbon isotopes to provide a record of the animal’s feeding and movement, giving a ‘biological passport’ for whale sharks.
Photograph: Simon Pierce/Marine Megafauna Foundation and University of Southampton
They also took photographs of the natural markings on each whale shark to identify and track individuals over 10 years. Every whale shark has a unique spot pattern, similar to a human fingerprint. The team recorded 4,197 encounters with 1,240 individual whale sharks in three countries.
Photograph: Simon Pierce/Marine Megafauna Foundation and University of Southampton
Only two sharks moved between sites, both swimming around 2,000 km north from Mozambique to Tanzania. The findings indicate that there are limited movements between these feeding sites over months to years.
Photograph: Clare Prebble/Marine Megafauna Foundation and University of Southampton
The results have implications for the conservation of this endangered species. ‘The best data available suggests that more than half of the world’s whale sharks have been killed since the 1980s. Although the western Indian Ocean remains a global hotspot for the species, even the largest feeding areas only host a few hundred sharks. Our results show that we need to treat each site separately, and ensure good conservation management is in place, as the sharks may not re-populate if they’re impacted by people’s activities,’ Prebble added.
Photograph: Clare Prebble/Marine Megafauna Foundation and University of Southampton
The study stresses the need to protect these filter-feeding sharks at the areas where they come together in numbers, particularly where human pressures are also present. Whale sharks are an incidental catch in coastal gillnets, which are frequently used in Mozambique and Tanzania. The Arabian Gulf is a huge oil shipping area where vessel strikes pose a major threat to the sharks when they are feeding near the surface.
Photograph: Simon Pierce/Marine Megafauna Foundation and University of Southampton
‘Whale sharks are fully capable of swimming across oceans, but it seems like the juveniles, at least, are choosing not to,’ said Dr Simon Pierce, principal scientist at the Marine Megafauna Foundation and a study co-author. “They like coming back to the same sites each year to take advantage of predictable feeding opportunities. Looking on the bright side, that emphasizes that local protection can have a major benefit for the recovery of this endangered species.’
Photograph: Clare Prebble/Marine Megafauna Foundation and University of Southampton
Whale shark tourism is estimated to be worth more than $100m each year around the world.
Photograph: Clare Prebble/Marine Megafauna Foundation and University of Southampton