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‘Biological passports’ help researchers track the world’s biggest fish

The world’s largest fish is something of a homebody, rarely wandering far from its favorite feeding waters, a new study reveals. And that has big implications for efforts to protect endangered whale sharks (Rhincodon typus), which have seen their numbers fall by half over the past 3 decades.

The behemoths typically glide slowly through the world’s oceans, grazing on plankton. They grow to 20 meters long and 40 tons. Previous research revealed they can swim more than 10,000 kilometers in a year, and dive to 2000 meters beneath the surface. And genetic studies suggested the sharks are divided into distinct regional populations.

Those groups appear to be even more distinct than previously realized, researchers report today in Marine Ecology Progress Series. To trace how far whale sharks moved, scientists sifted through nearly 4200 photographs of some 1200 whale sharks taken in three areas in the western Indian Ocean and the Persian Gulf. Each shark has distinctive markings that allow researchers to identify individuals, so they were able to see whether the fish were moving between the three areas.

They also measured isotopes—or forms—of nitrogen and oxygen present in tiny patches of skin taken from sharks living in the three areas. Each region has its own signature ratio of the two isotopes, which is reflected in the plants and animals that live there. So the skin patches essentially became “biological passports,” the researchers note, recording where the sharks had traveled.

Combined, the two sets of information show the sharks—many of which were young males—didn’t travel far: Most swam just a few hundred kilometers from their feeding grounds. Just two made the 2000-kilometer journey from one territory off the coast of Mozambique to another off Tanzania.

The results highlight the need to protect specific shark populations, the researchers write, and not assume that sharks from healthy groups will repopulate lost groups. And shark conservation could have economic benefits, they add, because the chance to see a whale shark has become a big tourist draw in many coastal regions.