A new study published Aug. 9 in the journal Marine Ecology Progress Series that used chemical signatures found in whale shark tissue, combined with photographic identification, revealed that most members of the species, Rhincodon typus, don’t swim more than a few hundred kilometers from a specific feeding site, at least when they’re young.
“Whale sharks are fully capable of swimming across oceans, but it seems like the juveniles, at least, are choosing not to,” Simon Pierce, a marine conservation biologist with the Marine Megafauna Foundation in California and one of the study’s authors, said in a statement. “They like coming back to the same sites each year to take advantage of predictable feeding opportunities.”
Lead author Clare Prebble, a marine biologist at the University of Southhampton in the U.K., said whale sharks could swim 10,000 kilometers (6,200 miles) or more each year. And studies on the genetic makeup of whale shark populations hint that some sharks, at least at some point in their lives, peel off from the known gatherings of hundreds of sharks around favorite feeding sites to mix with other populations.
Thus far, though, scientists have struggled to use tags to track individuals over long distances or time periods. One team had a breakthrough earlier this year when it traced the movements of a female on a two-and-a-half-year journey of 20,142 kilometers (12,516 miles).
Prebble and her colleagues decided to look at the ratio of different forms, or isotopes, of two elements, carbon and nitrogen, in the sharks’ skin and muscle. This sort of “biological passport,” they thought, could help them track where a shark has been. For example, a ratio with beefier isotopes would suggest that a shark has spent more time along the coast than out further into the ocean. The team collected tissue samples from sharks in the waters around Tanzania, Mozambique and Qatar, and compared them to each other.
To backstop the research, they also examined photographs of whale sharks taken at the sites over a 10-year span, which allowed them to identify individual sharks from their unique spot patterns. In all, the team photographed more than 1,200 individual whale sharks.
When they compared the isotope ratios between the populations near the three countries, they found that, generally, the ratios suggested that most of the sharks had stayed within a radius of a few hundred kilometers. The team photographed only two individuals in different locations. Both swam about 2,000 kilometers (1,240 miles) from Mozambique to Tanzania.
Marine ecologist Clive Trueman of the University of Southampton, also a co-author, said the sharks at the study sites were mostly young ones.
“To truly assess how populations are globally structured and distributed, we need to learn more about where the sharks go once they reach adulthood,” Trueman said in the statement. “They may well move out of our sight to feed and breed in deeper offshore waters.”
Still, the study has significant implications for protecting whale sharks, said Prebble, who also works for the Marine Megafauna Foundation.
“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,” she said.
The IUCN listed whale sharks as endangered in 2016. The world’s largest fish faces threats from ship strikes in the Arabian Gulf (also known as the Persian Gulf). It’s also sometimes snared incidentally in nets intended for other species along the coasts of Tanzania and Mozambique.
But some countries are beginning to find that living whale sharks are a boon for the tourism industry. A recent study found a previously unknown “hotspot” in the Indian Ocean around Madagascar, where sharks seem to return to filter feed on plankton. That’s spurred an influx of visitors eager to spend time swimming alongside the sharks, which can grow to 20 meters (66 feet) long. Whale shark-centered tourism benefits from the sharks’ preference for the same feeding grounds.
“Looking on the bright side, that emphasizes that local protection can have a major benefit for the recovery of this endangered species,” Pierce of the Marine Megafauna Foundation said. “The rewards can also be felt locally, with whale shark tourism now worth over $100 million each year around the world.”
Banner image of a whale shark in Tofo, Mozambique © Clare Prebble/Marine Megafauna Foundation/University of Southampton.
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Guzman, H. M., Gomez, C. G., Hearn, A., & Eckert, S. A. (2018). Longest recorded trans-Pacific migration of a whale shark (Rhincodon typus). Marine Biodiversity Records, 11(1), 8.
Pierce, S., & Norman, B. (2016). Rhincodon typus. The IUCN Red List of Threatened Species, 8235(2307–8235), 26.
Prebble, C. E. M., Rohner, C. A., Pierce, S. J., Robinson, D. P., Jaidah, M. Y., Bach, S. S., & Trueman C. N. (2018). Limited latitudinal ranging of juvenile whale sharks in the Western Indian Ocean suggests the existence of regional management units. Marine Ecology Progress Series, 601, 167–183.
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