Published online 7 May 2009 | Nature | doi:10.1038/news.2009.452

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Basking sharks run for the sun

Satellite tags solve mystery of huge disappearing fish.

Basking sharkBasking sharks winter in a warmer climate.D. Burton / Naturepl.com

Every winter the second biggest fish in the oceans disappear. Now a group of researchers in the United States has found out where some basking sharks take their vacations.

Back in 1954 marine biologists proposed that basking sharks (Cetorhinus maximus) — which are second only to the whale shark in the fish-size league — might make their way to the bottom of the ocean and hibernate through the winter. Despite finding a place in much of the scientific literature on basking sharks, this idea was recently debunked by satellite tags showing they remain active in winter1.

However, only sharks in the northeast Atlantic were tagged in that study. The true wintering place of western sharks remained a mystery.

Now, by placing 25 satellite tags on basking sharks off the coast of Massachusetts, Gregory Skomal of the Massachusetts Division of Marine Fisheries in Oak Bluffs has shown that basking sharks on the west side of the Atlantic swim south for the winter2.

When the tags began transmitting their data back to him, their locations came as something of a surprise, Skomal says. The first one popped up in a not unexpected location, off the coast of North Florida. Later the same day another signal was received from the Cayman Trench, in the Caribbean.

"That floored me," says Skomal. "I said, 'That has to be a malfunctioning tag.'"

After establishing that that tag had travelled attached to a shark, not on board a fishing boat, Skomal was in for another shock.

"Just when I thought I couldn't be any more surprised, I was," he says. One of his tags had surfaced off the coast of Brazil.

Heading south

Eventually 18 tags reappeared, from near New England down to the coast of Brazil. It is the first time basking sharks have been recorded moving into tropical waters.

This is rather different to what has been observed on the northeast side of the Atlantic, where tagging studies have not found basking sharks crossing the Equator.

"What we're seeing is a complete surprise to us, as these animals are not just going offshore, they're going far away," says Skomal.

“What we're seeing is a complete surprise to us.”

Gregory Skomal
Massachusetts Division of Marine Fisheries

In their paper, published in Current Biology, Skomal and his colleagues suggest these migrations might be linked to reproduction, with females seeking a tropical nursery where they can give birth2.

There was no hint of the western sharks' adventurousness in the earlier findings of David Sims of the Marine Biological Association, UK, who led the team behind the northeast Atlantic tagging1.

"What seems clear from the northwest is, from all the tags that Gregory has deployed, it's pretty consistent that animals move south when the water gets colder," he says. "We don't see that in the northeast. The idea there are a good number of individual migrations is a fantastic finding."

But Sims thinks Skomal's sharks are more likely to be searching for plankton to feed on than looking for love.

Global conservation

Key questions remain: why do the northeast Atlantic sharks differ in their holiday plans from their transatlantic cousins? What are they actually doing on their trips: gorging on foreign food or indulging in a holiday romance?

One thing that is clear, say both Sims and Skomal, is that the research shows that basking sharks — which are listed as 'vulnerable' on the International Union for Conservation of Nature (IUCN) 'red list' of endangered species — travel internationally and therefore need global protection.

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"It throws into sharp focus that there needs to be more international attention on the protection of these animals," says Sims.

Sarah Fowler, outgoing chair of the IUCN shark specialist group, said that these "fantastic results" reinforce the importance of the international Convention on the Conservation of Migratory Species of Wild Animals (CMS).

She says that finalizing a memorandum of understanding and action plan to conserve migratory sharks under the convention is vital.

"This instrument will enable range states [with jurisdiction over the shark's waters] to coordinate actions for the conservation of the basking shark, which is listed on the appendices of CMS," says Fowler, managing director of NatureBureau International, a UK-based ecological consultancy. "We hope that the memorandum of understanding will be completed and opened for signature at the end of this year." 

  • References

    1. Sims, D., Southall, E. J., Richardson, A. J., Reid, P. C. & Metcalfe, J. D. Mar. Ecol. Prog. Ser. 248, 187–196 (2003). | Article
    2. Skomal, G. et al. Curr. Biol. advance online publication, doi:10.1016/j.cub.2009.04.019 (2009).
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