420 Whale Sharks Swarm Mexican Coastline

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(Whale shark aggregation off the coast of Mexico; Credit for images: Oscar Reyes)

Up to 420 whale sharks gathered off the coast of the Yucatán Peninsula in Mexico, forming the world's largest known assembly of this species, according to a press release issued by the Smithsonian National Zoological Park.

The discovery counters the widely held belief that whale sharks, which can weigh more than 79,000 pounds, are solitary filter feeders that prefer to be alone in the open ocean. The impressive shark assembly proves they will gather for the right reasons. Food now appears to be the draw.

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“Whale sharks are the largest species of fish in the world, yet they mostly feed on the smallest organisms in the ocean, such as zooplankton,” Mike Maslanka, biologist at the Smithsonian Conservation Biology Institute and head of the Department of Nutrition Sciences, says in the press release. “Our research revealed that in this case the hundreds of whale sharks had gathered to feed on dense patches of fish eggs.”

Maslanka and his team identified the whale shark assembly using both surface and aerial surveys. Considering these sharks can grow to more than 40 feet long, the surface-level surveying must have been extraordinary.

In spite of their enormous size, whale sharks are not aggressive and move very slowly. Usually they're seen in the ocean with their up to 5-foot-wide mouths open, waiting for food to float in. These sharks occur in all tropical and subtropical ocean regions around the globe.

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Researchers are calling the huge shark assembly the "Afuera" aggregation. As part of the study, the scientists used fine nets to collect food samples inside and around the school of feeding whale sharks. Tests determined that the coveted fish eggs were from little tunny (Euthynnus alletteratus), a member of the mackerel family.

“Having DNA barcoding is an incredibly valuable resource for this research,” says Lee Weigt, head of the Laboratories of Analytical Biology at the Smithsonian’s National Museum of Natural History. “It not only allowed us to know what exactly this huge aggregation of whale sharks were feeding on, not readily done from only physical observations of eggs, but it also revealed a previously unknown spawning ground for little tunny.”

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In addition to the "Afuera," the scientists found yet another, less dense aggregation of whale sharks off the northern tip of the Yucatán Peninsula. This second collection was named the Cabo Catoche aggregation. In this region, the draw for the sharks mostly consisted of small crustaceans and shrimp. Both groups of sharks had the same sex ratio, indicating that entire shark families must have "heard" nature's dinner bells.

“With two significant whale shark aggregation areas and at the very least one active spawning ground for little tunny, the northeastern Yucatán marine region is a critical habitat that deserves more concerted conservation effort,” Maslanka concludes, noting that whale sharks are listed as "vulnerable" by the International Union for Conservation of Nature, primarily due to harpoon fisheries in Southeast Asia and likely incidental capture in other fisheries.