Schools of Whale Sharks Off Mexico’s Coast Feed on Fish Eggs

New insights into the world's largest fish and their behavior

Whale sharks (Rhincodon typus) are the largest living species of fish, capable of growing to 41.5 feet in length. They are often thought to be solitary giants, living and feeding in the open ocean. Scientists based at the Smithsonian Institution and their colleagues, however, have now discovered that this is not necessarily the case. Whale sharks can actually be gregarious and amass in the hundreds to feed in coastal waters.

Schools of whale sharks have been witnessed in the past, ranging from several individuals up to a few dozen in number. However this new research, which involved both surface and aerial surveys, has revealed an enormous aggregation of whale sharks — the largest ever reported. There were 420 individuals counted in a group off the coast of the Yucatán Peninsula in Mexico. What brings them together is food.

“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,” explained Mike Maslanka, biologist at the Smithsonian Conservation Biology Institute and head of the Department of Nutrition Sciences. “Our research revealed that in this case, the hundreds of whale sharks there had gathered to feed on dense patches of fish eggs.”

While whale sharks may seem conspicuous, being not just the longest but also the heaviest living fish, weighing more than 39 tons in some cases, there is still much to learn about them. They have a very widespread distribution, occurring in all tropical and sub-tropical oceanic regions around the world. Understanding this filter-feeder’s diet is especially important, since food sources determine much of the whale shark’s movements and whereabouts.

Barcoding Analysis
During the dozens of surface trips that Smithsonian team members made to this group off Mexico, which has become known as the ‘Afuera aggregation,’ they used fine nets to collect food samples inside and immediately outside the school of feeding whale sharks.

Scientists then used DNA barcoding analysis to examine the collected fish eggs to determine the species. They found that the eggs were from little tunny (Euthynnus alletteratus), a member of the mackerel family.

“Having DNA barcoding is an incredibly valuable resource for this research,” confirmed 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, which is very difficult from just a physical observations of eggs, but it also revealed a previously unknown spawning ground for little tunny.”

Shifting Populations
The team of scientists examined a nearby, less dense assembly of whale sharks, described as the ‘Cabo Catoche aggregation’, off the northern tip of the Yucatán Peninsula. They found that the prey of this group mostly consisted of copepods (small crustaceans) and shrimp. Increased sightings at Afuera coincided with decreased sightings at Cabo Catoche, and both groups had the same sex ratio, implying that the same animals were involved in both aggregations.

“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,” said Maslanka.

The whale shark is listed as ‘vulnerable’ with the International Union for Conservation of Nature and Natural Resources. Populations appear to have been depleted by harpoon fisheries in Southeast Asia and perhaps incidental capture in other fisheries over recent years.

Rafael de la Parra Venegas, Robert Hueter, Jaime González Cano, John Tyminski, José Gregorio Remolina, Mike Maslanka, Andrea Ormos, Lee Weigt, Bruce Carlson, Alistair Dove. An Unprecedented Aggregation of Whale Sharks, Rhincodon typus, in Mexican Coastal Waters of the Caribbean Sea. PLoS ONE, 2011; 6 (4). It is online here <10.1371/journal.pone.0018994>


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