Scientists amazed by large groups of feeding whale sharks

It was a rare and amazing sight for marine biologists: hundreds of whale sharks congregated in waters off the east tip of the Yucatán Peninsula. In 2009, scientists were astonished to find about 420 whale sharks in a single aerial survey, within an elliptical patch of ocean just 3 by 6 kilometers (1.9 by 3.7 miles). It was the largest-ever congregation of whale sharks ever observed.

What were the fish doing there? The answer led to yet another discovery. The whale sharks, which are filter feeders, were feasting on an enormous abundance of tiny eggs from a fish called the little tunny, an Atlantic tuna species that, until then, had not been known to spawn in these waters on such a large scale.

A team led by Rafael de la Parra Venegas, at Proyecto Dominó, Comisión Nacional de Áreas Naturales Protegidas at Quintana Roo, Mexico, documented their discoveries in a paper published in the online journal PLoS ONE.

The top photo shows 220 whale sharks (and four tourist boats) from 600 meters (1,968.5 feet) above the water. The photo below was taken at a lower altitude, showing 68 whale sharks, one tourist boat, and two snorkeling tourists. Image Credit: Al Dove/Georgia Aquarium.

Marine biologist and paper co-author Al Dove, at the Georgia Aquarium, described it in his blog as “hands-down the coolest biological phenomenon I’ve ever seen..” While the scientific paper he and his colleague produced is fascinating, it is a formal just-the-facts description of the phenomenon. If you want to get a taste of the scientists’ excitement and amazement over this discovery, as well as some stunning photos of whale sharks, check out Dove’s blog.

In this video, Al Dove is seen photo-documenting whale sharks.

Whale sharks, also known by their taxonomic name Rhincodon typus, are found in tropical and subtropical oceans throughout the world. (There are just two other filter-feeding sharks, megamouth and basking shark.) They hold the distinction of being the largest fish species in the world. Adults are thought to be around 9 meters (29.5 feet) in length and larger; the biggest specimen ever caught was 12.65 meters (41.50 ft) long and weighed over 21.5 tonnes (47,000 lb), found in the Indian Ocean off Pakistan in 1947.

Female whale sharks give birth to live pups, each about 40 to 60 centimeters (16 to 24 inches) in length. Juveniles are thought to reach sexual maturity around 30 years of age. Whale sharks have a long lifespan, about 70 years, perhaps longer.

Most sharks we’re familiar with, like the great white shark, are finely-tuned hunters that feed on large prey. In contrast, whale sharks are docile giants that feed by filtering plankton and small fish out of the water. It’s done using a sieve-like structure, small teeth-like scales, that line the gills and part of the throat. Whale sharks swallow a mouthful of water and moments later, expel it out their gills. During this action, plankton are trapped in the sieve-like filters, then swallowed.

Whale sharks, generally thought to be solitary creatures, sometimes congregate to feed on localized bounties of eggs from fish or coral spawning, and at waters abundant in zooplankton.

Videographer Bruce Carlson captured this hauntingly beautiful footage of whale sharks.

There are two locations in the waters off the Yucatán Peninsula of Mexico where whale sharks gather to feed. Close to shore, in turbid waters tinged with green off Cabo Catoche (at the northernmost point of the peninsula), whale sharks are found in large numbers feeding on abundantly available crustacean zooplankton. The sharks arrive in early May, peak in numbers between late July and mid-August, then disperse in mid-September.

The second location, known as “Afuera,” is in waters off the east tip of the Yucatán Peninsula. The arrival and departure schedule of the whale sharks is similar to the Cabo Catoche site. Here, aggregations of whale sharks were first scientifically recorded in 2006, though fishermen had long known about it. In 2007, for reasons unknown, the sharks were not seen. But they reappeared in 2008.

It was the events of 2009 that stunned marine biologists: the numbers of fish kept rising till on August 12th, up to 420 whale sharks were seen during one aerial survey. It was the largest whale shark gathering ever observed.

But what were they eating in those clear blue waters? A towing net was dragged through the water to obtain samples. The scientists found an abundant supply of clear fish eggs – which explained why the waters looked clear blue – in such large quantities that the towing net would get clogged with eggs in less than 40 seconds. DNA analysis of the egg samples indicated they came from a tuna species called little tunny (Euthynnus alletteratus). That in itself was a discovery since little tunny were not known to breed at that location. The biologists speculate that 2009 must have been a good year for little tunny spawning since it appears that some whale sharks at nearby Cabo Catoche waters also showed up to feed at Afuera.

“Vertical Feeding” by whale sharks. Just … WOW!

There is a rich diversity of marine life in waters surrounding the northeast coast of the Yucatán Peninsula. Billfish, dolphin fish, tuna, groupers, and snappers are common, and scientists recently learned that besides whale sharks, manta rays, devil rays, cownose rays, and sea turtles also gather to feed in these waters.

This richness in marine life is due to a tropical upwelling that brings nutrients from deep waters to the shallower, sunnier waters of the Yucatán shelf where photosynthesizing phytoplankton flourish. Phytoplankton form the base of the marine food chain that attracts, successively up the chain, other creatures in the food chain.

A whale shark inhales little tunny eggs at the Afuera aggregation in Yucatan Mexico. Image Credit: Al Dove/Georgia Aquarium.

Whale sharks are known to aggregate at two important sites in waters off the Yucatán Peninsula. One of the sites, called Afuera, off the east tip of the Yucatán Peninsula, was where marine biologists observed the largest-known gathering of whale sharks, up to 420 on one single aerial survey in August 2009. The whale sharks were attracted to an enormous quantity of fish eggs in the water, which led to another discovery: the site was also a major spawning area for little tunny, a small species of tuna. To quote the authors of the paper, “Extraordinary biological phenomena of the sort we report here deserve extraordinary conservation measures.

The whale sharks of Quintana Roo, Mexico. Image Credit: Al Dove/Georgia Aquarium.

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