Nearly a decade ago, scientists, with the help of local villagers, discovered a large aggregation of whale sharks that gathered from May to mid-September off Mexico’s Yucatán Peninsula. A researcher named Rafael de la Parra founded a group called Proyecto Dominó (because whale sharks have white spots on their backs just like a domino block) in 2003 to protect the sharks, and American scientists soon joined the cause. Environmental reporter Juliet Eilperin wrote this month’s Smithsonian cover story about the researchers’ efforts to learn more about the sharks, adapted from her new book Demon Fish: Travels Through the Hidden World of Sharks.
In 2009, De la Parra and other scientists, including two from the Smithsonian Institution, reported a record 420 whale sharks in an area of just seven square miles. The phenomenon is now called “the Afuera.” (Literally, a gathering of sharks that have come from “outside,” or from distant places.) In a recent study, Smithsonian scientists Mike Maslanka, a biologist at the Smithsonian Conservation Biology Institute and head of the Department of Nutrition Sciences, and Lee Weigt, director of the Laboratories of Analytical Biology at the National Museum of Natural History, provided some insight into what causes the sharks to congregate. It turns out the answer is pretty simple: food.
Whale sharks are the largest fish in the sea, but their food is quite small, so they have to eat a lot of it. “Where the sharks appear seems based on the abundance and type of food available,” says Maslanka. In the area where the Afuera takes place, sharks are feeding on fish eggs, while in other gathering locations, they are drawn to plumes of zooplankton. Maslanka has been down to the Yucatán to work with the sharks five times since 2006. When he first got involved with the project, he was working with two whale sharks at the Georgia Aquarium, but even that did not prepare him for seeing 10 or 20 animals at a time near Isla Holbox off of the Yucatán, or hundreds at a time during the Afuera. “To see an aggregation of such large animals in close proximity is amazing,” says Maslanka.
When Maslanka is at the study site, he uses fine nets to harvest zooplankton or fish eggs. ”We use these samples to determine the nutrient and energy content of what the sharks are consuming. We also observe behavior to assess how long the sharks feed throughout the day and what mode of feeding they are using to provide an estimate of what actual intake may be,” says Maslanka. “At the same time, other folks on the boat are tagging or otherwise identifying individuals, taking water measurements, collecting additional water or tissue samples, filming and doing a variety of other activities. It can be a very busy operation.”
Back in the United States, Lee Weigt analyzes the collected fish eggs. Using DNA barcoding, all the eggs collected to date have been identified as a fish in the mackerel family and called little tunny (Euthynnus alletteratus). The species was not previously known to spawn in the area.
“I think the biggest take-home message so far is the need to protect a larger area within the northeastern Yucatán marine region, based on the known aggregation sites and an apparent tunny spawning ground near or within that region,” says Maslanka. “We still have so much to learn about these animals, their habitat and what habitats and conditions are ultimately linked together based on their extensive migration patterns. We are only just starting to piece that together, and we’re excited to be a part of the discovery process.”