Largest-ever aggregation of giant fish discovered off the Yucatán Peninsula
By Summit Voice
SUMMIT COUNTY — Far from being solitary behemoths of the sea, whale sharks sometimes gather in large schools to feed on fish eggs or tiny shrimp, according to recently published research by the Smithsonian Institution.
Whale sharks are the largest fish species, but they’ve been less studied than many other types of fish. Until recently, biologists had only seen them gather in groups of a few dozen, but in the past few years, researchers have learned that they can be gregarious, gathering by the hundreds to feed of Mexico’s Yucatán Peninsula.
“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,” said 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 had gathered to feed on dense patches of fish eggs.”
Whale sharks have a very widespread distribution, occurring in all tropical and sub-tropical regions of the ocean around the world. Understanding this filter-feeder’s diet is especially important since food sources determine much of the whale shark’s movement and location.
During the dozens of surface trips that team members made to the aggregation, called 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 and 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,” said 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.”
The team of scientists also examined a nearby, less dense aggregation of whale sharks, known 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.
Filed under: biodiversity, endangered species, Environment, Marine biology, wildlife Tagged: | biodiversity, Environment, marine biology, Smithsonian Institution, Summit County News, whale sharks, Yucatán Peninsula