Mote Marine Laboratory

World-Traveling Whale Sharks

By: Nadine Slimak

When you’re the biggest fish in the sea, most of the world’s ocean is your playground — at least that’s the way it’s starting to look according to information gathered about the whale sharks that aggregate by the hundreds each year off Isla Holbox, Mexico.

These sharks have been the focus of a Mote research and conservation study since 2003. Although fishermen and some local government officials knew that whale sharks gathered there each summer, conservation efforts were just beginning back then and the aggregations had never been studied by scientists.

“We now think that this may be the largest feeding aggregation of whale sharks in the world — or at least the largest known to science,” said Dr. Robert Hueter, director of Mote’s Center for Shark Research and a principal investigator in the project. “At least several hundred whale sharks — and perhaps as many as 1,500 or more — gather here each year from May to September to feed on the plankton that’s associated with a seasonal upwelling. What’s really neat is that we’re seeing the same fish each year. We’re also finding out where they go when they leave Mexico at the end of the season.”

And go they do.

Each year, Mote scientists work with collaborators in Mexico’s Proyecto Domino and the Comisión Nacional de Áreas Naturales Protegidas, the University of South Florida, the Georgia Aquarium and ecotourism guides and fishermen from Quintana Roo, Mexico, to study these aggregations. They gather photos of the fish so the sharks can be identified using the unique patterns of dots on their bodies (whale sharks are called “domino” by the locals because of these dot patterns). Sometimes, genetic samples are collected. They also tag the fish with numbered tags and, in some cases, deploy satellite tags that will give a more in-depth record of a fish’s diving and traveling behavior. The satellite tags are designed to stay on the fish and record information about its location and diving patterns, then release at a pre-programmed date.

“Some of our most exciting findings have come from these satellite tags,” Hueter said. Since 2003, the Mote team has deployed 22 satellite tags on whale sharks in Mexico. In 2007, Hueter tagged a male and two female whale sharks with satellite tags. One was to release after 90 days, another after 120 days and the third after 150 days.

“The male shark’s tag released after 120 days,” Hueter said. “It recorded dives to 1,720 meters — that’s about a mile deep — which is about the deepest dive recorded for a whale shark to date. The female whose tag popped off after 90 days also made deep dives, to just less than a mile. These findings raise some interesting questions about why the sharks dive so deep.”

Just as interesting was the information that was beamed to Hueter’s computer when the third tag popped off the other female whale shark. “She traveled at least 7,213 kilometers from the Yucatan Peninsula through the Caribbean Sea to south of the equator between Brazil and Africa,” he said. “That’s almost 5,000 miles if you drew a straight line from where she was tagged to where the tag popped off. Since she didn’t travel in a straight line, she swam an even greater distance during that
150-day period.”

The information, combined with a genetic analysis done by USF Ph.D. student Andrey Castro, provides more insight into the world-traveling nature of whale sharks, Hueter said. “Andrey’s study showed that there may be a single population of whale sharks in the Atlantic that is distinct from the whale sharks that live in the Indo-Pacific region,” he said. “What that means is that a discussion about the conservation of whale sharks has to be a discussion that crosses many international borders. Clearly these sharks are world travelers.” 

WHEN A TAG COMES BACK

Most satellite tags used in shark research are gone for good once they transmit their data at a pre-programmed time. But once in a while, the tags wash ashore, are found and are returned to some extremely happy scientists.

“It’s almost like winning the lottery when that happens,” says Dr. Robert Hueter, director of Mote’s Center for Shark Research. But it happened in January 2008 when a tag that Hueter had deployed on a nearly 23-foot male whale shark in July 2006 washed up on South Padre Island, Texas.

“Normally, the information we receive from the satellite tags are data summaries,” Hueter says. “But with the tag in hand, we could look at a minute-by-minute recording of what the shark was doing.”

Mote Biologist John Tyminski and Hueter were able to correlate the deep dives with the time of day. “It appears that the deep dives often coincide with sunrise and sunset,” Hueter said. “And the more extreme dives were made in a fairly steady rate of ascent and descent. There are a few possible explanations for why that might be.”

The sharks could be following prey or using the temperature change to get rid of parasites or release body heat they build up at the water’s surface. The dives could also indicate a “metabolic resting” period.

“Sharks don’t sleep the same way we humans do,” Hueter said. “But we know they go through periods when they’re more active and less active. These dives could indicate that whale sharks are resting during these slow and steady descents. It’s a plausible explanation, but we need more information to fully explain this strange diving behavior.”

That information could be forthcoming through external archival tags deployed on three whale sharks in 2007. These tags, which log depth and temperature, need to be physically retrieved so their data can be gathered. Since the sharks may return to the same place each year, that’s pretty likely. “I think the odds of winning another whale shark lottery will be on our side this time,” Hueter said. 

Learn more about: Dr. Hueter's Research

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