KINGSTON, R.I. — June 11, 2015—Tiger sharks are among the largest and most recognizable sharks on the planet, yet many of their habits remain mysterious because they are long-distance travelers that are difficult to track. But a new study co-led by a University of Rhode Island biologist, reported in this week’s issue of the journal Scientific Reports, has yielded the first long-term satellite tracking of the animals, revealing previously unknown migration patterns that are more similar to birds, turtles and marine mammals than other fish.
According to URI shark researcher Brad Wetherbee, tiger sharks have long been believed to be a mainly coastal species. But the sharks he and his colleagues from the Guy Harvey Research Institute at Nova Southeastern University tracked made 7,500 kilometer, round-trip journeys every year between two vastly different ecosystems — the coral reefs of the Caribbean and the open waters of the North Atlantic. And they returned reliably to the same areas each year, a discovery with significant conservation implications.
“Our success in tracking tiger sharks over multiple years has demonstrated a number of aspects of their ecology that are both surprising and unprecedented,” said Wetherbee. “They undergo extensive movements on a very broad scale and yet have remarkably consistent behaviors from one year to the next. Rather than just observing behaviors once, as is common in many studies, our research greatly expands understanding of movement ecology of highly migratory species on a long-term basis, which is crucial for the most effective management of marine animals.”
The details of the movements and migrations of tiger sharks and most other large shark species have been a mystery because they are difficult to track for more than a few months due to the limitations of satellite tags and other logistics. For this project, the tags the team attached to sharks near Bermuda lasted in many cases more than two years and in some cases more than three years.
One tiger shark the researchers named Harry Lindo traveled more than 44,000 kilometers, the longest distance documented for a tiger shark and possibly the longest ever published for a shark.
The researchers showed that adult male tiger sharks in the Atlantic spend their winters in Caribbean island locales, including the Bahamas, Turks and Caicos Islands, and Anguilla. During summer, they travel into the North Atlantic, often more than 3,500 kilometers and as far north as southern New England, though well offshore in nearly the middle of the ocean.
“These repeated journeys were very unexpected,” said co-author James Lea of the Marine Biological Association of the United Kingdom. “The tiger shark has traditionally been considered a coastal species, and it is rare among sharks to so easily and habitually switch between the two vastly different environments.”
Remarkably, the sharks followed the same pattern each year and returned to almost the same small area in the Caribbean each time. “Even though they’ve got a whole range of islands to choose from,” said co-author Mahmood Shivji, “it seems like each animal has its favorite winter spot.”
What makes the tiger sharks so committed to particular areas is still an open question. At the south end, the story may be fairly simple. Female tiger sharks are common in the Caribbean in the winter, so the Caribbean may just be the best place for male tiger sharks to find mates. Why they go so far north is more complicated. How far they go seems to be guided by avoidance of colder temperatures —information that offers one example of how migration research can help to predict the implications of climate change for sharks.
Wetherbee said that there must be something beneficial to the animals in the mid-Atlantic that makes it worthwhile for them to make the long trek north. One possibility is that they go to feed on young loggerhead turtles that also migrate north — indeed when the researchers examined stomachs of some tiger sharks killed by long liners in the region, they found turtle remains. But there are also turtles much farther south.
The only other instance when researchers have found a broadly similar, repeated migration pattern between coastal and distant open water regions is with the warm-bodied, great white and salmon sharks in the Pacific. White sharks migrate in the winter from the California and Baja coasts to a mid-Pacific open water area dubbed the White Shark Café. “We joke that tiger sharks, not being media stars like white sharks, wouldn’t be comfortable in a ‘café’ and prefer to hang out in their ‘truck-stop’ in the mid-Atlantic,” said Wetherbee.
The new research has major implications for conservation. “Understanding how these animals use the oceans is the first step toward effective conservation,” said co-author Guy Harvey. “Protecting migratory species is a great challenge because they can be found in such a wide area. But, protecting the areas where animals such as tiger sharks spend the most time is a tractable goal once those areas have been identified.”
All of the satellite tracks for tiger sharks in this study, as well as ongoing tracks for other species including mako sharks, oceanic whitetip sharks and marlins, can be found at: www.ghritracking.org.
Tiger shark. (Photo by Nick Filmalter/Danah Divers)