More than 200 species of sharks appear on the IUCN Red List, ranging in status from critically endangered to near threatened. Scientists suspect some shark populations have declined by as much as 90 percent. Loss of these apex predators ripples through entire marine ecosystems; some coral reefs have died after sharks were fished out. Declines also hurt the commercial and recreational fisheries and tourism industries.
In order to protect sharks, though, scientists need to know where they hang out and the routes they travel in the world’s oceans. One of the best ways to find out is to tag and track the animals.
Scientists at Mote Marine lab in Florida have put tags on more than 12,000 sharks to date. Recently, Robert Hueter, director of Mote’s Center for Shark Research, and scientists from New College of Florida, Eckerd College, and The Nature Conservancy spent five days tagging another 34 sharks off the west coast of Florida. Tagging sharks is, essentially, fishing, but more like the commercial rather than recreational kind in terms of the amount of work involved.
On the cramped back deck of a research vessel, the team spends the first half of each day setting lines. One person baits large circle hooks — easier to remove from a caught shark than conventional J-shaped ones — and another attaches them to a sturdy longline spooling off a motorized winch under the watchful eye of a third person. Each day, between 60 and 80 hooks go into the water, 399 in all on this trip.
The second half of the day involves checking those lines, running the process in reverse, pulling in the hooks, and removing any bait left, pausing each time a shark turns up on the line. Working with a large, square net, Hueter and colleagues lift the catch onto the boat, placing a modified hose in the shark’s mouth to push water through its gills. It takes several people to hold a shark still while the team collects blood samples and measurements and applies tags before removing the hook and releasing the animal.
The smell of bait and the wind-driven seas can easily turn the stomach. Those handling the sharks end up with hands bloodied by the animal’s rough skin. Everyone gets wet. The team works as quickly as possible with no breaks, as sharks don’t do well hanging on lines. In fact, occasionally one doesn’t make it, but the value of data to be gleaned from tagging makes it worth that risk. Such mortality also provides a valuable lesson for those engaging in catch-and-release shark fishing, Hueter points out, stressing the importance of handling the animals as little as possible.
“If you catch a hammerhead and bring it onto the beach to pass around for selfies before you release it,” he says, “that shark probably won’t make it.”
As with all fishing, success varies. One day, the team tags 17 sharks, the next day, only three. Hueter calls the expedition’s total catch of 54 sharks a positive sign, though. In the 1980s and 90s, fisheries data showed significant declines in large coastal and oceanic shark populations due to intense fishing pressure. In 2003, scientists estimated that scalloped hammerhead, white, and thresher shark populations dropped more than 75 percent in the previous 15 years alone. Recent data indicate that some depleted populations are on the rebound, though, Hueter reports, and this expedition may help confirm those trends.
The team fitted 34 sharks with a conventional or dart tag, a numbered streamer attached to a small dart inserted into muscle near the dorsal fin. These only cost about 50 cents per tag and are easy to apply (so easily, in fact, that NOAA provides them to recreational fishermen). These tags only provide data when tagged sharks are recaptured, though, and then only point-to-point tracks from place of application to wherever recreational or commercial fishermen made the catch. With roughly a 4 percent return rate, it takes a lot of conventional tags to generate useful data. (Anyone who sees or catches a tagged shark is asked to report the details to NOAA.)
In addition, the expedition fitted several bull sharks with acoustic tags, which send out signals picked up by strategically placed receivers. Receivers at entrances to several Florida bays, for example, will show whether bull sharks go into those bays. In 2008 acoustic tags provided the first proof that individual whale sharks move between Belize and the northern Gulf of Mexico, an important finding for efforts to protect that endangered species.
The scientists also deployed two satellite tags, one on a tiger shark and another on a bull shark. These tags transmit their location to satellites, making almost real-time tracking of the animal possible. They only transmit when above the water, though, so work best on species that spend more time near the surface, such as tiger, great white, bull, whale, or basking sharks. These tags cost about $1,500 each and were funded by The Nature Conservancy of Texas.
Research institutions have standard procedures for capture, tagging, handling, sampling, and release of animals, including sharks. “Once a shark is caught, we work very quickly,” says Hueter. “We don’t like to handle the animal any more than we have to.”
After all, the overall goal of all this work is to help sharks, by providing resource managers and policy makers with data to help them determine what areas to protect and where to limit fishing. For scientists, the possibility of more and healthier sharks makes the grunt work of tagging them worth it.