Shark Files: Shark 'Hotspots' at Risk for Overfishing

Sharks in key oceanic zones have less and less room free of fishing vessels, a new study finds.

An international team of researchers that tracked sharks by satellite has found that so-called "hotspots" of shark activity are at a far greater risk of being overfished than previously imagined.

According to research just published in the journal Proceedings of the National Academy of Sciences by a team out of the United Kingdom, United States, Spain and Portugal, when more than 100 sharks from six different species were tracked and their paths compared against the GPS movements of nearly 200 fishing vessels, the sharks and the vessels were overwhelmingly in the same places.

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While it's not surprising in and of itself to see fishing vessels and fish in the same spot, it was the sheer scale of the overlap that indeed surprised the researchers and pointed toward an increased chance the fish could be taken out of the sea in overly large numbers.

"Although we suspected overlap might be high, we had no idea it would be this high," said lead author Nuno Queiroz, of the University of Porto, Portugal, in a release. "Space-use overlap on this scale potentially increases shark susceptibility to fishing exploitation, which has unknown consequences for populations."

Two of the most-fished sharks - mako and blue - saw around 80 percent of their range overlapped by the presence of fishing vessels, the team observed.

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The researchers found the greatest risk of overfishing was in waters such as the Gulf Stream, the mid-Atlantic ridge southwest of the Azores, and the North Atlantic current/Labrador current convergence zone near Newfoundland.

There is little in the way of regulation regarding oceanic shark fishing, the scientists said, owing in part to a dearth of information about the places where fishing vessels and sharks overlap.

"Many studies have tracked sharks, and many studies have tracked fishing vessels, but fine-scale tracking of sharks and fishing vessels together is lacking, even though this should better inform how shark fisheries should be regulated," said senior author David Sims, of the University of Southampton.

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With tens of millions of sharks already being hauled from the sea on a yearly basis, the authors suggest that catch quotas or size limits be instated in the hotspots.

"This highlights how broadly the fishing exploitation efficiently ‘tracks' oceanic sharks within their hotspots year-round," Sims said. "In the North Atlantic, it seems threatened sharks have few places left to hide in the face of industrialized, high-seas fishing of the last 50 years."

Great whites and other sharks travel on "ocean highways," according to new research that is helping to define the routes. Mapping out the routes is enabling scientists to better understand the behavior of these apex ocean predators and to improve conservation tactics. Ongoing research also shows how busy both the sharks and these highways can be. "The sharks do show repeated travels along the same general paths, which is quite amazing since there are few 'road signs' in the open ocean," Mahmood Shivji, director of Nova Southeastern University's Guy Harvey Research Institute and the Save Our Seas Shark Research Center, told Discovery News. As for how the sharks know where to go, he added, "The main current hypothesis is that they use the earth's magnetic field to navigate, like some birds do." Great whites are among the most well-traveled sharks, swimming for thousands of miles with relative ease. In the winter, they migrate from the California and Baja coasts to a mid-Pacific open water area dubbed "The White Shark Café." Precisely why great sharks do this, however, remains a mystery.

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For the research projects, Shivji and his team are affixing satellite tags to multiple shark species. The tagging itself can be arduous, since the scientists are so close to the toothy predators. "This type of tracking research needs to be long-term to really reveal and understand patterns of shark migrations," he said. "Such long-term studies are difficult and expensive to conduct, in part because working on the ocean and with big animals poses considerable challenges."

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Once tagged with a satellite-tracking device, the sharks often need a bit of stimulation to get them back on their particular ocean route. This can be a dangerous hands-on process, as this researcher demonstrates.

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Shortfin mako sharks continue to astound researchers, given the duration and miles involved in their migrations. "Our long-term satellite tracking data is showing that individuals can travel over 10,000 miles in a single year, and they also are starting to show indications of round-trip, repeated and predictable migration patterns," Shivji said.

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Along their highway-like migration routes, sharks seem to primarily stop for three things: food, sex and babies. Here, lemon sharks stop for their babies. Research conducted in Bimini in The Bahamas, spanning almost two decades, shows that female lemon sharks that were born there returned 15 years later to give birth to their own young. Some sharks never seem to forget home sweet home, even if it is a temporary safe-feeling spot before traveling begins again.

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Tiger sharks have been in the news lately because Shivji and his colleagues determined that these sharks undergo remarkable, bird-like migrations. The findings were published in the journal Scientific Reports. "The tiger shark migrations are bird-like in the sense that they seem to be following the same general round-trip, repeated migrations from warm over-wintering areas to temperate summer areas," he said. One tiger shark, named Harry Lindo, was tracked traveling more than 27,000 miles, possibly the longest-ever recorded distance traveled by any species of shark. "It is truly remarkable," Guy Harvey, a renowned marine artist, conservationist and fisheries ecologist said. Harvey has been working with Shivji on the shark tagging projects. Brad Wetherbee, another researcher on the team, noted that tiger sharks seem to have their own "café" as well, except they have given it a different name. "We joke that tiger sharks, not being media stars like great white sharks, wouldn't be comfortable in a 'café' and prefer to hang out in their 'truck stop' in the mid-Atlantic," Wetherbee, who is based at the University of Rhode Island, said.

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As they travel, sharks may go solo or with companions. Here, an oceanic whitetrip journeys on its own. This species has been in decline around the world due to overfishing, the researchers said. Oceanic whitetip sharks used to be one of the most common sharks seen close to the water's surface, but that's no longer the case. In terms of its journeys, Shivji said, "The initial data are suggesting that, although they are highly migratory, they don't travel as far out into the open ocean as the shortfin makos and tiger sharks do in the Atlantic -- an unexpected finding."

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This oceanic whitetip shark is definitely not traveling solo. Accompanying it are rainbow runners and pilot fish. The shark serves as a moving bodyguard for the much smaller, weaker fish. Oceanic whitetips, in turn, benefit from the fishes' presence, because pilot fish eat health-threatening parasites, dead skin and other undesirables off of the sharks. This kind of win-win arrangement is called "a mutualistic relationship."

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Sand tiger sharks live very deep in the ocean, unlike more surface dwelling, or "pelagic," tiger sharks, shortfin makos and oceanic whitetips. In fact, sand tiger sharks prefer to stay close to the bottom of ocean floors. "Migrations are seasonal in the western Atlantic, with migrations along the coast into cooler waters in the summer and warmer waters in the winter," Shivji said.

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Caribbean reef sharks also undergo migrations, with females traveling to "nursery" waters northwest of the Brazilian coast. These sharks, dubbed "the silent sea predators," are formidable hunters, but thankfully pose little threat to humans because they tend to avoid interactions with us. In this case, a bait ball containing fish and other edibles was intentionally dropped into the Caribbean reef shark's territory, leading to a feeding frenzy. Shivji said that tracking all migratory sharks poses major challenges, especially because sharks cover so much territory. This further poses conservation challenges, since waters off the coasts of multiple countries are involved, all linking up into the shark highways. "The shark long-term tracking will show where and when they spend the most time during their travels on a predictable basis," Shivji said. "Once we know this, fishery regulation and protection become more tractable since they provide geographic areas to focus management and conservation efforts on."

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