International fleets are putting pressure on the world’s shark populations, whose natural habitat is largely located in industrial fishing areas — where no legal protection exists, according to a new study published in the journal Nature.

Credit: Enrico Strocchi (Flickr)

Researchers mapped the activity of 23 shark species and fishing vessels around the globe. To do that, they tracked up to 1,500 sharks with satellite tags, combining the data with data on ship movements obtained from safety technology used by the vessels. The results help us better understand some behaviors of sharks, but they also shed light on the threats we are causing them.

“This paper shows that pelagic sharks converge on what we call ‘hotspots’ in the oceans, where there are high concentrations of prey,” Prof. Rob Harcourt, who participated in the study, said. “Unfortunately, for the very same reason, fishing fleets head there, too. This makes the sharks more vulnerable.”

The research found that sharks spend between two and six months in high-risk zones annually. They migrate throughout the year, swimming in areas near boundaries in the sea between different water masses that attract sea life. Fishermen also go there, seeking to catch as many fish as possible.

The overlap between the two happens in many areas identified by the study, including the southern Great Barrier Reef, the Gulf Stream, and the California Current. Sharks exploited commercially such as North Atlantic blue sharks are more concentrated in those high-risk zones.

Overall, around 24% of the space used by sharks in an average month overlapped with long-line fisheries, according to the study. When looking at specific species, 76% of the space used in an average month by blue sharks in the North Atlantic overlapped, and 62% in the case of the North Atlantic shortfin mako sharks.

“Tens of millions of these pelagic sharks are being caught by industrialized fisheries in areas where there’s little or no management, and some populations have declined as a result,” says David Sims, a UK-based marine ecologist at the Marine Biological Association in Plymouth, who participated in the study.

The research can now help as a blueprint for where large-scale marine protected areas aimed at conserving sharks could be set. The United Nations is currently working on a high seas treaty for protecting ocean biodiversity, which will consider large-scale marine protected areas for the high seas.

“I really hope that the results from this paper are able to inform whether it’s time-area closures or more dynamic management tools that can allow us to fish for the species that we want to catch and avoid the shark species that are often making our seafood choices unsustainable,” concludes Elliott Hazen, a research ecologist with NOAA.