Tens of millions of oceanic sharks are caught and killed by fishing vessels every year. Originally they were simply seen as bycatch, as the fleets in the Atlantic Ocean were looking for other species such as tuna, but now they have become a target in their own right. And yet the shark fishing in these waters remains largely unregulated and little is known about what species of oceanic shark tend to move where, hampering the efforts to manage and conserve them.
A new study has, however, at least begun to answer some of these questions. By tracking both the sharks that are targeted and the longline fishing vessels that catch them, they have been able to map out where the sharks' range and the fisheries overlap, and which species are most at risk. Longline fishing involves, as the name suggests, laying down hundreds of kilometers worth of line with thousands of hooks attached. 
The research, carried out by a team from the University of Miami, involved tagging and tracking over one hundred sharks from six different species throughout the Atlantic over a period of four years. While the most common species tagged was the blue shark, they also caught shortfin makos, longfin makos, tigers, great hammerheads, and scalloped hammerheads. In addition to this, during the same period of time they also tracked two entire Portuguese and Spanish longline fishing fleets using onboard GPS.
What they found was worrying. For two of the most heavily caught species of shark in the Atlantic, the blue and mako, the researchers found that the commercial fisheries overlapped with 80 percent of their known range, putting the majority of the population at risk from overfishing. Not only that, but some individual sharks were found to be spending around 60 percent of their time around the longline fishing vessels, further increasing the chance that they would be caught.
“Although we suspected overlap might be high, we had no idea it would be this high,” says Nuno Queiroz, the lead author of the study published in the Proceedings of the National Academy of Sciences. “Space-use overlap on this scale potentially increases shark susceptibility to fishing exploitation, which has unknown consequences for populations.”
They found that sharks from all the species gathered in various “hotspots” that were characterized by thermal fronts in the ocean and areas of productivity. The longline fishing vessels are targeting these areas, effectively tracking the shark’s movements as they seasonally migrate from hotspot to hotspot. This is worrying because the boats and the sharks return to the same areas year on year, increasing the probability that the populations will be severely depleted or even eradicated.
The researchers suggest that, considering that there are still currently no quotas on the number of sharks that can be fished in the Atlantic, this should be put in place. In addition, due to the considerable amount of overlap between the sharks' range and the fisheries, there should also probably be some no-take zones, or Marine Protected Zones, where the animals are free from the threat of fishing.
Image in text: A blue shark on a commercial longline in the Altlantic. Marine Biological Association.
