Oceans cover 70% of our planet, extending for hundreds of millions of kilometres. Despite their vastness, oceans have not escaped the effects of human activity, and evidence has steadily accumulated in recent decades that disturbances such as overexploitation1, plastic pollution2 and climate change3 have had major negative consequences for marine life. Writing in Nature, Queiroz et al.4 add another dimension to this story by showing that vessels fishing on the high seas (the regions of oceans beyond national boundaries) overlap substantially with areas of the ocean that are frequented by sharks, leaving these wide-ranging animals with scant refuge from fishing pressure.
As some of the oceans’ fiercest predators, sharks were once presumed to be safe from overfishing5. Yet when shark-targeted commercial fisheries were developed in the mid-twentieth century, this presumption was soon proved incorrect. Most of these fisheries underwent a swift cycle of boom and bust, lasting only a decade or so before shark populations plummeted and the fisheries collapsed6.
The expansion of industrial fishing across the high seas in the latter half of the twentieth century subjected sharks to another threat. Most of those fisheries target tuna and billfish (such as swordfish). These fast-moving fishes have high population growth rates, which allow them to withstand greater fishing pressures than the sharks that are taken alongside them as by-catch (species caught unintentionally) or as secondary targets. Despite the risk of overfishing sharks, regional fisheries-management organizations have been reluctant to develop management plans or catch limits for sharks, and have little incentive to collect data that could be used to demonstrate the negative effects that fishing is having on these species.
However, assessments of available regional data have reinforced concerns about sharks, painting a stark picture of populations that have declined precipitously7,8. Sharks, along with their relatives, are now thought to be one of the most threatened groups of marine species, with one-third of them assessed as being at risk of extinction9. Nevertheless, the patchy availability of fisheries-dependent data has meant that the full extent to which sharks interact with fishing fleets on the high seas — and the impacts of these fisheries on them — has remained unknown.
Scientists are increasingly using satellite-derived data to fill in such knowledge gaps about the human ‘footprint’ in the world’s oceans. For example, the automatic identification system (AIS) — a locator system used by many boats as a safety feature to prevent collisions — provides data that enable boat movements to be monitored globally. Analyses of AIS data have revealed that fishing-vessel tracks are found across much of the oceans10.
Queiroz and colleagues paired AIS data with satellite-tracked movements of 1,681 tagged sharks to provide a global estimate of the extent to which areas of the ocean frequented by sharks overlap with active zones of industrial fishing. Focusing on vessels using fishing gear called pelagic longlines, which are responsible for the majority of catches of oceanic sharks globally11, the authors report that almost one-quarter of the average space that individual sharks move through monthly overlaps with the footprint of these fleets.
White sharks (Carcharodon carcharias) and porbeagles (Lamna nasus) are listed as being at risk of extinction on the International Union for Conservation of Nature’s Red List of Threatened Species. Worryingly, of the shark species studied by the authors, these two had some of the greatest overlap between the areas they prefer and those targeted by the longline fleets. Spatial overlap between the locations of fishing vessels and sharks was also high for commercially valuable shortfin mako (Isurus oxyrinchus) and blue (Prionace glauca) sharks (Fig. 1).
Underlying the high degree of spatial overlap between sharks and industrial fishing vessels is the mutual targeting of areas of the oceans that attract fish because of their favourable productivity and temperature profiles. Unsurprisingly, congregating in such areas enables both the fishing vessels and the sharks to enhance their catch rates.