Over the past half century, oceanic, or open-ocean, shark and ray population sizes have shrunk by 71 percent, according to a study published January 27 in Nature, a trend that coincides with increased fishing of these species.

“Knowing that this is a global figure, the findings are stark,” coauthor Nick Dulvy of Simon Fraser University says in a statement. “If we don’t do anything, it will be too late. It’s much worse than other animal populations we’ve been looking at. It’s an incredible rate of decline steeper than most elephant and rhino declines, and those animals are iconic in driving conservation efforts on land.”

There are 31 shark and ray species in the open ocean, 24 of which are now endangered, according to the Red List Index standards of the International Union for Conservation of Nature (IUCN). Three species—the oceanic whitetip, scalloped hammerhead,...

Shark declines have been documented previously, but to get a global sense of the animals’ situation, the team used long-term data from the Red List along with the Living Planet Index, a venture between the World Wildlife Fund and the Zoological Society of London that tracks the abundance of species. These data sets quantify species’ subpopulation sizes in different geographical areas over time, and the team used these to extrapolate global numbers.

For instance, the great hammerhead experienced a resurgence in two areas after protections were introduced in 2005. Other regions have seen a decline, including a 99.9 percent decrease in the Mediterranean since the mid-1980s. Accounting for all regions where this species is found, there was a more than 80 percent reduction over the span of three shark generations, according to IUCN data included in the analysis. Fishery numbers indicate how much ships bring in and how many sharks were included as bycatch, giving the team an idea of how fishing correlates with population sizes. 

These cartilaginous fish are harvested for their meat and fins, though most die as bycatch in other fisheries, particularly large fish such as tuna, which the sharks themselves hunt.

“You drop a fishing line in the open ocean, and often it’s sharks that are there first—whether or not they’re the primary target,” marine biologist Stuart Sandin of the Scripps Institution of Oceanography who was not involved with the research tells the Associated Press.

New technologies have made it easier for fishers to meet the demands of the market, including improved sonar technology to find fish and stronger, faster boats with an increased capacity to used baited hooks and nets. As a result, annual shark catches are 18 times the amount in 1970, the authors write in the paper, putting an enormous strain on these populations that are slow to become sexually mature and don’t produce many offspring at once.

“That’s the driver for the 70% reduction in the last 50 years,” coauthor Richard Sherley of the University of Exeter tells BBC News. “For every 10 sharks you had in the open ocean in the 1970s, you would have three today, across these species, on average.”

To combat these nosediving numbers, the authors recommend that governments around the world enact and enforce science-based protective measures and catch limits so irreversible population collapse does not become a reality.

“The US is one of the only countries that’s been really successful in reversing sharp population declines through management,” coauthor Sonja Fordham, the president of Shark Advocates International, tells Science. “It’s difficult; there’s intense pressure from the fishing industry to protect short term economic interests.”

Moving forward, the team will study the status of sharks and rays that occupy reefs and coastal waters. 

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