The number of sharks and rays roaming the world’s open seas and oceans plummeted by 71% during the half-century ending in 2018, according to an unprecedented global analysis indicating that overfishing pushed three-quarters of the ecosystem-balancing species toward extinction.

Researchers behind the dire findings published Wednesday in Nature, which showed an 18-fold increase in the proportion of 31 species of cartilaginous fish caught relative to their global population, implored governments to implement catch limits to prevent entire populations from collapsing.

“I was expecting it was going to be bad, but just to see that it was this bad … yeah,” reflected Nathan Pacoureau, the study’s lead author and a postdoctoral scholar at the Department of Biological Sciences at Canada’s Simon Fraser University, in an interview with The Academic Times. “We show steep declines and rapidly rising extinction risk.”

Between 1970 and 2018, those declines occurred at a steady pace, averaging 18.2% each decade, the study says, and were not driven by any one or handful of species. In fact, the only species whose numbers haven’t dwindled is the smooth hammerhead.

Three previously abundant and wide-ranging species — the scalloped and great hammerhead and oceanic whitetip — were thinned out to the point where they’re now critically endangered, Pacoureau and his colleagues found. Another four — the pelagic thresher, dusky shark and shortfin and longfin mako — are endangered. And 24 species are now threatened with extinction, compared to just nine in 1980.

Experts joining the globe-spanning project also came from Australia, the U.K., Italy, South Africa, the U.S., Brazil, Sri Lanka, New Zealand, the United Arab Emirates, Taiwan and France, bringing along perspectives from different regions that have assessed local population declines that had not yet been comprehensively analyzed. 

For this study, researchers calculated two indicators: the Living Planet Index, which measures changes in abundance for 18 of the species, and the Red List Index, which gauges changes in extinction risk for all 31. Both indicators are often used to track progress toward the Convention on Biological Diversity’s Aichi Biodiversity Targets and the United Nations’ Sustainable Development Goals, although they’ve historically been primarily focused on terrestrial animals.

These two key international policy frameworks do contain specific objectives regarding sharks and rays: to reverse population declines and use marine resources sustainably. But those meant to be achieved by 2020 were missed, the study shows, as the authors found that 77% of species qualified as threatened with extinction under the Red List criteria, which are determined by the International Union for Conservation of Nature.

The effects of overfishing have varied by region, according to the study, and tropical species have been harder hit than temperate-zone ones. Overall declines were determined to be 46.1% in the Atlantic Ocean, 67% in the Pacific Ocean and 84.7% in the Indian Ocean.

Sharks provide important ecological functions as top predators, but they grow slowly and produce few pups. And they are vulnerable to exploitation for their meat, fins, gill plates and liver oil. 

Catches peaked at 63 million to 273 million animals in the early 2000s, the study says, but then began to fall because of overfishing.

“We demonstrate that — despite ranging farther from land than most species — oceanic sharks and rays are at exceptionally high risk of extinction, much more so than the average bird, mammal or frog,” Simon Fraser marine biodiversity and conservation professor Nicholas Dulvy said.

Not only does overfishing jeopardize ocean ecosystems, it threatens ecotourism dollars and food security for some of the world’s poorest countries, such as African nations along the Indian Ocean, Pacoureau explained.

The practice was found in the study to follow the “serial depletion” phenomenon, whereby harvesters clear out the largest animals first, before exploiting and ultimately depleting medium-sized and then smaller fish that are less economically important. Most often, the issue is bycatch, with the sharks being caught as collateral damage when harvesters are hunting something else, rather than targeted fishing.

The study notes that two vulnerable species, the white shark and porbeagle, appear to be recovering in several regions, largely because of fishing bans on possessing or retaining them. Hammerhead sharks are also rebuilding in the Northwest Atlantic because of U.S. quotas that are strictly enforced, the study says.

“It is possible to reverse shark population declines, even for slow-growing species, if precautionary, science-based management is implemented throughout the range of the species before depletion reaches a point of no return,” the authors said in the study. That should include both domestic and regional catch limits for less-threatened sharks that can support sustainable fisheries, they say, as well as retention bans and bycatch mitigation measures for endangered and critically endangered species.

In the U.S., advocates are hopeful President Joe Biden will “return [the country] to its role as a global shark conservation leader” after America "strayed from that posture and even opposed important shark conservation advances" in recent years, such as efforts to alleviate fishing pressure on the North Atlantic shortfin mako, said Sonja Fordham, a co-author of the study and president of The Ocean Foundation’s Shark Advocates International in Washington, D.C.

Next, Pacoureau said, researchers may shift their focus to emphasize the positive, analyzing what’s working in oceanic shark and ray conservation and potentially producing a sort of practice-based report card.

“What is a bit sad is there is actually a lot of knowledge and scientific analysis and advice, it’s just that they are not fully taken into account,” he said.

Fordham is eager to step in on that front.

“The U.S. can play a critical role as one of the few countries that has been successful in reversing shark population declines,” she told The Academic Times. “We have invaluable expertise, lessons and practical solutions to share with other countries.”

The study “Half a century of global decline in oceanic sharks and rays,” published Jan. 27 in Nature, was authored by Nathan Pacoureau, Simon Fraser University; Cassandra L. Rigby, James Cook University; Peter M. Kyne, Charles Darwin University; Richard B. Sherley, University of Exeter; Henning Winker, European Commission Joint Research Centre and South Africa Department of Environment, Forestry and Fisheries; John K. Carlson, NOAA National Marine Fisheries Service; Sonja V. Fordham, Shark Advocates International; Rodrigo Barreto, Centro Nacional de Pesquisa e Conservação da Biodiversidade Marinha do Sudeste e Sul do Brasil Instituto Chico Mendes de Conservação da Biodiversidade; Daniel Fernando, Blue Resources Trust; Malcolm P. Francis, National Institute of Water and Atmospheric Research; Rima W. Jabado, Elasmo Project; Katelyn B. Herman, Georgia Aquarium; Kwang-Ming Liu, National Taiwan Ocean University; Andrea D. Marshall, Marine Megafauna Foundation; Riley A. Pollom, Simon Fraser University; Evgeny V. Romanov, CAP RUN — CITEB; Colin A. Simpfendorfer, James Cook University; Jamie S. Yin, Simon Fraser University and Rutgers University; Holly K. Kindsvater, Virginia Polytechnic Institute and State University; and Nicholas K. Dulvy, Simon Fraser University.