Sharks and their relatives face an existential crisis unprecedented in their 420 million years on the planet. A global trade in products from these animals fuels the capture of tens of millions of individuals a year. Strong demand combined with poor fishery regulation and high levels of incidental catch have resulted in many populations being overfished, with some now facing extinction.

Many activists argue a total ban on shark fishing is the only solution to slow or halt the decline. But a 2016 study found the majority of shark researchers surveyed believe sustainable shark fisheries are possible and preferable to widespread bans. Many reported they knew of real-world examples of sustainable shark fisheries. But a global roundup of empirical data exploring which species are being fished sustainably was lacking.

New research, appearing in the February 6 issue of Current Biology, is filling that gap, and the findings bolster the idea that around the world, some sharks are being fished sustainably. Nicholas Dulvy, a marine conservation biologist at Simon Fraser University in British Columbia, and shark ecologist Colin Simpfendorfer of James Cook University in Australia recently examined global stock assessments of 65 shark populations of 47 species. They found 39 of the populations, representing 33 different species, are fished sustainably—that is, they are harvested at levels that allow them to remain stable in size and not edge toward extinction. Although these 33 species account for only a small fraction of the world’s sharks, rays and their kin the chimeras (collectively referred to as sharks), which in total number more than 1,000, they are proof of concept that sustainable shark fishing is possible.

Cross-referencing stock assessments sourced from the scientific literature, government agencies, known experts and internet searches with other data sets including United Nations Food and Agriculture Organization catch statistics and International Union for Conservation of Nature (IUCN) threat categories, along with trade records, Dulvy and Simpfendorfer calculated the take of biologically sustainable sharks comprised 7 to 9 percent of global totals. But there are two components to sustainable fishing: the biological capability of the fish to withstand harvesting and the careful management of that harvesting by humans. The researchers found only 4 percent of global trade in sharks was directly sustainably managed.

Dulvy, who co-chairs the IUCN Shark Specialist Group, says the science “comes straight out of population modeling theory,” and the idea of maximum sustainable yields. To set limits on what can be harvested sustainably, researchers need to know the proportion of old versus young fish in a population as well as the speed with which individuals can reproduce—factors that affect the entire population’s ability to grow, decline or remain stable in numbers. Many shark species are so poorly studied that scientists do not yet know these basic parameters. But, theoretically, any species whose biology is well understood can be managed sustainably.

As expected, Dulvy and Simpfendorfer determined some species that reproduce very slowly, including deep-water gulper sharks and cownose rays, cannot sustain fishing pressure. These creatures produce a maximum of one pup per year, on average, so they must be protected to maintain their numbers.

But the researchers found something that may come as a surprise to conservation advocates: other relatively low-productivity species could be sustainably fished. One example is the Pacific spiny dogfish Squalus suckleyi, a type of small shark. In 2011 a fishing industry group in British Columbia obtained Marine Stewardship Council (MSC) certification for this species, a process that validates for consumers the product was fished sustainably. It was the first such certification in the world awarded for a shark, explains Michael Renwick, executive director of the British Columbia Dogfish Hook and Line Industry Association who spearheaded this certification process.

The Pacific spiny dogfish can live 70 years and does not reach sexual maturity until age 40. Females gestate their babies for two years, and may take a year off from breeding after pups are born. This shark thus has one of the longest reproductive cycles of any animal. “You look at that and you go, ‘how on Earth can [harvesting] that be sustainable?’” Dulvy says. But it is possible because people have invested in figuring out its productivity, in population monitoring and in good management with carefully calculated quotas.

Dogfish, a group of species from the Squalidae family of sharks, have not always been managed sustainably. They are the fishes of choice in the traditional British dish, fish and chips. When Europe overfished its own stocks, dogfish fishing shifted overseas to the northeastern coast of the U.S. , where it again took its toll. There, overharvesting of Atlantic spiny dogfish (Squalus acanthias) throughout the 1990s led officials to significantly restrict that fishery, explains Michael Pentony at the National Oceanic and Atmospheric Administration’s Greater Atlantic Regional Fisheries’ Gloucester, Mass., office.

Now that Atlantic dogfish stocks have recovered and the U.S. dogfish industry has obtained MSC certification, British Columbia’s industry has, for multiple reasons, declined, and its industry group has deemed renewal of its MSC status too expensive to pursue.

Perhaps the most controversial finding from the new study is that shark fins, too, can be harvested sustainably. Shark fin is a delicacy in some Asian cultures. But the traditional way of harvesting the fins—in which the fins are hacked off of the live animal, which is then tossed back into the sea to suffocate or die from bleeding—has prompted public outcry. The uproar over this practice, called “finning,” has been a major driver for shark conservation. Against that backdrop, sustainable shark fin is an “unthinkable notion for many,” Dulvy and Simpfendorfer acknowledge. But their study suggests it is indeed possible. In fact, they found nearly 9 percent of fins on the market originate from sharks whose populations are being fished sustainably.

Obtaining shark fins need not involve finning at all, however. “There are absolutely ways to get fins into the fin trade without finning,” says David Shiffman of the University of Miami, who led the 2016 study that surveyed shark scientists’ attitudes toward shark fishing. He notes great strides in legislation that have reduced the number of sharks finned at sea in at least 17 countries.

Indeed, by definition, exploiting a resource sustainably requires whole animal use, Simpfendorfer explains. In the case of MSC-certified Atlantic dogfish, the heads become lobster and crab bait; back meat becomes British fish and chips; belly flaps are a German delicacy; liver supplies nutraceuticals; fins and tails headline east Asian soup; and leftovers become agricultural fertilizer, says Massachusetts-based attorney John Whiteside, Jr., who helped east coast U.S. dogfish fisheries achieve MSC status.

But for sustainable shark fishing to work, its products have to be labeled and traceable back to a well-managed source—a requirement that very few of the sustainably harvested fins currently on the market now meet. Traceability depends on careful management of product “chain of custody” with specific information carried through from capture vessel to retailer. Ideally the products have a closed chain of custody, meaning no uncertified products come in from the sidelines, explains Glenn Sant, Fisheries Trade Programme Leader at TRAFFIC International, the wildlife trade monitoring group co-founded by World Wildlife Fund and the IUCN, who was not part of the study. “Industries have been doing this for a long time,” he observes, with bar codes making traceability of products easy and cheap. Traceability challenges are not technological. They lie in gaining sufficient transparency to discern whether the fishery had adequate management within the product’s country of origin. Currently, unless a product is MSC-certified or has permits from the Convention on International Trade in Endangered Species (CITES) attached, its traceability “needs great improvement,” Sant says.

Determining how to harvest some sharks sustainably while protecting others that cannot be harvested at all will require further work. Dulvy and Simpfendorfer suggest developed countries must support developing ones in improving traceability and negotiating international treaties for fisheries and trade. Bycatch remains a problem, too. Tuna fisheries, for example, often hook pricey species like blue sharks and shortfin makos and then sell them, rather than releasing them. Meanwhile bycatch sharks that are not economically valuable and thus released back to the sea may not fare well either. New satellite tagging research by Steven Campana, a shark biologist at the University of Iceland who was not affiliated with the research, says a quarter of those live-released sharks may die from the stresses of capture and handling.

Another concern: legal shark fishing could hide illegal trade. But “illegal unsustainable shark fishing is happening regardless,” Shiffman notes. In his view it is better to have at least some sustainable, scientifically well-managed products in the marketplace. Without them, he says, “whatever fills the gap that we leave is going to be worse.”