Reef sharks are disappearing from oceans around the world—and they’re at a much higher risk of going extinct than researchers previously thought, according to a new paper published last week in the journal Science. The sharp decline of these species, which dwell in coral reefs, could further upset the delicate balance of these vulnerable marine ecosystems as they grapple with the effects of human-caused climate change.
In an ambitious, collaborative effort involving more than 150 scientists, researchers surveyed nearly 400 coral reefs across the globe. Five common reef shark species have declined by as much as 60 to 73 percent over the last 50 years, according to their analysis. Worse still, some shark species appeared to be totally absent from 34 to 47 percent of the reefs they surveyed.
Overfishing is likely to blame for the sharks’ demise, which is “absolutely jaw-dropping,” as David Shiffman, a marine biologist at Arizona State University who was not involved in the project but wrote an accompanying commentary to the paper, tells the Washington Post’s Dino Grandoni. In some instances, fishermen are targeting sharks directly for their fins, which command high prices for use in shark fin soup, a popular dish for celebratory events in parts of Asia. In other cases, fishermen accidentally contribute to sharks’ decline by overfishing their prey.
The results were not a surprise to researchers, as they build on a 2020 study that also documented a dramatic reef shark depletion around the world. More broadly, they also align with the International Union for the Conservation of Nature’s (IUCN) 2021 finding that more than a third of all shark and ray species are at risk of extinction.
Our results show that reef shark populations can be recovered by implementing well enforced marine protected areas (#MPAs) and regulating fishing. Using these tools can effectively recover populations in relatively short time frames. pic.twitter.com/R9bH7rd5oF— Colin Simpfendorfer (@SharkColin) June 15, 2023
Though the latest study reveals disheartening trends overall, it highlights some patterns that could help save the sharks moving forward. Shark populations were more robust in protected areas or regions with fishing limitations in place, per the researchers. Their numbers were also higher off the coasts of wealthier nations with strong governments.
Together, these findings suggest that implementing—and enforcing—conservation rules could help the sharks rebound in areas where they’re struggling, as study co-author Colin Simpfendorfer, a marine biologist at James Cook University in Australia, tells New Scientist’s Madeleine Cuff.
But that’s easier said than done. Many lower-income countries simply cannot afford to spend time, energy and money on natural resources management, per Shiffman’s commentary.
“Conservation problems involve solving human problems as well as those associated with ecology,” he writes. “A country that lacks the resources to feed its people is less able to sustainably manage and protect its biodiversity.”
To get a handle on reef shark populations across the planet, the researchers undertook a “herculean effort,” as Simpfendorfer tells the Washington Post. They dropped nearly 23,000 baited remote underwater video stations (BRUVS)—submerged GoPro cameras with food to entice sharks—near 391 coral reefs off the coasts of 67 countries and territories.
The cameras recorded more than 20,000 hours of video footage, which volunteers then painstakingly reviewed. They noted the number of sharks they saw, as well as rays, which tend to take over as the top predator when sharks disappear. In particular, they looked for five common species of reef sharks: Caribbean reef shark, nurse shark, grey reef shark, blacktip reef shark and whitetip reef shark. As a result of the findings, grey reef sharks have already been reclassified as endangered by the IUCN, per New Scientist.
With this new data in hand, researchers used models to estimate the overall changes in shark and ray populations. As they suspected, ray numbers were higher in areas with fewer sharks.
Though rays are also predators, they’re not a one-for-one replacement for sharks—and the lack of sharks can lead to a host of negative ripple effects. For example: Without sharks feasting on them, herbivorous fish begin to thrive. Their booming populations consume more algae, which ordinarily sequesters carbon dioxide in the ecosystem. So, reefs without sharks are storing lower amounts of heat-trapping greenhouse gases, per New Scientist. That’s bad because it makes coral reefs less of a defense against climate change—and reefs are already suffering because of rising ocean temperatures.
“People need healthy coral reefs,” says study co-author Mike Heithaus, a marine ecologist at Florida International University, in a statement. “We are seeing that when sharks disappear, that causes other changes in these ecosystems. Keeping shark populations healthy, or rebuilding them, is important for maintaining their roles for healthy reefs.”