Reef sharks are having a challenging time as they confront threats from fishing, denser human populations and climate change.
An extensive survey has found that adults have effectively become extinct in nearly a fifth of the world’s tropical oceans, while a separate study reveals that, even when protected, shallow shark nurseries are affected by warming water and diminishing oxygen levels.
Removing such predators from ecosystems can fundamentally change energy flows and nutrient recycling and allow growth of smaller predators with flow-on effects down the food chain, says co-author of the survey, Mark Meekan from the University of Western Australia.
It reveals a fall in reef shark numbers that, until now, has been undocumented – even though coastal sharks comprise two thirds of species traded globally, according to lead author Aaron MacNeil from Dalhousie University, Canada.
The survey included 371 reefs in 58 countries, using observations from more than 15,000 baited video stations (BRUVS) over four years. At each reef, they conducted 30 to 50 BRUVS and analysed the maximum number of sharks seen at any one time on hour-long videos.
Using a statistical model, the researchers used these data to estimate the numbers of sharks expected to be seen on any given reef, along with factors that could explain why their numbers vary.
They found no sharks at 63% of the stations and 19% of reefs. More than half the nations had abundance scores 50% less than would be expected, which the authors say suggests “loss of reef sharks is pervasive among reefs globally”.
Nations with the lowest rank included Qatar, the Dominican Republic, Colombia, Sri Lanka, Jamaica, Kenya, Vietnam, India, French West Indies and Guam.
On the positive side, they found several countries where shark conservation is working. Those with robust populations compared to their region’s average included Australia, the Bahamas, the Solomon Islands, New Zealand, French Polynesia, the Maldives, and the US.
Successful conservation strategies include strong shark fisheries management and low human pressure. Regions with depleted populations, on the other hand, had poor governance, are close to markets and use destructive gillnets and longlines.
While the results are sobering, MacNeil says they also give hope that reef shark populations can be successfully conserved, even where people are present and fishing them.
Shark sanctuaries were associated with 50% higher abundance of sharks compared to nations without them, for instance, while introducing limits on catches could increase numbers by 15%, on average. Other successful strategies include catch limits and banning certain equipment.
Conservation measures would need to address local fishery operations, social norms and cultures. “By engaging with local communities,” says MacNeil, “we have a chance to make shark fishing sustainable across a wide range of contexts.”
The study also gives a baseline of shark numbers for future evaluation of conservation initiatives.
But we can’t ignore the effects of climate change, says Jodie Rummer from James Cook University in Australia, lead author of the other study published in the Journal of Experimental Biology.
Tackling overfishing and exploitation, their number one threat, is critical, she says. “But then, we need to address the fact that no matter what dotted lines we put around these countries to declare marine protected areas or shark sanctuaries, climate change crosses those lines.”
This is particularly important for baby sharks that rely on shallow coastal waters for protection and food while they grow, learn to hunt and stay clear of predators.
With lead author Ian Bouyoucos, her team investigated the impact of temperature and oxygen levels on baby blacktip reef sharks (Carcharhinus melanopterus) in the world’s largest shark sanctuary, spanning 4.8 million square kilometres in French Polynesia.
They found the pups are faring well, so far. “[W]e found the growth rate and metabolism of baby sharks is resilient to the temperature changes they currently face in these shallow habitat,” Bouyoucos says.
“We also found the sharks with a greater tolerance for higher temperatures had a greater tolerance for low oxygen levels, which is really promising.”
But the team is concerned the pups are reaching their biological limits, which would pose a dilemma.
“If they choose less harsh habitats, they lose their food and protection,” says Rummer. “If they remain within the safe, shallow nurseries, they suffer the effects of warming waters and decreasing oxygen levels.”
She says that despite token gestures to save reefs, such as culling crown-of-thorns starfishes or putting fans or shade cloth over them, they don’t really get to the heart of the problem – human activities that are warming the planet.