A new study has assessed how susceptible sharks and rays are to climate threats around the world including warming waters, plastic pollution, ocean acidification and habitat destruction. The scientists found that coastal and freshwater sharks were disproportionately more vulnerable to threats.
The study also raised the alarm about the threat of overfishing to all shark and ray species.
“Overfishing still persists as the dominant threat and this was with every single species,” says Ava Graham, a marine biologist studying at the University of Exeter’s Deep Shark Research Group whose dissertation paper is now published in Aquatic Conservation: Marine and Freshwater Ecosystems. “Of all my 100 species… it was quite shocking to see that every single one is being overfished,” she says.
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In the study, Graham looked at the biological traits of 95 different shark and ray species to predict how well they might cope in a changing world.
As well as fan-favourites such as great whites, great hammerheads, oceanic whitetips and blue sharks, she also reviewed some of the more weird and wonderful species, including gulper sharks, wobbegongs and porcupine rays.
“It was more interesting looking at the less-heard-of shark species around the world,” she says. “Out of the 100 species I did, most of them you don't really hear about.”
The most vulnerable of all the species she analysed was a baffling-looking animal called a largetooth sawfish (Pristis pristis), a freshwater ray that lives in estuaries and can grow more than six metres long.
This weird fish looks like it has a hedge trimmer strapped to its face. “It has this long, protruded almost-nose with spikes,” she says. “They're amazing to look at.”
Sawfish are also sometimes known as carpenter sharks because of their shark-like bodies and tool-laden face. This long snout is covered in sensory pores to find prey. When the sharks hunt down a snack, they turn their ‘nose’ into a weapon and slice at the water to stun or injure their prey.
But the saw-like snout that should help them find their prey can put them in harm’s way. “Because of this large, protruding nose, they're very susceptible to fishing pressures,” says Graham. As well as being targeted by some fishers, they are also caught as bycatch. “They get caught on fishing nets,” she says.
As well as being easily caught in nets, their habitats and nursery areas are being destroyed and it takes a long time for populations to recover because of their slow reproduction rates. “Everything is against them in a way,” she says.
The unusually shaped guitarfish (whose bodies are reminiscent of a guitar) and diamond-shaped wedgefish are also highly vulnerable, according to the study.
Graham based her analysis on an existing risk assessment framework (IRACC) – developed to assess how vulnerable different sharks and rays in Australia were to climate change – but expanded it to look at species around the world. “It was a brave, bold approach to look at this more globally,” she says.
Their analysis combined several elements, rather than just looking at one threat or one species. “The thing that really excites me about this study is the fact that it is taking into account multiple stressors in one assessment in a framework that's already been robustly tested and shown to work really well,” says Graham’s supervisor Dr. Daniel Moore, a marine biology lecturer at the University of Exeter.
Many shark species are grappling with how to survive with water temperatures rising globally. “This is causing some species to shift their range more northward to cooler waters,” says Graham. But the sharks aren’t the only ones following prey to these new regions –fishing vessels are also led here by the fish stocks. “So, then they're going into new fishing grounds, and that is the reason that they're then becoming depleted,” she says.
Coastal and freshwater sharks, such as sawfish, were by far the most vulnerable to this array of threats because of their nearshore habitats. “Proximity to humans just isn't a good thing for sharks,” says Moore. “They then coincide with so many human activities that are detrimental.”
The most threatened species – such as sawfishes and guitarfishes – prefer a very specific coastal habitat. Whereas open ocean sharks can move to different regions to avoid threats, these species can’t relocate. As a pelagic shark, “you've got options,” says Moore. “When you're a sawfish, you have no options.”
Although the study paints a bleak picture in many ways, it also highlights the importance of shark habitats – such as mangroves, seagrass meadows and salt marshes – to coastal communities. “They're nurseries for fish so that supports fisheries, they’re really important for carbon sequestration, for storm protection,” Moore explains.
Protecting or restoring shark nursery habitats will also support populations who live by the coast as well as playing a key role in acting as a buffer for the impacts of climate change. “It's a win for sharks and win for humans if we protect [them],” says Moore.
This can sometimes be as simple as not disturbing these habitats. “We know that nature is so good at being able to bounce back if we leave it alone,” says Graham. “There is hope.”
