Shark Culling And Overfishing May Be Contributing To Climate Change

ABC - Sarah Sedghi

Research finds shark culling is contributing
to climate change (AAP: ScreenWest)

 New research has found that sharks play an important role in preventing climate change, warning that overfishing and culling sharks is resulting in more carbon being released from the seafloor.

A paper published in the journal Nature Climate Change has found that the culling and fishing of sharks and other large fish is leading to an overabundance of their prey, such as turtles, stingrays and crabs.
Larger numbers of these marine creatures means that vegetation which stores carbon is being eaten in greater quantities.
"Sharks, believe it or not, are helping to prevent climate change," said Dr Peter Macreadie, an Australian Research Council Fellow from Deakin University and one of the paper's authors.
Several years ago researchers found that carbon is stored in blue carbon ecosystems in the marine environment.
"They are the seagrasses, the salt marshes, the mangroves and they're among the most powerful carbon sinks in the world," Dr Macreadie said.
"So they will capture and store carbon at a rate 40 times faster than tropical rainforests like the Amazon and they'll store that carbon in the ground for millennial time scales."

He said as predators were culled and overfished, other marine life consumed more and more vegetation.

"Turtles, crabs, certain types of worms, stingrays — these animals that are overabundant to do with loss of predators used to keep their numbers in check," Dr Macreadie said.
The researchers used Cape Cod in Massachusetts as an example of where this process had been observed.
"There had been overfishing in the region, so a lot of the big fish had been removed and then what we saw was an increase — a remarkable increase, a huge increase — in the number of crabs that bury and borrow down in the system, in the salt marsh which sequestered all this carbon," Dr Macreadie said.
"And we'd found that in an area there, the crabs had become so abundant that they had pretty much destroyed the salt marsh, and it was a small area, it was only 1.5 square kilometres, but it liberated 250,000 tonnes of carbon that had been stored in the ground."

Release of ancient carbon would have 'catastrophic' effect
He said with the culling of huge numbers of sharks and other top ocean predators, researchers had discovered many other examples of this occurring.
"There's been some 90 per cent loss of the oceans' top predators and so we've learnt this link between sharks and other top predators and the cascading effects they will have down to other animals in those ecosystems that are eating themselves out of house and home.
"They're eating the blue carbon ecosystems that have sequestered so much carbon and this is causing release of ancient carbon as a consequence."
Dr Macreadie said it would have a catastrophic effect on the environment.
"We've only just scratched the surface here," he said.
"These blue carbon ecosystems are so critical for sequestering carbon and they support these important food webs, and when these food webs are disrupted it's a bit like playing a game of Jenga — you pull out a few pins and the whole thing falls apart.
"If we just lost 1 per cent of the oceans' blue carbon ecosystems, it would be equivalent to releasing 460 million tonnes of carbon annually, which is about the equivalent of about 97 million cars.
"It's about equivalent to Australia's annual greenhouse gas emissions.
"So I think it's time to take a good look at the way in which nature helps mitigate climate change for us and trying to do everything we can to let that natural process operate in full force, and if sharks are a part of that, if predators and a part of that we need to take that into consideration."