Killing the ocean's apex predators could have a major effect on climate change, with new research raising questions about the consequences of shark culling.
Getting rid of sharks disrupts the circle of life and means their normal prey such as sea turtles are in abundance and therefore eat more seagrass, researchers from four Australian universities say.
That's a problem because seagrass holds a lot of oceanic carbon within sediments and with more turtles eating the vegetation, more carbon is released into the earth's atmosphere accelerating climate change.
In an article published in Nature Climate Change today, the authors say sharks should be protected to help mitigate the effects of climate change and believe it is something which needs to be urgently included in policy.
"Coastal vegetation such as seagrasses, saltmarsh and mangroves, is incredibly efficient at capturing and storing carbon, 40 times more efficient than tropical rainforests in fact," Sydney's University of Technology and Deakin University marine biologist Dr Peter Macreadie says.
"This 'blue carbon' can be stored in sediments for millennia. Despite occupying less than 1 percent of the seafloor it's estimated that coastal blue carbon ecosystems sequester more than half the ocean's carbon."
Getting rid of the ocean's predators, including large fish, leaves the populations of herbivorous fish and other animals unchecked and they can't self-regulate.
"In the case of sharks and turtles, sharks eat turtles, which in turn eat seagrasses. But when sharks disappear, the turtles have a tendency to run wild and the seagrass ecosystems cannot sustain the turtle populations."
This was noticed in Shark Bay, Western Australia, where hunting had left fewer sharks in the area and carbon storage rates were less than half of other areas which had more sharks.
"At the extreme level, we see turtles without predation pressure eating themselves out of house and home and destabilising carbon stocks that have been locked away for millennia," Dr Macreadie says.
Co-author Prof Rod Connolly, a marine biologist from Griffith University's Australian Rivers Institute, warns of the potentially catastrophic implications of shark culling and unsustainable harvesting of large fish for humans.
"We are already aware of the need to manage how many fish we take and from where. But we should also know that our decisions affect climate change," he says.
"Predators play an important and potentially irreplaceable role in carbon cycling. The effect of the disproportionate loss of species high in the food chain cannot be underestimated."
Coastal wetlands are accountable for taking in a quarter of a trillion kilograms of carbon from the atmosphere each year, Prof Connolly says.
The authors say data is limited on this area of research, but believe further studies will back up their findings.
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