Overfishing of large parrotfish can lead to withering reef systems, according to a new report.
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A bicolour parrotfish (Cetoscarus bicolor).
Credit: Wikimedia
SYDNEY: The depletion of large fish due to overfishing throws the functioning of coral reef ecosystems off kilter, say researchers, who offer new insight into the plight of the world's reefs.
Big fish are the reef housekeepers of the tropical fish world. A subfamily of large, colourful fish known as parrotfish perform the equivalent of mowing and weeding, clearing away the debris caused by cyclones and bleaching that if left unattended can affect the reef's regeneration process.
According a study in the current issue of Proceedings of the Royal Society B: Biological Sciences by researchers led by David Bellwood from James Cook University in Queensland, overfishing can result in a reef containing only small fish that aren't built to clear away dead coral, which will prevent new coral from sprouting and thriving.
Coral gardeners
Overfishing and its severe impact on large fish populations has been long acknowledged both within the reef ecologist and fishermen communities, specifically by older fishermen who may once have made a living from undamaged reefs. But until now, no one has calculated the extent to which a depletion may impact key ecosystem functions performed by parrotfishes.
The researchers investigated groups of parrotfish across 18 locations from the western Indian Ocean to the Central Pacific, encompassing a broad range of human population densities with different levels of overfishing.
Areas that were lightly fished or protected, such as the Hilder and Carter reefs on the Great Barrier Reef, had more than 50 times the biomass of large fishes compared to overfished locations. "Nonetheless, many reef systems are operating with compromised or fragmentary ecosystem processes, and are increasingly unable to absorb the impacts of fishing, pollution, climate change and ocean acidification," said the researchers.
'Dusting' is not enough
Coral predation, or the 'weeding' done by big parrotfish, can be performed by other fish, but large parrotfish are much more efficient at removing coral over large reef systems because of their jaw size.
In the absence of big fish such as Bolbometopon muricatum - the largest Indo-Pacific parrotfish species - due to overfishing, the little fish can only do their light grazing activities known as 'dusting'. And more and more small fish proliferating means that dusting is overdone while the mowing and weeding is neglected.
"Intact parrotfish populations can scrape the surface of each square metre of reef every 18 days, removing up to 40 kg of sediment from each square metre per year," the team reported.
Humans threaten large parrotfish extinction
The more natural disasters there are around reefs, the more masses of big corals and dead corals will need to be removed and picked through to allow regeneration. More will pile up because of the lack of big fish, and reefs may not grow as they should.
The most intact reefs in the world lie in Australian waters, so it is especially important that we protect them from overfishing, the researchers said. They stressed that we should not settle our minds to the comfortable thought that if only large fish are vulnerable to humans, the small mowers would be able to sustain the reef's resilience regardless of overfishing.
"I agree with the authors in the points of caution they raised against settling our minds in the comfortable thought of, 'Oh well, if only large fish are vulnerable to humans , the small mowers will be there to sustain the reef's resilience regardless of overfishing'," said Sonia Bejarano from the Marine Spatial Ecology Laboratory at the University of Queensland, who studies the feeding ecology of parrotfishes and other reef grazers. "As populations of large species are depleted, fishers may be forced to maintain their profit by heavily exploiting small individuals, and then the mowing and weeding function could also be severely impacted."
