A reef with high coral cover. (Photo: Nick Graham, ARC Centre of Excellence)
Tipping point for coral reef collapse identified
Wednesday, September 28, 2011, 23:20 (GMT + 9)
An international team of scientists has discovered exactly how many fish can be taken from a coral reef ecosystem before it collapses. This could help save reefs through sustainable management.
The scientists spent 10 years studying the state of coral reefs around nine countries off East Africa and determined how overfishing can produce a predictable sequence of events that result in the collapse of reef ecosystems.
|Fishermen setting a net in the coral reef lagoon of Kenya. (Photo: Joshua Cinner)
The findings were published this week in the journal of the US National Academy of Sciences.
Australian scientist Dr Nick Graham, from the ARC Centre of Excellence for Coral Reef Studies of James Cook University, said this is proof that we need marine parks and no-catch zones, ABC reports.
According to the study, in well-protected areas, typically 1,000-1,500 kg of reef fish can be found per ha of coral reef.
As this amount is reduced below 1,000 kg per ha, detectable early warning signs include increased seaweed growth and urchin activity, Graham explained, The Sydney Morning Herald reports.
"You see patches of weeds replacing coral, you see more sea urchins devouring the coral, you see a general decline in the species richness on the reef, and you see less coral cover," he said. “If you carry on fishing and you remove more fish from the reef, you cross a threshold where the predators in the fish assemblage, so the fish that eat other organisms do their job less effectively.”
The maximum sustainable yield (MSY) was found to be between 300 and 600 kg of fish per ha. Graham said that once fish stocks fell below 300 kg/ha, the reef was in real trouble.
Moreover, the loss of hard corals is not a warning sign, as previously thought, but actually the last stage in the collapse of a reef.
|1) A female fish trader in Kenya. (Photo: Joshua Cinner) /2) A recovering plate coral dominated reef in the central Indian Ocean. (Photo: Nick Graham)
“Further still and we start seeing declines in herbivore and interestingly the very last threshold that we found were collapses in the amount of hard coral cover. So this indicated that a lot of people assume the best indicator of the health of a coral reef is actually the very last sign that a reef is in trouble,” he pointed out.
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