Fishermen and scientists questioning how many fish can be sustainably taken from a reef believe they’ve quantified the tipping point.
Nick Graham, from the ARC Centre of Excellence for Coral Reef Studies at James Cook University, says ecosystems may take decades to recover from overfishing to feed the world’s growing population.
In a report released today, researchers have demonstrated how overfishing can generate a predictable sequence of events that lead to the collapse of reef ecosystems.
It also offers targets in order to keep fisheries sustainable.
As “fish biomass” - the number and weight of fish living on a reef - declines due to fishing, a number of tipping points are crossed, from which it is increasingly harder to return.
The study shows that in well-protected areas, there are typically 1000 to 1500 kilograms of reef fish per hectare of coral reef.
As this is reduced below 1000 kilograms, early warning signs such as increased seaweed growth and urchin activity begin to show up, Dr Graham says.
“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.
The researchers found that between 300 and 600 kilograms of fish per hectare was the “maximum sustainable yield”.
When fish stocks dropped below 300 kilograms/hectare the reef was in real trouble, Dr Graham said.
The loss of hard corals, which had been thought of as a warning sign, was actually the last stage in the collapse of a reef, the study found.
The report, Critical thresholds and tangible targets for ecosystem-based management of coral reef fisheries, has been published in today’s issue of the Proceedings of the National Academy of Sciences of the USA (PNAS).
