How many fish can you take from a coral reef before the ecosystem collapses?
This is the question a study by Tim McClanahan and coauthors and published in a recent issue of the Proceedings of the National Academy of Sciences of the United States of America sought to answer.
Compiling data from more than 300 surveys (at more than 150 sites) of shallow coral reefs from nine countries across the Indian Ocean, the authors focused on eight metrics as an indicator of reef ecosystem health: biomass of reef fish; species richness of reef fish; biomass of herbivorous reef fish; biomass of reef fish capable of preying on sea urchins; sea urchin biomass; a predation index based on tethered sea urchin assays; cover of macroalgae; hard coral cover; and cover of all calcifying organisms. With the data on hand, the authors then applied it in a series of mathematical models.
Based on their results, the authors found that well-protected reefs typically had 1,000–1,500 kg of fish per hectare. They also identified several key "tipping points" that function as warning signs of impending collapse, each one more dire than the last. Like a series of dominoes, each "tipping point" leads to another that lessens the chance of recovery if left unreversed.
The first sign of ecosystem change in the form of increased seaweed growth occurs as the fish biomass drops below 1130 kg per hectare.
A further decline to 850 kg per hectare leads to a sharp spike in the predominance of algae.
At 640 kg per hectare, a sharp decline in the predation rates of sea urchins occurs, while continuing decline to 300 kg per hectare leads to a massive loss of biodiversity (evidenced by a sharp drop in herbivorous fish biomass and rapid increase in sea urchin numbers).
The final collapse occurs below 150 kg per hectare, when the coral growth and cover rapidly drop to zero.
The authors found that the maximum sustainable yield of a reef fishery required maintaining the fish biomass at 300–600 kg per hectare. Dropping below 300 kg per hectare presages serious trouble, they say. Not surprisingly, they found no-take marine reserves to have the highest fish biomass, while many unregulated fisheries were operating to the point of ecosystem collapse.
The study also found that coral cover, previously thought to be a useful as an early-warning sign of reef ecosystem collapse, was ironically the final stage of the collapse. "y the time you see the loss of live coral cover, it may be already too late to save the reef", said coauthor Nick Graham.
The authors hope that the study would identify early warning signs to fisheries managers and aid the establishment of better management guidelines for coral reefs by providing tangible management targets.
For more information, see the paper: McClanahan, TR, NAJ Graham, MA MacNeil, NA Muthiga, JE Cinner, JH Bruggemann and SK Wilson (2011) Critical thresholds and tangible targets for ecosystem-based management of coral reef fisheries. Proceedings of the National Academy of Sciences 108, pp. 17230–17233.
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