Reef Overfishing Tipping Points

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The ARC Centre of Excellence for Coral Reef Studies at James Cook University compiled surveys of over 300 reef sites and determined several significant reference  points for gauging reef health in relation to fish biomass.  They found that every hectare of well-protected reef typically had 1000 to 1500 kilograms of fish biomass. As the numbers are reduced below 1130kg/ha, the first signs of ecosystem change is evidenced by increasing seaweed growth.  At 850kg/ha, the algae-to-coral ratio sharply increases to favor algae. At 640kg/ha, researchers observed a rapid decline in the predation of sea urchins.  The next reference/tipping point is crossed at 300kg/ha when biodiversity drops dramatically (with a severe decline in herbivorous fish coinciding with a rapid increase in sea urchin populations), raising alarm for ecosystem failure. Below 150kg of fish per hectare, calcification rates and hard corals breaches the final threshold where coral growth and cover rapidly descend to zero.

Coral coverage may not be the best indicator of coral reef health

Not surprisingly, the report found more stringent fishing regulations (such as total or partial fishing closures and gear restrictions) resulted in healthier fish populations.  Protected reefs with no-fishing zones (such as Cabo Pulmo National Park, deemed the most robust marine reserve in the world) had the greatest biomass and biodiversity.

The study also discovered something interesting about using coral cover as a metric for determining reef health:

Ironically, coral cover is one of the most commonly measured metrics for coral reef condition but was the least sensitive metric to losses in fishable biomass. There is broad correspondence between this observed pattern and the documented trajectory of Caribbean reefs, where severely overfished reefs were dominated by very high cover of live corals until their sudden shift to dominance by macroalgae following a die-off of the most abundant grazing sea urchin (1). Together, these studies indicate that reefs may appear healthy long after fishable biomass has been reduced to the point that ecosystem function is jeopardized. This emphasizes the need for reference points that ensure management actions are implemented before it is too late.

By monitoring fish biomass and correlating the data with the discovered "tipping points," the report hopes to help fisheries recognize early warning signs and establish better fishery management guidelines for coral reefs, including collection for the aquarium trade.

The report Critical thresholds and tangible targets for ecosystem-based management of coral reef fisheries is published in the latest issue of Proceedings of the National Academy of Sciences of the USA (PNAS).

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