Coral Reefs and a Scourge of Sea Urchins

January 11, 2011

A long-term WCS study off the Kenyan coast finds overfishing in coral ecosystems can stunt the growth of reefs.

Overfishing near coral reefs does not just keep fish populations down. It stunts the growth of the coral, too.

In sites off the coast of Kenya, WCS marine researchers found reefs in areas closed to fishing grew more in size and structure than those in areas open to fishing. The 18-year study shows how proper fisheries management encourages healthy coral ecosystems and sustainable fishing economies.

The researchers found that  fishers hauling in too many carnivorous fish results in a food web out of whack. That’s because without predators, sea urchins multiply. These spiny, asterisk-shaped invertebrates become the most numerous grazers on the reefs. Unfortunately, they eat a type of algae that produces calcium bicarbonate, an essential building block of the corals themselves. Coralline algae help coral formations grow, specifically into the massive flat reefs that fringe most of world’s tropical reef systems.

The influence of the urchins uber-grazing on the reefs, the long-term study found, was more serious and persistent than the strong El Niño that devastated coral reefs throughout the tropics in 1998.

These under-appreciated coralline algae are known to bind and stabilize reef skeletons and sand, as well as enhance the recruitment of small corals by providing a place for their larvae to settle,” said Tim McClanahan, head of WCS’s coral reef research and conservation program. “This study illustrates the cascading effects of predator loss on a reef system and the importance of maintaining fish populations for coral health.”

But urchin-eating predators, such as triggerfish and wrasses, are not the only fish integral to the stability of the food web. Vegetarian fish keep things in balance as well. Parrotfish and surgeonfish eat coralline algae, too, but they also keep the types of fleshy algae that compete with the coral-building algae in check.

And it’s not just corals that rely on healthy populations of fish to flourish. We do, too.

“The survival of the coral reefs is critical for hundreds of millions of people who depend on these complex systems for coastal protection, food and tourism revenue around the globe,” said Caleb McClennen, director of WCS-Marine. “This study demonstrates the importance of improving fisheries management on reefs so that corals can thrive, safeguarding some of the world's most fragile marine biodiversity and strengthening coastal economies.”

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