Good news for the world's vanishing corals: a new study shows that commercial fishing bans in Australia's Great Barrier Reef kept a lid on coral-gobbling starfish.
"This is definitely good news for coral," says John Bruno, an associate professor of marine science at the University of North Carolina (U.N.C.) at Chapel Hill.
Researchers found that there were as many as seven times fewer outbreaks of coral killing crown-of-thorns starfish—which can have up to 20 spike-covered arms and grow up to two feet (0.6 meter) in diameter—in so-called "no-take" regions as there were in those where commercial fishing is allowed. Lead author Hugh Sweatman, a marine ecologist at the Australian Institute of Marine Science in Queensland, says scientists are not sure why this translates into fewer starfish given that these fish (the ocean's top predators) such as coral trout, eat other fish, not the barbed starfish.
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But Sweatman speculates there may be a cascade effect down the food chain when the large fish are spared capture by commercial fisherman. The dominant piscine predators dine on the smaller fish around a reef, which, in turn, prey on the small invertebrates that chow down the starfish larvae (young). If the smaller fish's top predators are culled, the former's numbers will grow, and they will consume more of the invertebrates: suddenly the fledgling starfish have fewer creatures eating them. Given that an adult female starfish can produce 60 million eggs in a year, "just a slight change in the survivability of offspring can mean a lot more starfish," Sweatman says.
Tiny starfish larvae remain in the rubble on the seafloor for almost two years before emerging to feast on coral. "Once they're out, the starfish eat and spawn like hell," says Sweatman. The starfish will typically reduce the amount of living coral from around 25 percent to less than 5 percent of a reef, which comprises mostly dead coral as well as other kinds of aquatic life.
"You can actually track the paths of the starfish as they eat," Sweatman says. The starfish leave a "brilliant white" trail of bare coral skeletons that "skuzzy green" algae then grow on within a matter of days. To feed, the starfish wrap themselves around their coral prey, eject their stomachs, and then digest the coral outside their star-shaped body.
Reducing the amount of coral laid to waste by these starfish will greatly improve overall reef health, according to Sweatman. He estimates that crown-of-thorns starfish are responsible for about 40 percent of the drop in live coral coverage on the Great Barrier Reef. Storms and a phenomenon called bleaching, in which corals expel the algae they house in exchange for food due to elevated water temperatures, together make up less than a third of the coral deaths.
The future looks a bit less dismal for the Great Barrier Reef, given this demonstrated connection between commercial fishing bans and curtailed starfish outbreaks: Just 4.5 percent of the reef was deemed no-take in 1989, but in 2004 this protected area shot up to 33 percent. "This study is yet another argument for instituting marine protection programs," Sweatman says.
Other studies, however, have shown that pristine coral reef systems with a full complement of sea life can actually lead to corals getting sick more often from bacteria and fungus.
"Reefs with more fish tend to have more coral diseases," says U.N.C.'s Bruno. "Overall, it's tough-going for coral these days."