A major barrier for corals trying to recover from damage is the overgrowth of algae. Too much algae, and coral larvae aren't able to find suitable spots to settle down and grow up. But, with the help of a few certain types of fish, coral reefs stand a better chance of bouncing back from problems like storms or predation.
PhysOrg reports on research from University of California, Santa Barbara that found in some places, fish can be the key to corals' survival.The South Pacific island of Moorea, in French Polynesia, seemed to bounce back easier after the loss of live coral in the 1980s. The researchers wanted to understand why. Crown-of-thorns sea stars had fed on corals, and algae grew on the skeletons. Normally, that's a bad sign. But in this area, there were larger numbers of bigger, fatter parrotfish and surgeonfish. It turns out, that the coral reef nearest the island was a nursery for these fish, which could grow fat on the algae, and in turn, help the algae stay mowed down, like a well manicured lawn. This gave space for coral larvae to take hold and rebuild the outer reefs that suffered the bulk of damage from both the sea star infestation and cyclones.
Because of the help of these "thousands of tiny lawnmowers," the reef bounced back more quickly, and it explains why many reefs in the Caribbean, which don't have healthy populations of parrotfish and surgeonfish due to overfishing, have a far more difficult time recovering from damage. The study shows yet another reason why protecting reefs from overfishing is a key solution for coral conservation. But it's not about protecting just any part of coral reefs -- it is vital to protect those that act as nurseries too.
Managers have tried to reverse the trend of overfishing through the creation of Marine Protected Areas (MPAs), where fishing is severely restricted or prohibited. "Our results suggest that this strategy may not be enough to reverse the trend of coral reefs becoming algal reefs," said Brooks. "Our new and very novel results suggest that it also is vital to protect the fringing reefs that serve as nursery grounds. Without these nursery grounds, populations of parrotfishes and surgeonfishes can't respond to increasing amounts of algae on the reefs by outputting more baby herbivores."
In short, the research team found that by using MPAs, managers can help protect adult fish, producing bigger, fatter fish. "But if you don't protect the nursery habitat –– the babies produced by these bigger fish, or by fish in other, nearby areas –– you can't increase the overall numbers of the important algae-eating fish on the reef," said Brooks.
The researcher have uncovered another important clue that could make marine protected areas all the more successful if implemented correctly.