Researchers from the University of California Santa Barbara have found that certain fish may help coral reefs recover from cyclones and predators.
Coral reefs around the world are under threat from rising sea temperatures and over fishing. However, researchers have discovered one location in the South Pacific Island of Moorea where the coral seems to be protected by parrotfish and surgeonfish.
Coral reefs that suffer large losses of live coral often become overgrown with algae and never recover. However, the reefs around Moorea experienced large losses of coral in the past — most recently in the 1980s — and each time they have recovered.
The research team, operating under the Moorea Coral Reef Long-Term Ecological Research project, was keen to find out what ecological factors might be responsible for the dramatic patterns of recovery in Moorea’s coral reefs.
What they found was that the biomass of herbivores on the reef — fish and other plant-eating animals — increased dramatically following the loss of live coral. Andrew Brooks, co-author of the study, said: “What was surprising to us was that the numbers of these species also increased dramatically. We were not simply seeing a case of bigger, fatter fish — we were seeing many more parrotfish and surgeonfish, all of whom happened to be bigger and fatter. We wanted to know where these new fishes were coming from.”
The team also discovered that neither the predatory crown-of-thorns sea stars nor cyclones affected the corals on the fringing reef (the one that grows against the island) nearly as badly as the barrier reef, which separates the shallow lagoons from the deeper ocean.
This is because the fringing reefs act as a nursery ground for baby fish, which feast on the algae. Because there is more food available, the parrotfish and surgeonfish proliferate.
Brooks explained: “In effect, the large numbers of parrotfish and surgeonfish are acting like thousands of fishy lawnmowers, keeping the algae cropped down to levels low enough that there is still space for new baby corals to settle onto the reef and begin to grow.”
As a result Moorea’s reefs seem to be recovering, with tens of thousands of baby corals growing. Coral reefs in the Caribbean don’t recover from similar serious disturbances or damage because the populations of parrotfish and surgeonfish have been depleted through overfishing. Without these fish, the algae becomes over grown and it becomes much harder for the corals to regrow.
The results suggest that simply trying to protect the reef areas from overfishing may not be enough to reverse the trend. It is also important to protect the fringing reefs that provide the nursery habitat to the fish so that their populations can grow to handle the large amounts of algae.