Marine biologists may have found a simple way of preserving tropical coral reefs that are threatened by pollution and climate change – ban fishing.
A study of a coral reef in the Caribbean has found the coral improves significantly when fishing is outlawed. Bans allow big predators such as sharks and groupers to prey on smaller parrotfish which graze on seaweed living on the coral.
The findings are surprising because grazing parrotfish help prevent seaweed from smothering the coral and more of their natural predators should in theory make it more difficult for coral to grow.
However, research by a team led by the marine biologist Peter Mumby, of Exeter University, found that although the presence of big predators caused parrotfish to decline, it results in some parrotfish growing bigger, allowing the coral to be grazed more efficiently. “Marine reserves are used as a tool to manage fish populations. The key finding from our results is reserves can also protect corals from the effects of climate change,” Dr Mumby said.
Many tropical coral reefs around the world have in recent years suffered from pollution running into the sea from rivers, and rising sea temperatures, which causes “bleaching” when the photosynthetic algae in the coral dies off. Reefs in the Caribbean suffered the additional problem of losing their main grazing animal, the long-spined sea urchin, which helped to keep potentially stifling seaweed in check. This meant that in this part of the world it was the seaweed-eating parrotfish that would have to keep the coral from being smothered and yet this species was the main prey of larger, commercially important fish.
Dr Mumby said: “More than 20 years ago sea urchins in the Caribbean were wiped out by disease, leaving parrotfish as the main grazer of reef surfaces. The fish use their teeth to remove seaweed from the reef which allows new corals to settle and grow. This grazing process is essential to the health of the system.”
The scientists studied a marine park in the Bahamas to see what would happen when a ban on commercial fishing allowed the reef’s top predator, the Nassau grouper, to return to its former hunting grounds.
“While an increasing number of larger predators is essentially good news, we had concerns this might result in a decrease in the numbers of parrotfish, which could ultimately damage the health of the reef,” Dr Mumby said.
The study, published in the journal Science, found that although parrotfish numbers went down, some individuals lived long enough to grow longer than 28cm – too big to be swallowed by predators.
“This ‘escape’ from a risk of predation means most reserves are unlikely to reduce the amount of grazing even after the number of predators rises,” Dr Mumby said.
“We found there was a doubling of grazing and a fourfold reduction in the amount of seaweed. For the first time we have shown that protected marine reserves create the conditions that are needed for corals to recover.”