No-take areas might protect reefs against overfishing, but they seem powerless to help corals fight the effects of global warming.

Coral bleaching refers to the loss of colour caused by the expulsion of symbiotic algae which usually live inside coral tissue. It is caused by unusually high ocean temperatures, and scientists are concerned that bleaching events will become more common as a result of climate change.

A major coral bleaching event in 1998 damaged reefs worldwide, but little was known about its effects on fish populations and the long-term recovery of the reef ecosystem.

To investigate, Nicholas Graham at Newcastle University, UK, and his colleagues studied reef fish numbers and diversity at 66 sites across the Indian Ocean including so-called no-take areas, where fishing is not allowed.

No benefit

“Although these areas weren’t designed to protect against bleaching, there is an expectation that they will protect an assemblage of fish fauna which will hopefully assist the reef to recover,” says Graham.

Unfortunately this was not the case. They saw the same extent of coral loss and impact on fish numbers in protected areas, as in no-take zones, and there was also no difference in the amount of time it took the reefs to recover.

Some reefs fared better than others, though. For example, the Chagos archipelago – south of the Maldives – is completely protected from fishing, but also has no pollution or sediment-producing building construction on the islands, and the area seems to have recovered faster than other areas.

Meanwhile pockets of the Seychelles surrounded by deeper water and strong currents are also recovering quickly.

“[The currents are] probably flushing the reefs and removing any excess sediment, which can discourage coral from settling,” says Graham.

Clean-living corals

He doesn’t advocate doing away with existing protected zones, though. “They are still working in terms of fisheries,” he says.

But additional protected zones that focus on reefs that show the greatest ability to recover are also needed, he says. And a reduction in land-based pollution and sedimentation should also help damaged reefs to bounce back.

DeeVon Quirolo, of the coral protection organisation Reef Relief, agrees, saying that reducing pollution will have more bearing on the health of coral reefs than “drawing lines in the water called no-take zones”.

“Advanced wastewater and storm-water treatment, reduction of sedimentation and runoff from agricultural areas, and no discharge zones for boater sewage are all ways to ensure that corals can survive,” Quirolo says. “Merely expanding protected areas without cleaning up the water will not.”

Journal reference: PLoS One (DOI: 10.1371/journal.pone.0003039)