Jan. 11, 2006 — Saving the big fish that eats the little fish, which eats the seaweed, is now helping to revive once-dying tropical coral reefs.

Marine ecologists studying coral reefs in a Bahamas marine reserve were surprised to discover that when fishing was banned from the reserve and the number of big predatory Nassau groupers was allowed to grow, the reef got healthier.

It turns out that by eating the smaller seaweed-grazing parrotfish on the reef, the groupers are freeing up room for larger kinds of parrotfish that are better at mowing down reef-choking seaweed.

So, by eating the little fish, grouper are indirectly causing more seaweed to be eaten, which opens up the water for coral to flourish.

The discovery underlines the powerful and complex effects that can start at the top of a food chain and trickle down, what's known as a "trophic cascade."

"We were quite surprised to find this subtle effect," said marine ecologist Peter Mumby of the University of Exeter in the U.K. It's a great example of how important it is to study actual reef ecology and not rely to heavily on fisheries models, he said.

"Until you actually study (reefs), you can't predict the effects," Mumby said. He and several colleagues published a report on the discovery in the Jan. 6 issue of the journal Science.

Caribbean coral reefs have been in trouble for at least 20 years — ever since an unknown disease wiped out sea urchins that used to help keep the seaweed under control, said Mumby.

The only effective seaweed grazers that remain on the reefs are several species of parrotfish, he said.

When fishing was banned in a Bahamas marine reserve and the parrotfish-dining Nassau groupers bounced back, it wasn't expected to have a positive effect on the reefs — but it did.

"Clearly the removal of a large predator from the system has a trickle-down effect," said marine ecologist John Ogden of the University of South Florida.

It's an idea that was ignored by the previous generation of fisheries managers, he said, which is one reason so many marine fisheries are in trouble today.

On the other hand, Ogden says overfishing, like that which removed the groupers from the Bahaman reefs, is only one of the "big three" factors affecting fisheries and reefs worldwide. The other two are land-based pollution reaching the oceans and global warming.

The pollution that causes the most trouble is run-off fertilizers and sewage that cause plankton blooms, which suck up oxygen from the water, creating vast, putrid marine "dead zones" like that found where the Mississippi River empties into the Gulf of Mexico.

Global warming, for its part, is gradually raising water temperatures and stressing coral reef organisms, Ogden explained. It could even have been a factor in, or at least is analogous to, the sudden unexplained die-off of the Caribbean sea urchins, he said.

"It's entirely possible that the accumulation of stress allowed what was otherwise a harmless organism in the (skins) of sea urchins to become pathogenic," Odgen said.

As for how the Bahamas study reflects other reefs worldwide, Mumby said it depended on how many seaweed grazers there were at a given reef.

In the Pacific Ocean, for instance, coral reefs have many kinds of grazers, so it's less likely that the demise or re-introduction of one species or grazer or predator would cause as dramatic an effect.