Mangroves Are Nurseries for Reef Fish, Study Finds

John Roach
for National Geographic News
February 4, 2004
Mangroves—forests of tropical trees and shrubs rooted in saltwater
sediments between the coast and the sea—are crucial nurseries for
coral reef fish, according to a new study.

The finding highlights the importance of the rapidly dwindling habitats to reef communities.

"Beyond showing they are important, we showed they are much more important than even assumed," said Peter Mumby, a marine biologist at the University of Exeter, England.

Mumby and his colleagues found that mangroves serve as a vital, intermediate nursery as coral reef fish journey from their cribs in seagrass beds to the large coral reef ecosystems that fringe coastal communities.

Coral reef fish were up to twice as abundant on reefs adjacent to mangrove forests compared to reefs that weren't, researchers found. They also learned at least one species, the rainbow parrotfish (Scarus guacamaia), depends on mangroves for its very survival.

The study will appear in tomorrow's issue of the science journal Nature and was supported by a grant from the National Geographic Society's Committee for Research and Exploration.

Mangrove Conservation

Mumby and his colleagues believe that conservation efforts are necessary to protect connected corridors of mangroves, seagrass beds, and coral reefs to maintain the resiliency of coral reef ecosystems—and their productivity for fisheries.

Ivan Valiela, a marine biologist with Boston University's Marine Biological Laboratory in Woods Hole, Massachusetts, agrees. He said the research reinforces the concept that individual ecological units—mangroves, reefs, land—are crucially intertwined.

"Maintenance of these important environments therefore has to be done from a wider perspective," he said. "This whole set of concepts bears on the issue of setting up coastal reserves, national parks, maintaining commercial stocks, and a host of other management issues."

Nursery School

Mangrove forests are home to an abundance of wildlife. Above water, butterflies, birds, and mosquitoes zip around the canopy. Snakes, crocodiles, and crabs scurry and swim about the forest floor. And in India, Bengal tigers (Panthera tigris) laze in forest branches.

"You tend to find mangroves form this very dense network of channels and creeks that are very, very calm and peaceful but also [teem] with all sorts of life," said Mumby.

Researchers have long known that fish often mature in the murky saltwater amid the tangled labyrinths of roots created by mangroves. But according to Mumby, the importance of these nurseries to reef fish communities had never been quantified.

Factors such as fishing pressure and larval supply were thought to be more important to the structure and abundance of reef fish than the presence or absence of mangrove forests.

Since juvenile fish are known to hang out in other habitats like seagrass beds and small, protected patch reefs before venturing out to large reefs, researchers sought to answer a key question: In the absence of mangroves, wouldn't these other habitats suffice?

To find out, they sought coral reefs so isolated from mangroves that it would be impossible for fish from mangrove habitats to reach them. They found such reefs in Belize.

"In Belize, we have the unusual situation of offshore reef atolls with massive amounts of mangroves as well as atolls with nothing at all," said Mumby.

The researchers contrasted the populations of 164 fish species in the two different habitats. They found that mangroves serve as an intermediate nursery, making for much healthier and robust coral reef fish communities.

Mumby explained that the fish start out in seagrass beds, but once they grow two to three inches (five to eight centimeters) they are too big to hide from predators there. At that point they move into mangroves, which offer murky hiding spots and abundant food.

"They survive well in the mangroves until they are a bit larger," said Mumby. "But at some point they need to move to the reef. We are not sure why they move to the reef, but [we] suspect it's a good place to reproduce."

Once they grow big enough in the mangrove, the fish swim out to patch reefs in the lagoon. There, they co-exist with thousands of other juvenile fish, packing on girth in order to reach and survive on the larger, fringing reef.

In the absence of mangroves, fish swim directly from the seagrass beds to the patch reefs. But because they are smaller, predators catch them more easily, said Mumby.

Mangrove Destruction

Previous research by Valiela indicated mangroves are being destroyed more than twice as quickly as the well-publicized destruction of tropical rain forests. "We, by no means, expected to find the rates we in fact calculated," he said.

In the past, mangroves were deemed a mosquito-infested nuisance to waterfront home development and razed. Today, they are mostly cleared to make way for shrimp farms.

Mangroves provide important functions, including processing land-derived nutrients, serving as a buffer against pollution runoff, and filtering food for marine mammals.

The finding that mangroves serve as crucial nurseries for coral reef fish highlights another reason to conserve these rapidly disappearing habitats. "To really sustain fish, one thing you should aim to do is conserve a certain amount of mangroves," said Mumby.

According to Valiela, this is easier said than done. "We are dealing with Third World, marginal economies. There are few choices for these people. We are sure they do not want to damage the very environment in which they live, but there are few other crops that yield as much [as shrimp farms]," he said.

Mumby said he hopes the findings will strengthen the importance of mangroves to fishermen who have a key political voice in many tropical regions. Valiela is calling on the international community to better understand the ecology of these connected ecosystems and pose conservation incentives and sustainable development alternatives for affected local communities.

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