Mangrove Might: Nearby trees boost reef-fish numbers

Mangrove forests are “unexpectedly” important to the fish on neighboring coral reefs, says a research team that took a new approach to assessing the tree-fish connection.

Biologists already knew that coastal groves of the tough trees, which thrive with their roots in salt water, serve as a refuge for young fish at some point in their development, explains Peter J. Mumby of the University of Exeter in England. These youngsters move from the groves to a reef when they grow up.

Mumby and his colleagues worked out a way to measure the importance of the mangroves to the fish. Belize reef locations near mangrove stands have an abundance of several common fish when compared with mangrove-poor locales, the researchers report in the Feb. 5 Nature.

“We expected the effect to be significant, but we didn’t expect it to be so big,” Mumby says.

The finding holds lessons for conservation, comments John C. Ogden of the Florida Institute of Oceanography in St. Petersburg. “What it says of most importance to me is that if we are to conserve and manage coral reefs, we have to make our plans at a much larger scale than we have been, taking into account the whole seascape and the adjacent land areas.”

That’s an urgent issue, says Ivan Valiela of Boston University. He estimates that at least 35 percent of the mangrove forests worldwide have disappeared in the past 2 decades, predominantly because of development of coastal lands.

Along the coast of Belize, Mumby and his team selected six islands adjacent to reefs. Three of the islands had abundant mangroves, and the others had few. The researchers could treat each island and its nearby reefs as a relatively self-contained system.

The researchers surveyed six common fish species that migrate from mangrove stands to reefs. The team reports that, on deep reefs, the biomass of commercially fished yellowtail snapper in the mangrove-rich areas is double that in the mangrove-poor areas. Three other species had roughly 50 percent more biomass near mangroves, and the other two showed no effect.

At shallow reefs, four of the species were at least twice as plentiful near mangroves. The blue-striped grunt showed the biggest effect, a seven-fold boost in biomass.

Valiela calls the magnitude of effects in the new study “notable.”

Statistical tests and information on fishing practices led Mumby’s team to reject the possibility that the biomass increases came from differences in predators, other habitat characteristics, or fishing.

Mumby suggests that the young fish grow to several centimeters in length in sea grass beds and then hide in the mangroves to get some extra heft before they head to the reefs.