Diseases on reefs? Overfishing and maybe butterflyfishes involved

Disease is killing off corals faster, in some parts of the world, than sedimentation, chemicals flowing from the land, damaging anchoring procedures and destructive fishing techniques.

And what's promoting disease? Perhaps it's overfishing and a high population of butterflyfishes.

(Image: Complex marine communities appear healthier than ones missing key players, research indicates. Credit: National Marine Fisheries Service.)

A new study suggests one thing can help protect reefs from coral disease: a diverse fish population.

This odd-seeming result, argues, the authors say, for protecting coral reefs from overfishing.

To make sense of this research, it is necessary to think of coral reefs as interlocked communities rather than simply collections of living rock. And fish are part of the community.

The paper that reported the study is in the October 6, 2009, Proceedings of the National Academy of Sciences. (www.pnas.org) It is entitled, “Functionally diverse reef-fish communities ameliorate coral disease,” by University of Guam Marine Laboratory researchers Laurie J. Raymundo, Andrew R. Halford, and Alexander Kerr, and University of Hawai'i Department of Zoology researcher Aileen P. Maypa.

The researchers studied reefs in the Philippines, including both fished reefs and marine protected areas. They looked at more than 20,000 coral colonies. In general, they found that reefs with diverse fish communities had less disease, and vice versa.

Their paper reports that fish are a major regulating force on reefs. Some fish, of course, eat corals, but other fish eat those fish, still others eat algae that threaten corals and so forth.

What happens if fishing pressure pulls out one piece of this three-dimensional jigsaw?

“If subject to sustained heavy fishing, entire functional groups can be lost, resulting in a cascade of effects,” the authors said.

Fish that are no longer being eaten can increase in population, building up the pressure on the things they eat, for example. The cascade of impacts threaten the stability of the reef and, “ultimately, the resilience of coral reefs is compromised.”

One additional and intriguing bit of information from the research was that areas with high butterflyfish populations tended also to have higher coral disease levels.

Anglers and spearfisherfolks tend not to target butterflyfishes, so they can actually increase in number after heavy fishing—both because they're not being killed by humans, but also because their predators are removed by human fishers.

Could these fishes be associated with coral disease spread? It looks like they might. The researchers studied data on Australian coral reefs and came up with the same results.

“Chaetodontids (butterflyfishes) again emerged as the single fish family significantly

and positively associated with disease prevalence,” they wrote.

© Jan TenBruggencate 2009