Herbivorous fish help reefs recover

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Having a diverse community of herbivorous fishes is crucial to the recovery of degraded coral reefs, according to research published in a recent issue of the Proceedings of the National Academy of Sciences.

Deron Burkepile and Mark Hay carried out experiments over a period of two years, where they enclosed equivalent densities and similar biomasses of single- and mixed-species groups of herbivorous fishes in large replicate cages on a reef in the Florida Keys to assess how herbivore species richness and species identity affected reef structure.

In the first year, the authors used redband parrotfish (Sparisoma aurofrenatum) and ocean surgeonfish (Acanthurus bahianus) to create treatments of: (i) two redband parrotfish, (ii) two ocean surgeonfish, (iii) one redband parrotfish and one ocean surgeonfish, or (iv) no enclosed fishes; in the second year, the authors used the redband parrotfish and the princess parrotfish (Scarus taeniopterus) instead.

Sparisoma aurofrenatum, Laszlo Illyes, Creative Commons.

Over the seven- to ten-month duration of the experiments, the authors monitored changes in macroalgal abundance, macroalgal species composition and coral survivorship and growth within the cages.

In the first year of the experiment, the authors found that algal cover was a significant 2.7"5.9 times higher and algal biomass a significant 3.5"17.4 times higher in the single-herbivore treatments than in the mixed-herbivore treatment.

Conversely, coral mortality was 0% in the mixed-herbivore treatment but was significantly higher (8"24% in 10 months) in the single- or no-herbivore treatments and coral cover increased 22% in the mixed-herbivore treatment but declined by 6"30% in all other treatments.

The results of the second year of the experiment were cut short by Hurricane Dennis, which wiped out all the cages.

According to Mark Hay, f the many different fish that are part of coral ecosystems, there may be a small number of species that are really critical for keeping big seaweeds from overgrowing and killing corals.

Our study shows that in addition to having enough herbivores, coral ecosystems also need the right mix of species to overcome the different defensive tactics of the seaweeds.

The authors conclude that ur findings suggest that a combination of (i) marine protected areas that help restore fish stocks and (ii) proactive management for critical components of herbivorous fish diversity could hasten recovery of coral reefs.

For more information, see the paper Burkepile, DE and ME Hay (2008) Herbivore species richness and feeding complementarity affect community structure and function on a coral reef. Proceedings of the National Academy of Sciences 105, pp. 16201"16206.