Humans affect the role of parrotfishes on the reef

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Kill all the reef fish and the reef dies with them, right? Not quite, according to new research.

Kill all the reef fish and the reef dies with them, right? Not quite, according to new research.

In the research, to be published in a forthcoming issue of the Proceedings of the Royal Society B, David Bellwood, Andrew Hoey and Terence Hughes sampled 18 reefs from Mauritius in the western Indian Ocean eastwards to Moorea in French Polynesia in order to examine the effect of fishing on ecosystem function (more specifically to what extent are rates of grazing, bioerosion, coral predation and sediment removal by parrotfishes influenced by human activity).

The authors first quantified the impact of human activity on parrotfish abundances and population structure before calculating the extent to which these changes in fish populations may impact the four key ecosystem processes performed by parrotfishes described above along a gradient of human population densities.

The authors found that rates of coral predation and bioerosion by parrotfishes are very sensitive to human population densities. This loss in ecosystem function was largely explained by the overfishing of large fishes. On the other hand, they found grazing and sediment removal by parrotfishes to be relatively resilient to human activity.

The most positive aspect of the findings is that even in the face of moderately high human population densities and intensive fishing, the reefs that the authors examined still retain enough grazing activity to prevent seaweeds from taking over the coral reefs. Furthermore, the resilience of the small- and medium-sized herbivorous fishes to fishing is critical for supporting the livelihoods of artisanal fishermen.

However positive this outlook, the authors warn that there may be hidden dangers. The coral reef may degrade to another undesirable state instead of a seaweed-dominated one, for one.

Secondly, the small-bodied, fast-growing parrotfishes that come to predominate the reef assemblage may be less capable of coping with future changes.

Thirdly, the loss of large parrotfishes may be slowly changing the species composition of corals.

Lastly and most importantly, the resilience of the small parrotfishes to harvesting encourages continued fishing, as the ability of degraded systems to support ongoing artisanal fishing makes it worthwhile to continue harvesting parrotfishes. 

This maintains unsustainable fishing pressure on larger parrotfish species long after they have collapsed.

For more information, see the paper: Bellwood, DR, AS Hoey and TP Hughes (2011) Human activity selectively impacts the ecosystem roles of parrotfishes on coral reefs. Proceedings of the Royal Society B: Biological Sciences doi:10.1098/rspb.2011.1906.

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