When little can be too much: Small-scale fishing and the vulnerability of coral reefs. PDF Print E-mail
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Written by Albert Norström   
Thursday, 22 April 2010 21:00

The world´s coral reefs are being ravaged by a range of human pressures, including climate change, pollution and overfishing. As a result, a substantial number of the worlds reefs are rapidly losing live coral cover. The implications of this coral loss are staggering, not only for associated reef fish, but also for the continued flow of ecosystem services like tourism, fisheries and coastal protection. Coral degradation can be a tough nut for managers to crack, especially if coral loss is associated with a phase shift to another community state that is dominated by another bottom-living organism. The classic example of a coral reef phase shift is the dramatic transition from coral dominance (52% coral cover, 4% macroalgal cover) to macroalgal dominance (2% coral cover, 92% algal cover) which occurred on Jamaican reefs in the 1980s as a result of the synergistic impacts of overfishing, hurricane damage and disease. Similar examples of coral-macroalgae phase shifts have been observed across the Caribbean region, throughout the Eastern-Pacific, Indian Ocean and on the Great Barrier Reef. This "alternative" reef state are problematic for managers since they can be long-lasting and difficult to steer back to a coral-dominated state.

 

Artisanal fisherman in Zanzibar. Photo by Jerker Lokrantz/azote.se

Managing for coral reef resilience has been promoted as building insurance against degradation and decreasing the likelihood of reefs undergoing shifts to such undesirable states. Resilience has been a great theoretical and communicative tool for understanding these dynamics and mobilizing conservation initiatives and has recently been the basis of developing early warning signals and/or operational indicators of resilience that can help managers predict whether or not a reef can absorb disturbances and rebuild itself.

 

However, little is still known about how small-scale, artisanal fishing can compromise coral reef resilience. In a new study, a team of Swedish and U.S scientists (including yours truly) show how this form of fishing can have significant impacts on the potential vulnerability of reefs by tinkering with a vital source of reef resilience: macroalgae-munching parrotfish. Parrotfish are herbivores and can be likened to lawnmowers that control the growth of fleshy macroalgae on a reef. Herbivorous parrotfish are susceptible to artisanal fishing gear and reducing their numbers could bring the reef to the brink of a shift to a macroalgal-dominated state.

The study, published in the journal Environmental Conservation, assessed the vulnerability of five coral-dominated reefs on the western side of Zanzibar Island. The team found that fishing pressure had a negative effect on parrotfish biomass, abundance, diversity and species richness. Moreover, highly fished reefs were dominated by smaller fish.

- Knowing when key ecosystem processes such as herbivory are approaching critically low levels is important because it could provide resource managers with an opportunity to avoid unforeseen and potentially persistent ecosystem changes, says lead author Jerker Lokrantz.

He recommends the use of routine monitoring measures such as species richness, biomass and size class distribution of key functional groups provide important information regarding potential vulnerability of coral reefs.

- These methods should be used widely as significant indicators for managers to avoid undesirable ecosystem changes, Lokrantz says.

 

 

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