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Traditional fishing practices — and not just large-scale commercial fishing — are destroying Fiji's coral reefs, according to new research.

A team of UK-based scientists has found that subsistence fishing in Fiji can promote outbreaks of a coral-eating starfish normally eaten by the fish caught by local people. They warn that this can lead to cascades of other ecological impacts as corals are replaced — perhaps permanently — by algae.

"This study suggests that even low levels of fishing may cause ecosystem meltdown," says Nick Polunin of the University of Newcastle Upon Tyne, who led the research.

Commercial fishing has long been identified as a major threat to coral reefs. But this study examined the impact of subsistence fishing for food only, using traditional methods such as spear or hook-and-line fishing.

The scientists studied 13 'relatively pristine' islands in Fiji's Lau Island group. For each island, they devised an index of fishing intensity based on the human population and the extent of the coral reef present. They then surveyed the islands' reefs for more than 100 species of predatory fish, as well as crown-of-thorns starfish, which feed on live corals.

They found that, as fishing intensity increased, predator densities fell by 61 per cent, while starfish densities increased — in some cases more than 1000-fold. Meanwhile, reef-building corals declined by a third and were replaced by non-reef building species (mainly algae) as a result of starfish predation.

This shift from coral to algae is also happening in the Caribbean and elsewhere. But there is debate among researchers about whether it is primarily attributable to nutrient inputs, to fishing or to climate-change effects.

"It is true [that coral reefs] are proving vulnerable to massive impacts such as coral bleaching linked to climate warming," Polunin told SciDev.Net. "However, we have the first good evidence for a strong link between fishing and a coral-to-algal phase shift."

Writing in the journal Ecology Letters, the researchers suggest that maintaining a minimum level of predators in protected areas across reefs may be a useful approach to conservation management.

Link to abstract of paper in Ecology Letters