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Removing just one fish species from a tropical river can have major effects on the ecosystem's health, according to research published in Science today (11 August).

The finding contradicts the general belief that the greater abundance and diversity of other species would compensate for the loss.

Researchers removed the flannelmouth fish (Prochilodus mariae) from a tributary of the Orinoco River and measured how this affected the movement and use of carbon in the ecosystem.

The fish is the dominant species in many South American rivers, where it feeds on algae and detritus on the riverbed.

As the fish moves, feeds, and excretes waste, it plays a key role in the cycle of carbon synthesis and degradation. It also removes particles that block the light needed by cyanobacteria that process nitrogen in the ecosystem.

The researchers found that the river's carbon cycle was disrupted within 48 hours of them removing the fish. The effect lasted for at least 40 days.

With the fish gone, the amount of organic carbon on the riverbed rose by 450 per cent. The amount of this carbon travelling downstream fell, suggesting a reduction in energy available to species there.

The researchers were surprised that a single fish species could have such a substantial impact in a highly diverse ecosystem. The river has more than 100 other fish species, but their combined activity failed to compensate for the loss of the flannelmouth.

Small detritus-eating fish such as the flannelmouth make up 50-80 per cent of total mass of fish in the Orinoco and Amazon basins but populations are declining due to overfishing, dams, deforestation and pollution.

The study also showed that flannelmouths have decreased in size over the past 25 years, a shift that tallies with changes in fishing nets.

"The sizes of fishing net mesh currently in use are illegal, but there is very little enforcement of the existing fisheries laws," says Brad Taylor, who led the study.

Taylor told SciDev.Net that the ecological consequences of overharvesting small fish such as the flannelmouth, compared to larger predatory fish species, have received little attention.

Yet the rapid growth, faster reproduction and greater abundance of smaller species make them targets for intensive harvesting.

Taylor called for better management of fisheries and additional research on the effects of losing small fishes on other ecosystem functions, bird populations and human welfare.

Link to full paper in Science

Reference: Science 313, 833 (2006)

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