Fish Ladders in the Tropics: A Trip to Nowhere

by Glenn Switkes
Tuesday, March 25, 2008

New studies confirm that fish ladders at dams in the tropics fail to meet their objective of guaranteeing the survival of migratory fish, and in fact could hasten the extinction of some species. Brazilian scientists found that ladders act as an "ecological trap," attracting schools of fish to poorer environments, and making it even more difficult for them to reproduce.

Researchers studying a major dam built on the Tocantins River found that 16 migratory species were attracted by the turbulence of the water at the Lajeado Dam´s 874-meter-long fish ladder, built to simulate the rapids that were destroyed when the dam was built. At the base of the ladder, the scientists found greater than natural numbers of predators provided a serious challenge to the fishes' ability to reach the ladder. Other predators stake out the ladder itself, attacking the fish as they maneuver through the barriers.

When the migrators finally reach the reservoir, they encounter clear, still waters that favor predators, including tucunaré and piranha. In the case of Lajeado, 282 times more fish climbed the ladder than those that descended. Larva and fry that return downstream encounter similar difficulties - lacking a strong current, their descent is slow, and predators await them in the reservoir.

In a study that analyzed fisheries impacts at a dam on the Paraná River and at a complex of dams on the Paranapanema River, scientists at the University of Maringá concluded that the fish ladders´ impacts were so great that "they should be de-activated immediately." At Canoas I and II Dams, operated by Duke Energy, the ladders caused a collapse in fish populations downstream. Huge quantities of fish swam upstream the initial year of the ladder´s operation, but the following year the migration collapsed - a sign that the fish that had climbed the ladder failed to descend.
Ladders can function relatively well in the northern hemisphere, but very few studies have assessed the effectiveness of these mechanisms in the tropics. The journal Neotropical Ichthyology published a special edition with studies about fish passage in June 2007. Researchers found the efficiency of the fish elevators at Yacyretá Dam (Paraguay/Argentina) in transporting migratory species to be only 2%, with nearly all the fish successfully transported from only three non-migratory species.

The elaborate fish passage at Itaipu Dam is being used as a model for new dams in the Amazon, but its effectiveness has
The elaborate fish passage at Itaipu Dam is being used as a model for new dams in the Amazon, but its effectiveness has
Photo: Caio Coronel/Itaipu Binacional
In Brazil, technical studies have often advised against construction of fish ladders, but environmental regulations required them as a symbolic mitigation measure. Studies are inconclusive about the effectiveness of the fish passage which was built at Itaipú Dam in 2002, 19 years after the dam began operation, but it is already being put forth as a model for the fish passage proposed for the Santo Antônio and Jirau dams on the Madeira River in the Amazon, which has one of the world´s highest diversity of fish species. The passage simulates diverse environments, and includes fish ladders, a natural fish passage channel, two artificial ponds and a lake for reproduction.

Although Amazon fisheries are a billion-dollar resource that sustains tens of thousands of families, Brazilian President Lula has made statements trivializing the issue, complaining that opponents of the Madeira dams "were trying to dump catfish in his lap."

In the fight to save the Amazon, activists are calling attention to the risks of relying on ineffective passage systems to protect such rich fisheries. "The construction of the Madeira dams would affect thousands of families here in Porto Velho, who earn a living fishing and selling fish. The poorer families are especially dependent on fish for their dietary and nutritional security. The proposed mitigation measures are a huge technological experiment, and the impacts of the dams would be grave and permanent," says Iremar Ferreira of the Living Madeira Institute.