Published online 17 January 2008 | Nature | doi:10.1038/news.2008.445

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Fish ladders of doom

Attempts to help river-spawning fish in Brazil may have led to their decline.

Catfish might go up a fish ladder to spawn, but their offspring may never find their way back down.NOAA

Throughout much of South America, fish 'ladders' designed to help fish swim up-river to breeding grounds are actually sending the animals to their death, with no chance of escape. That’s the conclusion of a review of river conditions by two researchers in Brazil.

In Brazil, fish populations started declining as hydroelectric dams were installed in the 1970s and 1980s, creating more and more reservoirs. In 1991, just over 600 tonnes of migratory fish were captured in two of the main Brazilian basins; in 2005, that number dropped to 200 tonnes, the researchers say. The problem was thought to be that the dams cut off access to normal breeding grounds for fish that swim upriver to spawn. It’s a problem of large proportions: some 700 large reservoirs now exist in Brazil alone, and 96% of the country's power is from hydroelectric sources.

Fisherman complained, and, as fisheries suffered, some Brazilian states forced hydroelectric companies to build fish ladders — a series of steps that allow the fish to swim up to and past a dam. Initial observations suggested that fish were using the ladders, but the decline continued. In fact, it accelerated.

Some ecologists suggested the acceleration was the result of increased fishing. But Fernando Pelicice and Angelo Agostinho at Maringá State University in Brazil suspected that the ladders themselves might be to blame.

Ecological traps

Pelicice and Agostinho analysed fish ladders on the Paraná river and found that they bore all the hallmarks of being ecological traps rather than helpers.

The ladders provide river-like flow conditions that attract migrant fish — such as the dourado and curimba — looking for spawning grounds. At the top of the ladders, the fish arrive in reservoirs, but because conditions in the reservoirs are not favourable (the waters are too clear and still to provide the cover the fish rely on to hide from predators, or the oxygen they enjoy in rivers), the fish bolt for tributaries to spawn. If swift-water tributaries are not available, the fish die. If they do manage to spawn, upon hatching their offspring travel downstream and hit the edge of the reservoir, where they often die in anoxic waters or are eaten by predators before finding the ladder that leads downstream to safety.

“Everyone was worried about the fish going up, nobody thought about them coming back down,” says Agostinho.

The team examined two areas in depth — the Porto Primavera Dam and the Paranapanema River. Papers published about fish in these areas helped to confirm that sightings of fish travelling up ladders were followed by low numbers of fish the next year.

Out of the frying pan

Before the ladders were built, the fish would reach the dams and when they could not go further upstream, they would go back down and look for smaller tributaries in which to spawn. It was not a good situation, but at least it was not a trap, he says.

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“It is frustrating to hear about this, especially because researchers have worked through very similar ecological problems in North America,” says Bill Pine, a fish ecologist at the University of Florida in Gainesville. In North America, some fish ladders have been made that bypass lakes completely. The specific conditions of different sites and requirements of different species need to be taken into account when designing systems, he says.

The best course of action, he says, is for development agencies to manage ladders experimentally to learn how to best support local fish populations. “Just because a design exists for a ladder that works for Pacific salmon doesn’t mean it will work for Amazon catfish. We need to do a better job of learning from our global successes and failures,” Pine says. 

  • References

    1. Pelicice, F. M. & Agostinho, A. A. Conservation Biology advance online publication doi: 10.1111/j.1523-1739.2007.00849.x (2008).
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