Fish ladders built into hydroelectric dams in South America can harm, rather than aid, migrating fish, a study to be published in the journal Conservation Biology revealed.
The paper by Fernando Pelicice and Angelo Agostinho analysed two case studies in Brazil (both in the upper Paran drainage): the Porto Primavera Dam and the impoundments on the Paranapanema River.
These are regions where large migratory fish such as the dourado (Salminus brasiliensis), pintado (Pseudoplatystoma corruscans), piracanjuba (Brycon orbignyanus), pacu (Piaractus mesopotamicus), and curimba (Prochilodus lineatus), migrate up- and downriver for spawning and nursery purposes.
The authors found that the four conditions necessary for the fish passages to serve as an ecological trap: (1) attractive forces leading fish to ascend the passage; (2) unidirectional migratory movements (upstream); (3) the environment above the passage has poor conditions for fish recruitment (e.g., the absence of spawning grounds and nursery areas); and (4) the environment below the passage has a proper structure for recruitment were met at these dams; the fact that the populations of migratory fishes in these areas have been declining lends support to the premise that the fish ladders are adversely affecting fish populations by transferring populations from high-quality (riverine) to low-quality (reservoir) environments.
In the Porto Primavera Dam, the authors observed that (1) fish that reach Porto Primavera Dam are regularly attracted by fishways, (2) except for those fish that immediately return thought turbines and spillways, all others tend to remain trapped between Porto Primavera and Jupi (the latter does not have fish passes), and (3) fish are removed from high-quality habitats (floodplain) and delivered into low-quality habitats (reservoir) while in the Paranapanema impoundments (1) the ladder in Canoas I actively attracts fish schools from Capivara Reservoir, (2) the fish flow is unidirectional and upstream, and (3) there is no evidence suggesting that these fishes are reproducing in areas above Canoas I Dam
The authors conclude: To avoid creating ecological traps and other negative effects, fish passages should be built for the purpose of conservation and not simply as a bureaucratic formality
Decisions of why, where, and how to install a fish pass must always be based on sound ecological information, and long-term monitoring and surveys are paramount after installation.
Management plans need a large-scale approach and a deep understanding of the whole system, information that requires much effort and financial support. Passages cannot be managed in a trial-and-error fashion and cannot be based solely on the assumption that fish have to migrate.
For more information, see the paper: Pelicice, FM and AA Agostinho (2008) Fish-passage facilities as ecological traps in large Neotropical rivers. Conservation Biology. DOI:10.1111/j.1523-1739.2007.00849.x