Environmental disturbances and fishes in the Amazon.

The Amazon is a multi-faceted biome extending across all of the countries in northern South America. Aquatic habitats include watercourses of all sizes, beaches, lakes, flooded forests and floodplain areas. White, black and clear water types differ in the amount and nature of their sediments and in their levels of dissolved organic carbon, ion content, pH, water density and temperature. This environmental diversity is added to the regular cycle of changes driven by region’s annual flood pulses. Over millions of years, the region has experienced challenges requiring continuous biological adjustments of all organisms. These natural constraints are claimed to be the cause of the unparalleled diversity of fish species in the region. The fishes have developed myriad adjustments at all biological levels to face the never-ending environmental challenges that naturally occur in their habitats (Almeida-Val et al., 1999). These challenges are now joined by recent human disturbances that are pushing fishes to their biological limits. The scientific community must therefore focus efforts on developing reliable strategies for biological conservation, and local, regional, national and transnational authorities must promptly adopt these strategies. River damming is one of the most controversial environmental challenges. Hydroelectric dams have both costs and benefits, but the costs (both monetary and non-monetary) are consistently understated and the benefits exaggerated (Fearnside, 2014). Blocking fish migration is a critical effect of dams. Many Amazonian fishes have an annual piracema, or mass stock migration ascending Amazon tributaries to breed in the headwaters. The most dramatic case is the blockage of migration of the ‘giant catfish’ of the Madeira River [including Brachyplatystoma rousseauxii (Castlenau 1855) and Brachyplatystoma vaillantii (Valenciennes 1840)]. These magnificent fishes were a major economic and nutritional resource for riverside populations in Brazil, Peru and Bolivia. The large catfishes did not succeed in locating and entering the fish passages installed at the two dams (Fearnside, 2014). Blocking fish migration also causes fragmentation of populations and loss of gene flow, which also affects fish yields.