South America has an area of 17.8 million square kilometers and a human population of almost 400 million inhabitants. South America harbours a wide range of aquatic habitat types, including many of the largest rivers of the world, extensive marshes, altitudinal lakes and rivers, and several others. Approximately one fourth of all free, surface freshwater of the world is in South America. Water chemistry of South American rivers is influenced by landscape features, especially headwater source, dominant vegetation cover, and soil types.
Rivers that drain the Andes are sediment-rich white-water rivers (e.g., Amazon, Meta, Marañon, Napo, Madeira), black-water rivers are those dark tea coloured that originate in the thickly forested lowlands (e.g. Atabapo, Japurá, Tefé, Negro) and are tannin-rich, having very low sediment loads. Finally, rivers that drain the ancient and well-weathered crystalline rocks of the Guiana and Brazilian Shields are the clear-water rivers, with low sediment and high transparency (e.g., Xingu, Tapajós, Tocantins, Ventuari). When total length or drainage basin area is considered, two South American rivers (Amazon and Parana) are among the largest 10 rivers in the world. However, if average discharge is accounted for, five South American rivers stand among the top 10 – Amazon, Orinoco, Madeira, Negro, and Parana, together discharging 393,000 m³/s in the Atlantic Ocean. The Amazon River alone is by far the world’s largest, with 219,000 m³/s – the second being Orinoco with 98,000 and Congo the third with 41,800 m³/s. As impressive as the Amazon discharge is, its the 250,000 square kilometers of periodically flooded lowlands that harbour an enormous diversity of specialized fishes.
The freshwater fish fauna of South America is the most diverse of all continents, with an estimated number of species above 4,000. In addition to that, fish taxonomists have described around 100 new species to the Neotropical Region every year, making an average of one new species every 3.5 days. The curve of species descriptions is clearly not approaching to an asymptotic, and it can be expected that the final number of fish species in South America may exceed 6,000 species.
Diversity of morphological adaptations and ecological requirements are also dramatic among the South American fish fauna. The armored-catfishes or Loricariidae, the largest catfish family with above 800 species, has remarkable examples. Some species live in highly oxygenated, fast flowing mountain streams while others dwell in almost anoxic lagoons in Amazonian lowlands, and are thus capable of using their intestines as an accessory respiratory organ. Other amazing adaptations can be found among the catfishes, as the phreatic habits of Phreatobius, a genus with a few species that live inside the soil in river banks and are most easily captured in existing water wells.
Possibly, the strangest feeding habits of South American fishes are those of the blood-sucking, parasitic candirus (Vandellia spp). These fishes are provided with special odontodes in their opercular area that allow them to firmly attach to the branchial arches of larger fishes and use their highly specialized premaxillary teeth to directly puncture the branchial artery of their hosts and feed on blood. These are the feared candirus that are known to mistakenly enter the human urethra causing painful injuries.
Also highly feared is the poraquê (Electrophorus electricus), one of the almost 200 electric eels of South America, or gymnotiforms. Most of the species in this order are small and use their electrogenic organ to build an electromagnetic field around themselves to communicate and to perceive their environment. Contrastingly, the poraquê grows to almost two metres long and can deliver electric shocks of up to 600 volts, which are used to stun their prey.
Several of freshwater habitats in South America are threatened by severe deforestation, water divergence for irrigation, hydroelectric damming, gold mining, which completely destroys the river beds and causes mercury contamination, and pollution in some restricted areas. Fish species are also threatened by invasive alien species and over exploitation. Deforestation and the consequent habitat degradation, especially the severe siltation of river beds, is one of the main threats to thousands of small to medium size rivers. Deforestation in South America occurs mainly for legal and illegal logging, cattle ranching, and especially for expanding the agricultural frontier on the Amazon borders and the Atlantic forest.
Hydroelectric damming affects fish populations in a three-fold manner. The transformation of a lotic environment into a lake either eradicates or extremely reduces reophilic species and at the same time provides conditions for lentic species to proliferate, thus severely changing the community composition locally. In a wider scale, it regulates the river flow below the dam, therefore disturbing the daily cycles of feeding and reproduction, and cuts the migratory routes of many large fishes. This is particularly important in South America because the reproductive biology of large, migratory fish is distinct from that of migratory North American or European fish species, for which fish passages where originally conceived. Contrary to those fishes, South American migratory species move up rivers during the high water season, passing the dam through fish passages, and spawn in the water current in the upper reaches. Eggs and then larvae are immediately carried down river by the turbid, well oxygenated torrent, and are expected to reach the floodplains in the low portion of the river courses, where they will feed and grow. If a dam is constructed between the spawning areas and the floodplains, eggs will sink as they reach the reservoir or will rapidly be eaten by the many tetras that usually dwell in lakes, and will not have the chance to go down the fish passages and reach the growing areas.
Gold mining in South American rivers is also an enormous environmental problem that adversely affects aquatic organisms. Rivers are heavily dragged, the process completely destroying river bed and banks. Further on the physical habitat degradation, the process usually employed to separate the gold from the substrate uses mercury, which is then vaporized causing severe poisoning to miners and the environment.
Other important threats for South American fishes is posed by the extensive amount of invasive alien species. This is not a severe problem in the Amazon yet, but many of the river drainages in Argentina, central and eastern Brazil, Chile, and other countries have many introduced alien fish species. Aquaculture is the main entrance for alien species and salmon has been introduced in south and central Chile, while Brazilian rivers harbour around 13 introduced species from other continents, especially tilapias, carps, trouts, ictalurid catfishes, clariid catfishes, among others. In addition to that, transposition of South American fishes between distinct river basins is also common, and commercially important fishes like the tucunaré (Cichla) and the tambaqui (Colossoma) from the Amazon basin have been introduced in the Pantanal and other places. The main problem caused by introduced fish species is competition for niches and resources, and sometimes their predatory habits. Predatory trout and salmon introduced in Chile and some parts of southern Brazil, respectively, have almost extirpated the local fish fauna in several places. Brazil’s Pantanal region, one of the world’s largest freshwater wetland ecosystems.
Despite the complex array of threatening factors that freshwater fishes are exposed to, there are limited on-the-ground conservation initiatives in South America. Perhaps the most important and successful conservation project is Project Piaba, created in 1989 by Labbish Chao in the Negro River north of Manaus, Brazil. The Project Piaba is a community based, sustainable ornamental fisheries project, that was founded on the perception that people living from harvesting ornamental fishes need to preserve the forest. The project’s slogan “Buy a fish and save a tree”; very well described this intention. Difficulties with the Brazilian environmental agency (IBAMA) and the fish exporters, however, have virtually shut down the Project, leaving hundreds of families in the Negro River with no other option than to become farmers. Other initiatives towards conserving fishes are the preparation of IUCN Red Lists for different countries or regions in South America. Probably the most ambitious of these is the partnership between the Brazilian Ministry of Environment and IUCN, with the aim of assessing the extinction risk of all Brazilian fish species. As this is a gigantic task, species are being assessed by region within Brazil.
Albert, J. S. and Reis, R.E. (eds). 2011. Historical Biogeography of Neotropical Freshwater Fishes. University of California Press, Los Angeles. 388p.
Malabarba, L.R., Reis, R.E., Vari, R.P., Lucena, C.A.S. and Lucena, Z.M.S. (eds). 1998. Phylogeny and Classification of Neotropical Fishes. Edipucrs, Porto Alegre, 603p.
Reis, R.E., Kullander, S.O. and Ferraris Jr, C.J. (eds). 2003. Check list of the freshwater fishes of South and Central America. Porto Alegre, Edipucrs, 729p.
Roberto is primarily interested in freshwater fish biodiversity, evolution and conservation. His research activities mostly involve discovering and describing the fish diversity of South America, and studying their phylogenetic relationships and biogeography. He has a Ph.D. from the University of São Paulo (1994), and is a Professor of Biology at the Pontifical Catholic University of Rio Grande do Sul, in Porto Alegre, Brazil, and a Curator of Fishes at the Museum of Sciences and Technology of the same University.
Roberto has extensive field experience collecting and studying fishes throughout South America, but in particular Brazil and Peru. He is also currently involved as Taxon Coordinator (FW Fishes) of the Brazilian Ministry of Environment initiative to assess the conservation status of all Brazilian vertebrates. He is a former President of the Brazilian Society of Ichthyology and has been a member of the Freshwater Fish Specialist Group since its beginnings.
E-mail: [email protected]