|Amazonia: Resiliency and Dynamism of the Land and Its People (UNU, 1995, 253 pages)|
|2. Environmental threats|
Disruption of fisheries as a result of dam-building could be a serious threat to the livelihoods of many rural and urban folk in Amazonia (fig. 2.2). A number of fish important in commerce and subsistence, such as jaraqui (Semaprochilodus spp.) and catfish (Brachyplatystoma flavicans, B. filamentosum), migrate from the Amazon to spawn in tributaries (Barthem, Brito, and Petrere 1991; Goulding 1981, 1989). Changes in water quality and flood cycles are likely to interfere with the reproduction and feeding of many of the 2,000 or more species of fish inhabiting the myriad waters of Amazonia.
Little is known about the impact of dams built thus far on Amazonia's fisheries (Bayley and Petrere 1990). On 2 October 1992, a school of jaraqui (Semaprochilodus sp.) was observed swimming back and forth along the foot of the dam at Curuá-Una in a fruitless at tempt to move upstream. The dam closed in 1976, so it is curious to see fish still trying to move upstream. Several other species have apparently disappeared above the dam, such as pirapitinga (Colossoma bidens) and jatuarana (Brycon sp.), but one interviewed farmer felt that fishing yields overall had not declined. Tucunaré (Cichla ocellaris) and charuto, in particular, are reportedly plentiful in the reservoir. The Curuá-Una reservoir spawned a population explosion of piranhas (Serrasalmus spp.) during the first two decades of operation (Ferreira 19B4; Junk et al. 1981). For all their fame as dangerous fish, piranhas are eaten by locals and sell briskly in urban markets.
Some fish were killed by the lack of oxygen and hydrogen sulphide when the Tucurui dam closed in 1984 (Sioli 1986), but the reservoir has become a significant fishery for the highly prized tucunare. Tucunaré from the Tucurui reservoir are marketed at least as far south as Carajás. The 2,100 km Balbina reservoir on the Uatumã River, completed in 1987 to provide electricity for Manaus, has also become a significant fishery for this spirited predator.
Known as peacock bass to English-speaking sport fishermen, tucunaré may accumulate mercury released by gold miners. Also, tucunaré in the Curuá-Una resrvoir near Santarém, Pará, became so heavily infested with parasitic nematodes that some locals declined to eat the highly prized fish (Junk and Nunes de Mello 1987).
Some fisheries downstream from the Tucurui appear to have suffered from the dam (Dwyer 1990: 44; Magee 1989). The productivity of fisheries appears to have declined mostly in the lower regions of the Tocantins in the vicinity of Cametá. One migratory species, Anodus elongatus, has virtually disappeared from the lower Tocantins (Merona, Carvalho, and Bittencourt 1987). Populations of Curimata cyprinoides have also diminished, at least temporarily. The Tucurui dam contributed to the collapse of the mapará fishery on the lower Tocantins by closing off a spawning route and reducing plankton biomass (Goulding, Smith, and Mahar in press).
Although the composition of fish communities has shifted below the Tucurui dam, the overall impact of this formidable barrier on fisheries has not proved especially serious from the perspective of local nutrition. A shrimp fishery based on Macrobrachium amazonicum was waning along the lower Tocantins well before the Tucurui dam closed; furthermore, this freshwater shrimp, which is used in a variety of regional dishes, thrives in the Tucurui reservoir (Odinetz-Collart 1987). The rapid turnover of water in the Tucurui reservoir, some six or seven times a year, helps avoid drastic changes in water chemistry and thermal stratification, thus reducing the danger of intoxicating fish (Barrow 1987).
Turtles, especially Podocnemis expansa, were abundant along the Tocantins in the seventeenth century and served as an important food for locals (Heriarte 1964: 30), but this resource had dwindled considerably long before construction of the Tucurui dam. The resulting reservoir, however, has covered many former nesting beaches and will probably preclude the chances of re-establishing sizeable populations of P. expansa along the Tocantins.
One concern about reservoirs in the Amazon is their potential role in favouring the population build-up of disease vectors. Soon after the Tucurui dam closed, some inhabitants and their livestock near the margin of the reservoir were plagued by swarms of mosquitoes (Mansonia titillans) and, to a lesser extent, horseflies (Lapiselaga grassipes). The former are known to carry two arboviruses, but no outbreaks of disease attributed to M. titillans have occurred near the Tucurui reservoir (Marques 1992). Populations of both flies appear to have dwindled, presumably as the new lacustrine ecosystem and surrounding areas have become more stable.
The Samuel dam on the Jamari River in Rondonia, which filled in 1989 to supply electricity to Porto Velho, has reportedly disrupted the upstream migration of some large catfish (João Paulo Viana, pers. comm.). Fisheries have also allegedly suffered downstream from the Balbina dam, but no quantitative data support such claims (Gribel 1990).
Some farmers have apparently benefited downstream from the Tucurui dam, whereas others have lost fertile planting ground. Regulation of water flow facilitates the irrigation of rice at the mouth of the Tocantins, but the reduced sediment load has resulted in a loss of flood plain for agriculture along the lower Tocantins (Barrow 1988).
No reservoirs created for hydroelectric dams in Amazonia are in imminent danger of losing electrical generating capacity because of siltation. The oldest dam, Curuá-Una, is nearly 20 years old and is still operational (fig. 2.3). All the other hydroelectric dams were built in the late 1970s and in the 1980s (table 2.1). Although the Tocantins appears to be getting cloudier as a result of forest clearing, "storage pockets" abound in the reservoir bed and this dead space will take some time to fill with sediment.
Table 2.1 Major hydroelectric dams operating in Amazonia
|Dam(a)||River||Operational||Capacity (MOO)||Reservoir |
Sources: Barrow (1988); Ledec and Goodland (1988); Serra (1992); Sioli (1986); field notes of NJHS at the Curuá-Una dam on 2 October 1992.
a. See fig. 2.1 for dam locations.
b. The Tucurui dam is expected eventually to generate 4,000 MW.
The Tucurui reservoir has drowned tens of thousands of Brazil nut trees (fig. 2.4). The Tocantins valley has always been the most important centre for the Brazil nut trade, and the 2,000 km reservoir has destroyed some valuable plant resources. It is difficult to measure the impact of such losses on yields, but some unique germplasm has surely been lost. As the Brazil nut's genepool shrinks, genes that could be useful for future improvement efforts also vanish.
Amazonia is relatively flat, so dam-building along major rivers leads to the drowning of substantial tracts of flood plain and upland forest. Iquitos on the Upper Amazon is 3,600 km from the Atlantic, yet only 80 metres above sealevel (Irion 1989). From Iquitos to the confluence with the Negro, the Amazon drops only 57 metros. Some dams are more "cost effective" in terms of kilowatts generated per flooded area (Ledec and Goodland 1988: 60). The Tucurui dam generates 30 kW/ha of reservoir, whereas Balbina delivers only 2 kW/ha of flooded area (table 2.1). Both dams are low in electricity generated per flooded area when compared with rivers with steeper valleys, such as the Itaipu dam on the Paraná (77 kW/ha) or the Grande Coulee on the Colorado (63 kW/ha).
The apparent and hidden ecological costs of building major dams in Amazonia must be weighed against the benefits hydroelectric dams bring to the region. Brazil's desire to tap the hydroelectric potential of waters in Amazonia is understandable in view of the burdensome bill for imported petroleum. Brazil produces less than one-third of its petroleum needs, and most of the electricity generated in Amazonia has historically come from diesel-powered turbines. The Tucurui dam has benefited Belem and environs with reliable electricity and has created jobs, such as at the aluminium smelting plant at Bacarena.
Nevertheless, a series of smaller, more environmentally benign hydroelectric projects might prove more suitable over the long term. To supply power to the pulp mill at Jari, for example, a proposed hydroelectric plant at the Santo Antonio Falls will divert part of the river through a turbine and thus will not involve any flooding.