Document(s) 6 of 11

Evoy Zaniboni Filho
Departamento de Aqüicultura/CCA
Universidade Federal de Santa Catarina
Florianópolis, SC, Brazil

Uwe H. Schulz
Fish Ecology
Universidade do Vale do Rio dos Sinos (UNISINOS)
São Leopoldo, RS, Brazil

CHARACTERISTICS OF THE BASIN

Geography

The Uruguay River rises in the Serra Geral Mountains as the Pelotas River, near the southern coast of Brazil, at an altitude of approximately 1,800 m. It runs inland along the southern border of Santa Catarina State (Figure 1) until it joins the Canoas River, which drains Central Santa Catarina State. Below this confluence with the Canoas the river is generally considered to become the Uruguay. Continuing along the border between Santa Catarina and Rio Grande do Sul states, the Uruguay flows 938 km to the mouth of the Peperi River.⁴¹⁸ From the Peperi River confluence the Uruguay flows south for 1,324 km, marking the borders between Brazil and Argentina,⁴¹⁹ and Uruguay and Argentina, until it meets the Paraná River to form the estuary of the Plata River, which flows into the Atlantic Ocean (Figure 1).

The watershed of the Uruguay lies between the temperate latitudes of 28^o10’ S and 37^o08’ S, with a total course of 2,262 km. For the purpose of this study the beginning of the Uruguay River is considered to be the confluence of the Canoas and the Pelotas rivers 1,816 km from the mouth.⁴²⁰

The Uruguay is one of three rivers that form the Plata watershed,⁴²¹ which has an area of 3.1 million km²; the other two rivers are the Paraná and Paraguay. Between the mouth of the Uruguay and the Atlantic Ocean lies an area of approximately 18,000 km² that includes the Rio de la Plata, a saline estuary whose depth varies between 4 and 18 m.⁴²²

The total area of the Uruguay watershed is approximately 365,000 km². One hundred and seventy-six thousand km² are in Brazilian territory

⁴¹⁸ Santa Catarina, 1997a

⁴¹⁹ ELETROSUL, 1979

⁴²⁰ Boschi, 1989

⁴²¹ OEA, 1969

⁴²² Boschi, 1989

(equivalent to 48% of the area of the watershed), 46,000 km² in Santa Catarina State, and 130,000 km² in Rio Grande do Sul State.⁴²³

Geology

The Uruguay is the youngest river of the Plata watershed.⁴²⁴ Its hydrographic basin rests upon the sedimentary and volcanic rocks that compose the Paraná Basin. Igneous extrusive rocks (in the form of lava beds) from the Serra Geral mountains, in the São Bento Range, predominate and cover Mesozoic and Neo-Paleozoic sedimentary rocks deposited in subhorizontal spills at depths varying from 300 to 1,000 m. Radiometric dating indicates that the principal volcanic activity took place in the Middle Lower Cretaceous, from 120 to 130 million years ago. The

FIGURE 1. The Uruguay River Basin showing hydroelectric plants⁴²⁵ and regions of greatest pollution (shaded in )

⁴²³ Santa Catarina, 1997b

⁴²⁴ Soldano, 1947

⁴²⁵ UHE = Usina Hidrelétrica

geotechtonic characteristics are associated with the two predominant lithological blocks of sedimentary rocks and basalt. The soil normally has a high clay content and, in general, has little depth.⁴²⁶

The river is a series of pools and rapids, formerly highlighted by the Augusto César Gorge (1,493 km from the mouth), just below the confluence with the Peixe River, which dropped 8 m in only 7 km.⁴²⁷ This gorge was flooded by the reservoir of the Itá Hydroelectric Dam, which began to fill in December 1999. The Yucumã (or Moconá) Falls, below the mouth of the Peperi River, marks a drop of 12 m through a diagonal crevice, forming rapids approximately 1,800 m long, the widest in South America. Below the mouth of the Quarai River, the former Salto Grande Falls (353 km from the mouth of the Uruguay) dropped 9 m in only 3 km⁴²⁸; these rapids were flooded in 1979 after construction of the Salto Grande Dam. The Yucumã Falls divide the Upper and Middle Uruguay, while the Salto Grande is considered the border between the Middle and Lower Uruguay (Figures 1 and 2).

FIGURE 2. Vertical profile of the Uruguay River, showing river sections and locations discussed in the text

⁴²⁶ ELETROSUL, 1981

⁴²⁷ ELETROSUL/CNEC, 1990

⁴²⁸ CARU, 1993

River Profile

As a result of its rather broken profile and abundance of rapids, the Uruguay is less navigable than the other rivers of the Plata Basin. Currently, after dredging stretches of the Lower Uruguay, a canal 7 m deep runs from the mouth to the port of Concepción del Uruguay (184 km). Commercial navigation with small boats extends 252 km up the river, and, except for at low water, boats with a draft of up to 2 m can reach the Salto region (390 km upriver).⁴²⁹

As suggested by the vertical profile (Figure 2), the different sections of the watershed have considerably different hydrological conditions. The upper river is steep, with an average drop of 1.76% and primarily fast water. The rocky terrain and the topography of the drainage basin result in considerable and sudden variations in flow. In the upper river the maximum average flow is 9,387 m³/s; the highest historic peak flow in the region exceeded 23,000 m³/s. Flooding occurs between June and October, although great annual variations in water level can be observed (Figure 3).

The Middle Uruguay, on the other hand, begins approximately 130 m above sea level and flows nearly 800 km with an average drop of only 0.16%, with some rapids. In the Lower Uruguay, the river runs nearly 350 km with a total drop of less than 1m.

In the lower basin, the average monthly variation in water level is less than 2 meters between dry periods and high water. In the upper and middle sections the combined average variation is approximately 10 m.

Hydrological conditions of the Lower Uruguay are strongly influenced by the Salto Grande Hydroelectric Dam. Historically the variation in the river level was small, dropping only 1.2 m during droughts. In spite of this, large floods exceeded 10 meters in height. Ports above Fray Bentos were for that reason built with two levels, to allow operation in times of drought and flood.⁴³⁰ Vast floodplains accompany the main river stem.

Water Quality

According to ELETROSUL/CNEC (1990) and the Administrative Commission for the Uruguay River, or Comissão Administradora do Rio

⁴²⁹ CENNAVE, 2000

⁴³⁰ CENNAVE, 2000

Uruguai (CARU, 1993), the water of the Uruguay has, on average, a low level of pollutants. However, near the large cities, contamination from untreated sewage, and, in the upper watershed, contamination by hog and poultry farming effluents, produce locally high levels of contamination. Dissolved oxygen is normally at near-saturation levels. The pH of the water is close to neutral, while electrical conductivity and alkalinity increase along the river (Table 1).

FIGURE 3. Multi-year mean of average, maximum and minimum monthly water levels of the Upper Uruguay River⁴³¹ and the Middle Uruguay River⁴³², showing representative water flow magnitudes⁴³³

⁴³¹ Water levels for Upper Uruguay River taken at Itá (1,529 km from the mouth), between 1940–1998.

⁴³² Water levels for Middle Uruguay River taken at Uruguaiana (580 km from the mouth), between 1931–1992.

⁴³³ DNAEE (www.dnaee.gov.br)

TABLE 1. Physico-chemical characteristics of the Uruguay River water⁴³⁴

In situations free of human impact, the low phytoplankton production of the Uruguay is due to a strong current, relatively low concentration of nutrients and high turbidity.⁴³⁵ The productivity of aquatic macrophyte communities is also quite low due to the scarcity of pools and the near-absence of marginal lagoons. The low primary production makes aquatic communities highly dependent on organic material originating on land, even more so than in other rivers of the Plata Basin.⁴³⁶

Social Aspects

Human occupation of the river basin

European colonization of the Uruguay River Basin began in the mid-sixteenth century, when Spanish and Portuguese settlers established small villages along the lower river and mixed with indigenous peoples. Difficulty navigating to the upper basin impeded colonization until 1620, when Jesuit priests led a migration of Guarani Indians south from the lands east of São Paulo.⁴³⁷ After 1633, Caboclos, an ethnic group that sprang from the mixing of indigenous people and Europeans, moved in. Their principal activities were subsistence agriculture, the cutting of yerba maté for tea, cattle-raising and transport. After 1894 incentified settlement began, intensifying after 1917 when the Brazilian government, with participation of the German and Italian governments, paid the travel expenses of European immigrants. Settlement was based on 20 to 30 hectare agricultural lots, which remain characteristic of the region.

⁴³⁴ ELETROSUL/CNEC, 1990; CARU, 1993

⁴³⁵ Quirós & Luchini, 1982

⁴³⁶ Di Persia & Neiff, 1986

⁴³⁷ ELETROSUL/CNEC, 1990

Agricultural development was accompanied by exploitation of the forests that covered the entire region. During the high water season, long rafts of cedars (Cedrella fissilis), angicos (Parapiptadenia rigida), grápias (Apuleia leiocarpa) and Brazilian pines (Araucaria angustifolia) were floated down the Uruguay to Argentina.⁴³⁸ In 1940 large sawmills were built, but after years of extraction the forests were depleted and the sawmills shifted to other regions.

After 1960, hog and poultry farms began to consolidate into large conglomerates, which have since dominated the Brazilian food market. Integrating with producers, these large agri-businesses have continued to stimulate the regional economy and have steadily increased productivity in hog- and poultry-raising.

In the Upper Uruguay agriculture revolves around soybeans, corn and black beans. In the Middle and Lower Uruguay, extensive cattle-raising and cultivation of soybean and rice prevail. Only fragments of the old forest remain along the boxed river valleys and on the steepest hillsides that people have been unable to occupy. In the Brazilian section of the river basin, primary and secondary vegetation cover nearly 17.5% of the land. Reforested areas, principally pines (Pinus elliottii), occupy another 3%. Compared with its original vegetation, with the exception of a few small remaining patches of primary forest nearly the entire region has been replanted to secondary vegetation, croplands and pasture.

The population density of the Uruguay River Basin is approximately 39 inhabitants per square kilometer. Nearly 45% reside in the rural areas.⁴³⁹ Despite the wealth and the stable economy, small economically depressed regions persist, mainly in the drainage basins of the Pelotas and Canoas rivers.

Habitats used by Migratory Fish

Upper and Middle Uruguay

The upper and middle sections of the Uruguay River occupy a fairly steep-walled valley with only a small floodplain that gradually flattens towards the headwaters. These characteristics directly influence the diversity and abundance of fish. The river bed is deeply channelled, broken up by waterfalls, rapids and narrows, and there are few islands or riparian

⁴³⁸ ELETROSUL/CNEC, 1990

⁴³⁹ Santa Catarina, 1997a ; Atlas Mirador, 1987

grasslands. The number of species found along the Uruguay River Basin probably surpasses the 150 species previously described.⁴⁴⁰ Data on fish productivity are not available, but the lack of floodplains in the Upper Uruguay suggests it is low.

Tributaries of the upper and middle section are normally short and broken by waterfalls. Migratory species and large fish are therefore generally restricted to the main river and to the lower section of the tributaries.

The migratory species present in the Lower Uruguay River normally rely on floodplain lakes for larval and juvenile rearing. As these lakes are absent from the steep valleys of the Upper and Middle Uruguay, the species appear to have adapted by using the mouths of tributaries as rearing areas. These areas of confluence take on lentic characteristics when the mainstem of the river floods, and backs up the waters of the smaller tributaries. Water transparency and temperatures tend to be significantly higher in these regions,⁴⁴¹ which leads to greater planktonic production and conditions favourable for larval and juvenile rearing.

Lower Uruguay

The Lower Uruguay resembles the Lower Paraná, which lies at the same latitude and a little farther west, and the fish species are practically the same. In both, species diversity and total fish biomass are high,⁴⁴² considering the sub-tropical climate. The high productivity may be due to a low profile and extensive floodplain, favouring the formation of shallow seasonal lakes and pools that accumulate nutrients.

Between the cities of Colón (236 km) and Fray Bentos (102 km) many large islands break up the Uruguay. 110 km from the mouth, at the outlet of the Gualeguaychú River, the islands disappear and the river widens substantially, reaching 8 to 12 km in breadth over a flat plain. A series of channels links the Uruguay and Paraná rivers in this stretch, with the Plata River strongly influencing the speed and direction of the currents in the Uruguay.⁴⁴³ During low water season, tidal influence is seen above the port of Paysandu, 204 km from the mouth.⁴⁴⁴ Since the construction

⁴⁴⁰ Di Persia & Neiff, 1986

⁴⁴¹ Zaniboni Filho et al., 2000

⁴⁴² Bertoletti, 1985

⁴⁴³ Sverlij et al., 1998

⁴⁴⁴ CENNAVE, 2000

of the hydroelectric dam at Salto Grande, these tidal effects have become more pronounced, particularly when water volumes released by the dam are reduced.

At Nueva Palmira, the Uruguay spills into the Rio de la Plata, a saline estuary covering approximately 18,000 km². The temperature of the Plata varies between 10 and 24^oC.⁴⁴⁵ Phytoplankton production is generally low and the ichtyofauna is composed primarily of sediment-eating, or iliophagic, fish.⁴⁴⁶ Euryhaline species predominate, and the presence of fresh-water species is low.⁴⁴⁷

MIGRATORY SPECIES AND MIGRATION PATTERNS

The fish community of the Uruguay is very similar to that of the Paraná River. Characiforms and Siluriforms predominate.⁴⁴⁸ In 1986, 150 species of fish were described for the Uruguay River Basin, including exotic, anadromic and estuarine fish.⁴⁴⁹ However, Hahn and Câmara (2000) identified 251 species in a brief bibliographic review of the Uruguay River.

Today, more than 100 species of fish are registered for the Upper Uruguay.⁴⁵⁰ The fish community of the Lower Uruguay is characterized by species of marine origin, such as Mugiliforms, Clupeiforms, flounders and rays. However, the species of the greatest biomass is the freshwater curimbatá (Prochilodus lineatus), a characid, which is fished intensely, sustaining industrial production of fishmeal and oil. As sampled by trawler, the curimbatá was found to be the most abundant species, occurring at a relative frequency of 23%. The voga (Schizodon nasutus) and the armoured catfish (Pterodoras granulosus) were second and third most abundant, at 4% and 3% respectively.⁴⁵¹ Other migratory species of commercial or recreational importance are the piava (Leporinus obtusidens), dourado (Salminus maxillosus) and Patí (Luciopimelodus pati).

⁴⁴⁵ Boschi, 1989

⁴⁴⁶ Quirós & Baigun, 1985

⁴⁴⁷ Boschi, 1989

⁴⁴⁸ Ringuelet, 1975

⁴⁴⁹ Di Persia & Neiff, 1986

⁴⁵⁰ Zaniboni Filho, et al., 1997

⁴⁵¹ Amestoy & Fabiano, 1992

Knowledge of the fish fauna of the Middle Uruguay is limited to the study by Bertoletti et al. (1989) of the stretch downstream from the Brazilian municipality of São Borja. In five sampling expeditions between 1988 and August 1989, 71 species were captured, with a composition intermediate to those of the Lower and Upper Uruguay. The species of the lower stretch, such as the ray (stingray) Potamotrygon brachyura, the clupeiod Lycengraulis sp., the sciaenoid Pachyurus bonearensis and the flounder of the Achirus genus are of marine origin. Siluridae of the Doradidae family and some Pimelodidae such as Zungaro zungaro (sometimes used as a synonym for Paulicea luetkeni) are also typical of the Lower Uruguay. Characiforms such as Steindachnerina brevipinna, Apareiodon affinis, and the siluriforms of the Ancistrus and Hemiancistrus genera are characteristic of the Upper Uruguay. Among the migratory fish, three species are considered commercially important in this upper reach: the piava (L. obtusidens), the curimbatá (P. lineatus) and the dourado (S. maxillosus). However, information on reproduction of the migratory fish in this region is not available. Near the city of Uruguaiana, in the Middle Uruguay, young of Pseudoplatystoma corruscans were observed in the small tributaries, which are probably feeding grounds of the young form of this species.⁴⁵²

Migratory Behaviour

Despite the similarity of the fish communities in the Uruguay and Paraná rivers, the life cycles of the fish and their migratory behaviour in the two rivers appear quite different. Bonetto and Pignalberi (1964), who tagged fish in the Argentine section of the Paraná, suggest that in this river there is a feeding migration downriver and a reproductive migration upriver. This behaviour appears common in the majority of tropical and subtropical rivers and is closely linked to the seasonal rains. Summer water levels in the Paraná allow upriver migration and the flooding of areas for the growth of larvae and fingerlings.

In contrast, the Upper Uruguay lacks the well-defined dry season typical of the Paraná headwaters. Rains fall throughout the year, and floods are swift and brief, due to the deep valleys and the absence of marginal

⁴⁵² Enrique Querol Chiva, personal communication

lakes and floodplains along much of the river. The migratory behaviour of the fish of the Uruguay, such as the dourado and the piracanjuba (Brycon orbignyanus), though still not well understood, is thus quite different from that of the same species in the Paraná. While the species form large schools that usually move upriver for reproduction and downriver to feeding areas, they may also be found moving in opposite directions at the same time.⁴⁵³

Monitoring of eggs and larvae in the Lower Uruguay region suggests that reproduction occurs between October and March, with one peak downstream of the Salto Grande Dam in the spring, and a second in the upper section of the Salto Reservoir at the beginning of the summer.⁴⁵⁴ Tagging studies in the Lower Uruguay indicate that the principal populations of migratory fish reproduce in the Middle Paraná River, rather than the Uruguay.⁴⁵⁵

There are also two forms of migration: mainstem, in which the fish remain in the main river, and tributary, in which the fish turn up the tributaries. The species can therefore be classified as either mainstem or tributary migratory species.

Principal Mainstem Migratory Species

Salminus maxillosus

The dourado (dorado in Spanish) is found throughout the Uruguay, as far as the Canoas and Pelotas rivers. Considered the best fishing trophy in the Uruguay River Basin, this species also brings one of the highest prices in the market. Despite its regional importance, however, there is a tremendous scarcity of information about its biology in the river basin.

The dourado is mostly restricted to the mainstem of the Uruguay, and is recorded only in the larger tributaries of the middle and lower basin such as the Negro, Quarai, Ibicui and Ijui rivers. In the tributaries of the Upper Uruguay, dourados are recorded only near their confluences with the Uruguay, except for the Canoas and Pelotas rivers. The dourado is found throughout the year along the whole of the mainstem of the river, and fishermen have developed specialized techniques for their capture. Most are taken during migration when they concentrate near the rapids.

⁴⁵³ Di Persia & Neiff, 1986

⁴⁵⁴ Espinach-Ros & Ríos, 1997

⁴⁵⁵ Sverlij et al., 1998

The dourado is carnivorous and an excellent swimmer. For most of the year it is solitary or moves in small schools, concentrating in fast waters where it ambushes slower-swimming prey. During the upstream migration to reproduce, large schools of dourado gather below natural obstacles, where they await higher water. Along the Uruguay only three rapids originally blocked the dourado during low water: Salto Grande (353 km from the mouth), Salto do Yucumã (or Moconá) (1,215 km from the mouth) and the Augusto César Gorge (1,493 km from the mouth). The other rapids can be easily passed by this fast-swimming species, which can leap small waterfalls. Nonetheless, dourados are commonly found gathered in stretches of strong current, probably attracted by the greater ease of capturing prey.

Dourado reproduce in spring and summer (November–February), a phenomenon closely linked to the rains. Gonadal maturation and the start of migration appear to depend on water temperature, and is earlier or later according to the coldness of the winter. After the water warms, movement upstream is limited only by the water level. Mature males and females have been recorded along most of the 1,500 km of the river, although no specific spawning grounds have been reported other than the stretch of the river below Salto do Yucumã.⁴⁵⁶ In surveys conducted between 1995 and 2000, fish in the final stage of gonadal maturation were found in different stretches of the river basin, indicating a variety of spawning locations.

Dourado tagged near the Salto Grande Dam travelled a maximum distance of 850 km in 41 days, corresponding to an average velocity of up to 21 km per day. The dourado is one of the two species tagged to move upriver in the reservoir.⁴⁵⁷

Brycon orbignyanus

The piracanjuba or bracanjuva (pirapitá in Spanish) was once recorded in the entire river basin. Reports from fishermen and residents of the Upper and Middle Uruguay indicate that large schools of piracanjuba once travelled upriver in the pre-reproductive period, when, gathered below natural barriers, many were easily caught. However, the species has

⁴⁵⁶ Fuentes & Ros, 1999

⁴⁵⁷ Delfino & Baigun, 1985

now practically disappeared from the Lower⁴⁵⁸ and Upper Uruguay.⁴⁵⁹ Despite its sporadic occurrence in small numbers over a few river stretches, the piracanjuba is now recorded consistently, but in low abundances, only near the Turvo Forest Reserve in Brazil. Currently, despite efforts to collect brood stock for this species, only three specimens are being maintained at the São Carlos Hatchery (in Santa Catarina State in Brazil).

Despite the value and abundance of the species in earlier times, the biology of the species in the Uruguay River Basin has not been studied. Some information on its ecology is, however, available from other river basins, and growing interest in cultivating B. orbignyanus in captivity has encouraged studies of its biology, summarized below.

B. orbignyanus is omnivorous, but prefers fruits and seeds. Studies indicate that the species has an enormous capacity to digest and assimilate vegetable proteins.⁴⁶⁰ Like the dourado, the piracanjuba produces semi-dense eggs that depend on flowing water during the entire incubation phase and the initial stages of larval development, a period of approximately two days. The eggs must hatch in flowing waters, which keep the eggs and larvae in suspension for that time. Larvae are carnivorous, like those of the dourado, selecting food from among zooplankton and showing a high propensity for cannibalism at the first feeding. The change to a more omnivorous and frugivorous diet probably occurs in juvenile fish.

Piracanjuba spawn in spring and summer (November–February), apparently in close relation to the rains and water temperatures. Maturation appears to depend on water temperature, and is delayed or advanced according to the coldness of the winter. Once temperatures are warm enough, migration depends on water flow adequate to allow the fish to move upriver; like the dourado, they also congregate below major rapids. There are no records of specific locations for spawning of piracanjuba along the Uruguay, although mature specimens have been caught in the Turvo Reserve (1,215 km from the mouth).

In fish surveys conducted in the Upper Uruguay between 1986 and 1987⁴⁶¹ and between 1995 and 2000,⁴⁶² no piracanjuba were captured. In

⁴⁵⁸ Espinach-Ros & Rios, 1997

⁴⁵⁹ Zaniboni Filho et al., 2000

⁴⁶⁰ Meurer, 1999, Cavalcanti, 1998

⁴⁶¹ Bertoletti et al., 1989

⁴⁶² Zaniboni Filho et al., 2000

addition, no piracanjuba have been captured by fishermen in the stretch between Itá (1,529 km) and the confluence of the Canoas and Pelotas rivers (1,769 km) since approximately 1980, though some have been recorded near the confluence with the Peperi River (1,225 km). This disappearance of the piracanjuba from much of the Uruguay indicates a lack of tolerance for an altered environment. According to Espinach-Ros and Rios (1997), the loss of forest along the river banks is likely one of the principal factors. The situation of piracanjuba in the Uruguay is critical, and urgent measures are required for its conservation.

Prochilodus lineatus

The curimbatá or grumatão (sábalo in Spanish) feeds on detritus and encrusting algal growth (phytobenthos). It thus prefers slower waters, where suspended material settles out and a robust benthic ecosystem can develop. These areas are most common in the lower stretch of the Uruguay, where the river bed flattens and the floodplain widens, resulting in extensive areas of still water and shoals. However, the species also occurs in pockets throughout the entire basin. In the upper part, where the river is boxed into a steep valley, pools with muddy bottoms and calm waters alternate with shoals and strong currents. Both areas are used by the species for feeding.⁴⁶³ The rapids form small areas of still water that remain more exposed to sunlight at low water levels, and phytobenthos on the rocky bottom is frequently abundant.

As with the other migratory species in the Uruguay, few studies of the biology of the curimbatá in the Uruguay River Basin have been conducted. It is, however, known that populations of the Middle and Upper Uruguay migrate upriver to reproduce. Mature fish are caught in different locations along the river; however, their spawning or nursery sites are not known. Where natural barriers impede migration the curimbatá also form large schools that mix with other species (such as the dourado) at the base of rapids and falls until floods arrive.

Curimbatá eggs are semi-dense and require flowing water during the entire incubation phase. Approximately 4 days after spawning, the larvae begin external feeding. In the first days they require planktonic organisms, then graduate quickly to consuming benthic organisms. Individuals from three to six years of age dominate Curimbatá populations in the Lower

⁴⁶³ Zaniboni Filho, et al., 2000

Uruguay. A maximum age of nine years was observed. There is also evidence for the co-existence of two populations of curimbatá with different growth rates in this area.⁴⁶⁴

Prochilodus species make extensive migrations. In the lower river basin, P. platensis tagged in the region of the Salto Grande Reservoir were recaptured at a maximum distance of 620 km from where they were released.⁴⁶⁵

P. lineatus in the Salto Hydroelectric Reservoir increased after dam construction, probably due to an increase in feeding areas and sedimentation of organic detritus.⁴⁶⁶ Abundant curimbatá eggs were found in the upper third of the reservoir, while larvae were distributed throughout the reservoir, indicating that the spawning areas were upstream.⁴⁶⁷ Considering that the reservoir is the only one in the Middle and Lower Uruguay with a long stretch of flowing water upstream, the growth of the curimbatá populations can be explained by more food having become available in the reservoir and by the spawning areas remaining accessible. New hydroelectric projects upstream could alter this situation irreversibly.

In the Lower Uruguay River Basin, tagging of the principal migratory species present indicate that their principal spawning areas are actually in the Paraná River.⁴⁶⁸ Considering the low declivity of the lower stretch of the Uruguay, saltwater influx at low water levels and the greater predictability of floods in the Paraná, it may be more advantageous for the species to feed in the Lower Uruguay and return to the Paraná annually to reproduce. This strategy appears to sustain large populations of curimbatá in the Lower Uruguay River Basin. According to Sverlij et al. (1998), the species has the largest biomass of fish caught in the region, and indeed sustains commercial fishing. However, the more recent study of Fuentes and Espinach-Ros (1999) found larvae of Prochilodus in the Lower Uruguay, although in low numbers.

Leporinus obtusidens

The piava or piapara (boga or boga común in Spanish) is restricted to the Middle and Lower Uruguay River Basin, though it was once abundant

⁴⁶⁴ Sverlij et al., 1992

⁴⁶⁵ Delfino & Baigun, 1985

⁴⁶⁶ Espinach-Ros & Rios, 1997

⁴⁶⁷ Espinach-Ros & Rios, 1997

⁴⁶⁸ Sverlij et al., 1998

throughout the basin. In the Upper Uruguay, the species has been recorded only near the Yucumã Reservoir. It is relatively easy to find in the fish market in the city of Itapiranga (1,240 km from the mouth). In the Lower Uruguay it is important to both commercial and recreational fishing. It reproduces in the spring, migrating longitudinally. Fuentes and Espinach-Ros (1999) found the highest piava reproduction intensity in the Lower Uruguay where 11% of all eggs and larvae counted were attributed to this species.

As with other migratory species of the Uruguay, regional biological studies of the piava are rare, despite the interest in fishing for the species. Through the analysis of eggs and larvae from fish culture, we can suggest that its reproductive behaviour resembles that of the dourado, piracanjuba and curimbatá, and piava are found exclusively in migrations together with these species. Its eggs are semi-dense and require flowing water for the entire incubation phase. Hatching occurs about 18 hours after fertilization, and external feeding begins four days after hatching. The post-larvae are able to select zooplankton and can eat benthic organisms.

The feeding of the young and the adults is diversified. They are considered omnivorous, although their small mouth allows the ingestion of small food items only. Seeds, aquatic insects and molluscs are the principal food; given this preference we can suppose that the piava was always abundant in the middle and lower river basin. In the Upper Uruguay, the geological and hydrological characteristics limit the availability of autochthonous food, and the species remains more dependent on fruits and seeds. Deforestation through unsuitable agricultural techniques can be cited as the principal cause of the near elimination of piava stocks in the Upper Uruguay.

In tagging studies near the Salto Grande Dam, the maximum distance between the area of release and recapture of the L. obtusidens was 540 km. The piava and the dourado were the only species that swam upstream in the reservoir.⁴⁶⁹

Long distance migratory catfish

Six species of migratory catfish are found in the Uruguay River: pintado (P. corruscans; surubí in Spanish); surubim (Pseudoplatystoma fasciatum, surubí atigrado in Spanish); jaú (Paulicea luetkeni; manguruyú in Spanish);

⁴⁶⁹ Delfino & Baigun, 1985

surubi (Steindachneridion inscripta; bagre cabezón in Spanish); and armado or abotoado (P. granulosus; armado in Spanish).

P. granulosus, an armoured catfish, is one of the most abundant species in the Lower Uruguay, constituting 3% of the total catch. Tagging studies indicate that the species uses both the Paraná and the Uruguay systems, and it is found throughout the Plata Basin, especially between October and March. However, during CARU’s monitoring, mature armados were caught only in the Paraná, and fish marked in the Paraná delta migrated as far as 700 km upriver. It appears that the Uruguay is used only as a feeding ground, while the reproductive areas of this species are in the Paraná.⁴⁷⁰

Other migratory catfish are abundant in the middle and lower river basin, and pintado, surubim and surubi are listed among the most frequent species in the catches of the Lower Uruguay and in the Salto Grande Reservoir.⁴⁷¹ All three species are fish-eaters, nocturnal, and prefer to inhabit deep pools of still water.

In contrast to the Lower Uruguay, interviews with fishermen and residents along the river indicate that pintado, surubim, and jaú were always rare in the upper river basin. Inventories of the fish fauna in the upper river basin between 1986 and 1987⁴⁷² and between 1995 and 2000⁴⁷³ caught no specimens of these species. The existence of pintado and jaú (~1,450 km from the mouth) can only be substantiated through the observation of single specimens of each species frozen by fishermen who keep the individuals as trophies. Eggs and larvae of migratory catfishes in the Lower Uruguay region are found throughout October to March, although in low densities. Larvae were caught in trawls at night, principally between Colon and Gualeguaychu, in the same nursery areas as other species such as the curimbatá.⁴⁷⁴

Through studies in fish culture, all the catfishes are known to have semi-dense eggs that must remain in flowing water until hatching (more than 17 hours). Larvae of pintado and surubim are smaller than larvae of other migratory fish such as the curimbatá, dourado, piava and piracanjuba, and in the first phases of life must rely on very small food items, such as rotifers. Despite this, the larvae are very cannibalistic in

⁴⁷⁰ Amestoy & Fabiano, 1992

⁴⁷¹ Sverlij et al., 1998

⁴⁷² Bertoletti et al., 1989

⁴⁷³ Zaniboni Filho et al., 2000

⁴⁷⁴ Espinach-Ros & Ríos, 1997

culture, although they are not able to ingest an entire larva, and commonly die themselves in the process. Nevertheless, cannibalism is still a principal source of mortality in the larviculture of these species.

The pintado and surubim produce large quantities of small eggs (between 0.7 and 0.8 mm in diameter), and a typical 10 kg female releases an average of 600,000 eggs at each spawning.⁴⁷⁵ Jaú and surubi have larger eggs, averaging 1.7 mm in diameter, and larger larvae at hatching. Jaú nevertheless have large ovaries and produce a large number of eggs at each spawning (a typical 10 kg female produces about 500,000 eggs). The surubi have small ovaries, and a 5 kg female releases approximately 40,000 eggs at each spawning.⁴⁷⁶

Migratory catfishes of the Uruguay apparently travel only short distances to reproduce. The maximum distance travelled by a tagged surubim in the Salto Grande Reservoir and recapture was 75 km, corresponding to maximum average velocities of 1 km/day. Tagged specimens were also recaptured downstream of the dam, indicating that they are able to pass through the turbines or over the dam.⁴⁷⁷

Principal Tributary Migratory Species

Pimelodus maculatus

The mandi-pintado or pintado (bagre amarillo in Spanish) is a small catfish found throughout the Uruguay River and in small and medium-size tributaries. It is an abundant species important to regional fishing and spawns throughout spring and summer. In studies of its life cycle in the Upper Uruguay region,⁴⁷⁸ adults were observed to migrate laterally, leaving the main river to spawn in the tributaries. Gonads mature between September and March when the water temperature is high, but spawning peaks occur in the winter, probably due to hydrological conditions.⁴⁷⁹ Gonadal maturation and spawning of the mandi-pintado in the Paraná River has been reported to depend on rainfall.⁴⁸⁰ After spawning, adult individuals concentrate in the main river where they feed intensely. The

⁴⁷⁵ Zaniboni Filho, 1998a

⁴⁷⁶ Zaniboni Filho, 1998b

⁴⁷⁷ Delfino & Baigun, 1985

⁴⁷⁸ conducted by Cassini,1998

⁴⁷⁹ Cassini, 1998

⁴⁸⁰ Basile-Martins et al., 1975

young are most frequent in the lower section of the tributaries and in the confluence with the main river.⁴⁸¹ In the Lower Uruguay larvae were found at highest frequency in the confluence of the tributaries with the main river and the tributaries and bays of the Salto Grande Reservoir.⁴⁸²

Mandi-pintado eggs resemble those of P. corruscans, being semi-dense and of small diameter. In culture, incubation requires flowing water until hatching. Larvae have a small yolk sac, and once they begin to feed depend on small food such as rotifers. They show cannibalism in the first larval stages in culture.

Rhamdia quelen

The jundiá (bagre sapo or bagre negro in Spanish) has been recorded throughout the Uruguay Basin, including the upper portions of the tributaries, and is important for fishing throughout the river. In the Upper Uruguay, spawning occurs throughout the year, though at greater intensity in the spring and primarily in the tributaries.⁴⁸³ R. quelen is carnivorous, feeding mostly on fish and crustacea, and feeds more intensely in the autumn and winter.⁴⁸⁴

Jundiá generally feed in the main river until the beginning of gonadal maturation, whereupon they migrate laterally up the tributaries, taking advantage of flood peaks. Final gamete maturation occurs in the tributaries. After spawning the fish return to the feeding grounds in the main river. In spring, at higher water temperatures and coinciding with the floods, the majority of the population spawns,⁴⁸⁵ but migration and spawning also occur throughout the year.

Jundiá eggs are semi-dense and remain suspended in flowing water after hydration. Fecundity is low compared with the majority of migrating fish, averaging 70,000 eggs/kg per female. The eggs hatch nearly two days after fertilization, and, in contrast to other migratory species described above, they hydrate less and can remain for some time on the bottom. Larvae begin feeding about 4 days after hatching, selecting food from the plankton and benthos. Unlike the other migratory catfish, jundiá larvae are not cannibalistic in culture.

⁴⁸¹ Cassini, 1998

⁴⁸² Espinach-Ros & Ríos, 1997

⁴⁸³ Silva, 1997; Cassini, 1998

⁴⁸⁴ Meurer & Zaniboni Filho, 1997

⁴⁸⁵ Cassini, 1998

IMPACTS ON MIGRATORY SPECIES

Fisheries Impacts

Commercial fishing in the Uruguay Basin is currently only important in the lower stretch of the river. An industrialized beach seine fishery of the curimbatá, for use in fishmeal and fish oil,⁴⁸⁶ landed between 1,000 and more than 5,000 tons annually between 1945 and 1988. The capture of 10 to 30 tons of fish per set in this fishery was common, with occasional catch of over 100 tons. The fishery is Argentinean, and is legislated to occur in the summer and fall, between October and April. It has declined since the late 1980s, when the similar fisheries of the La Plata River was shut down, and continues on only a relatively small scale.⁴⁸⁷ In the rest of the basin, fishing is primarily recreational, and, for small groups of residents, for subsistence. Associations that represent fishermen along the middle and lower section of the river basin are registered, though data on catches are not available. On the Brazilian section of the Middle Uruguay there is no official commercial fishing,⁴⁸⁸ though unregistered fishing may be substantial.⁴⁸⁹

Inland landings in the Brazilian states of Santa Catarina and Rio Grande do Sul, which includes the Upper Uruguay Basin and represents sport and artisanal fishing, were 2,414 tons in 1995. Only 7.5% were of the longitudinal migratory species such as S. maxillosus, P. lineatus, L. obtusidens, P. corruscans and S. inscripta. Species that conduct only lateral migrations, such as R. quelen and P. maculatus, made up more than 15% of the total catch.⁴⁹⁰

Despite the low importance of fishing for wild stocks in the upper river basin, fish culture in this region makes it one of the largest producers of fresh-water fish in Brazil. Fish culture production exceeded 7,000 tons in 1995 (although more than 97% of the total produced were introduced species).⁴⁹¹ Santa Catarina State, currently the second largest Brazilian producer of fresh-water fish, in 1998 produced 14,400 tons based equally on Chinese carp, common carp and tilápias.⁴⁹² Of the mere 2% of Brazilian

⁴⁸⁶ Espinach-Ros & Delfino, 1993

⁴⁸⁷ www.entrerios.gov.ar/produccion/dpesc07.htm

⁴⁸⁸ IBAMA, 1997

⁴⁸⁹ Hahn & Câmara, 2000

⁴⁹⁰ IBAMA, 1997

⁴⁹¹ IBAMA, 1997

⁴⁹² EPAGRI-CIRAM, 1999

fish used in fish cultivation in the region, the majority came from other hydrographic basins.

However, the regional market price of native migratory species in the Uruguay is 3 to 7 times higher than the price of cultivated exotic species. The major impediment to the development of native species culture is the lack of technology to produce fingerlings. Only in 1998 were the first fingerlings of migratory species from the Uruguay Basin produced by the Fish Hatchery of São Carlos, Brazil.⁴⁹³

Other Impacts

The greatest impact on the migratory fish community is from habitat alteration following dam construction and deteriorating water quality from pollution. Habitat changes have the gravest long-term impact on the fish fauna. Chemical contamination of the water and organic pollution are theoretically reversible.

It is important to emphasize that the changes in the upper stretch of the river will permanently affect the indigenous fauna. The Upper Uruguay appears to be an area with high levels of endemism, and each year new species are described. In general, the biology of these new species is unknown and the effects of the hydroelectric projects on them are unpredictable. Some of them may disappear before they are studied.

Dams

Dams block reproductive migration. They alter migratory and reproductive behaviour by changing the natural flow of waters that, among other factors, trigger migration. By retaining nutrients in the reservoir, they interfere in the river’s metabolism downstream. Adult and young fish passing through the turbines suffer high mortality. Dams alter the extension and location of spawning and growing areas, and they change the limnology by transforming a lotic environment into a lentic environment, altering temperatures, oxygen concentrations, current speeds, and sedimentation rates.

Three hydroelectric projects dam the Uruguay: the Salto Grande in the Lower Uruguay, inaugurated in 1979, and in the Upper Uruguay, the Itá and Machadinho, which began to operate in 2000 and 2002,

⁴⁹³ Zaniboni Filho, 1998a

respectively. Five more dams planned for the Upper Uruguay will transform a fast-running river into a chain of slow water environments.

In Espinach-Ros and Rios (1997) investigation of altered fish fauna in the Salto Grande Dam, the period from 1981–1984 (soon after dam closure) was compared with that of 1990–1995. The most visible changes were a greater abundance of pelagic species (Parapimelodus valenciennis), two iliophagous fish of the Prochilodus genus and cascudos of the Hypostomus and Loricaria genera. Bentophagous species (Iheringichthys westermanni, P. bonariensis) and small predators (Acestrorhynchus altus, Cyrtocharax squamosus, Astyanax spp.) also increased. A parallel study of the distribution of fish eggs and larvae indicated that these species are able to complete their life cycle in the tributaries that flow into the reservoir or in the reservoir itself, spawning in the area of transition from lotic to lentic.

The effect of the reservoir varies according to species. Some, such as P. corruscans, Oxydoras kneri and Sorubim lima disappeared gradually from catches after dam closure, and had disappeared altogether 15 years later. Two frugivorous species, the pacu (Piaractus mesopotamicus) and the piracanjuba (B. orbignyanus) were absent in experimental catches during reservoir filling, possibly due to the destruction of the vegetation along the Uruguay Basin. Agostinho (1997) reports the same observation for the Itaipu Reservoir on the Paraná River, where pacu and piracanjuba disappeared two years after dam construction. Nevertheless, some migratory species have maintained stable and abundant populations in the 15 years since the flooding of the Salto Reservoir, including S. maxillosus and L. obtusidens. Catches of others, such as P. lineatus and L. pati, have increased considerably.⁴⁹⁴

To mitigate the impact of the Salto Grande Dam, fishways have been built to the design of those used in Europe and the United States for the movement of salmonids (Borland type). A study of their efficiency revealed that dourado (S. maxillosus) and surubim (P. corruscans) do not enter the lock chambers.⁴⁹⁵ Only the curimbatá (P. lineatus) was found in small quantities in the entrance and exit chambers. Of the 62 species found in the reservoir only 23 were captured in the ladder chambers. The most abundant species were small Siluriformes (P. valenciennis, Auchenipterus nuchalis) and Clupeiforms (Lycengraulis sp.). All of these species are

⁴⁹⁴ Espinach-Ros & Rios, 1997

⁴⁹⁵ Espinach-Ros & Rios, 1997

considered pelagic. This may indicate that the fish, mainly the benthic species, have difficulty in finding the ladder entrances.

The Itá and Machadinho dams, on the Upper Uruguay, are the first hydroelectric projects in the upper river basin. The Itá Reservoir extends to the Machadinho Dam, obliterating the lotic stretch of river between them. The Itá Dam is 126 meters high, and ladders and other structures for fish movement downstream to upstream were not planned in the project. The planning of other new dams calls for the complete elimination of lotic stretches along the entire Uruguay, making it probable that current-dwelling species like dourado and curimbatá will no longer find the lotic stretches they need upstream. The importance of these lotic areas for their reproduction has been demonstrated by a study of the distribution of eggs and larvae in the Salto Grande Reservoir.⁴⁹⁶ Consequently, the populations of these species are likely to decrease much more in the new reservoirs of the Upper Uruguay than was observed at Salto Grande.

Agriculture

Hog- and poultry-raising presents a special problem on the Upper Uruguay, principally in Santa Catarina State, which has the largest production of these animals in Brazil. The average density of hogs in the region is 82 hogs/km². In some watersheds, as in the Jacutinga River Basin, up to 1,909 hogs are concentrated per square km.⁴⁹⁷ From 1993 to 1996 the growth in stock was 41%, and the hog population in the basin is now 3,120,000, or 69% of the hogs in the entire state. These animals produce some 26,000 m³ of waste per day.⁴⁹⁸ Based on the biochemical demand for oxygen (DBO), this corresponds to a population equivalent of 10.8 million people. The 1997 census shows a regional population of 1.8 million people.

The poultry population in the Catarinense section of the Upper Uruguay region is 58,100,000 birds, averaging 1,460 birds/km², although in the Jacutinga River Basin it reaches 34,800 birds/km². Between 1986 and 1996 poultry-raising grew by 57%. The quantity of manure produced by the birds is nearly 7,000 tons/day, or equivalent to the pollution of 6.68 million people. As of 1996 the watershed raised 68% of the poultry in Santa Catarina State.

⁴⁹⁶ Espinach-Ros & Rios, 1997

⁴⁹⁷ Santa Catarina, 1999

⁴⁹⁸ BRASIL-IBGE, 1997

Not surprisingly, the level of organic pollution is high in the Upper Uruguay, principally in the Peixe and Canoas rivers. High coliform levels are present in the stretch of the Upper Uruguay from near Concórdia (1,529 km) to the Peixe River (1,620 km), with an average coliform index of 10⁴ NMP/100 ml. This is probably caused by agriculture (hog-raising) and not by human sewage, given that the largest hog-producing region corresponds to the points with the highest concentrations of fecal coliforms.⁴⁹⁹

Industry and human habitation

Mercury levels nine times the acceptable limit for waters free from contamination were detected in the Upper Uruguay Basin, in the Canoas, Peixe, Pelotas and Uruguay rivers.⁵⁰⁰ The source is probably cellulose factories in these river basins.

Stretches of the river downstream from the large cities are the principal concentrations of contamination. Agricultural and industrial activities contribute to the dumping of pesticides, herbicides, nutrients and heavy metals (particularly chromium, from tanneries) and most domestic sewage is released into the river without any treatment. This has a number of consequences, such as the creation of an environment favourable to cholera. Once introduced, the Vibrio virus can remain virulent for decades. The organic overload of some stretches of the river also provokes eutrophication, resulting in toxic algal blooms and other similar problems.

Copper was detected only in some of the samples from the rivers with industry, such as the Peixe, and in the agricultural river basins, such as Forquilha, which have levels above the established standards (20 mg/l). Copper is associated with agricultural chemicals, principally fungicides. High levels of sediment contamination by copper were noted. Some regions have nickel concentrations two times greater than that permitted for environments considered to be free of pollution, while the levels of zinc exceeded by seven times the standard value and that of copper was nine times higher than the limit for waters free of pollution. The principal origin of these products is probably agricultural pesticides and herbicides, galvanization plants and metallurgy.⁵⁰¹

⁴⁹⁹ ELETROSUL/CNEC,1990

⁵⁰⁰ CONAMA, 1986; EPA, 1972

⁵⁰¹ ELETROSUL/ CNEC, 1990

CARU, which initiated a water-quality monitoring program in the Middle and Lower Uruguay in 1987, considers this section of the river to be generally clean.⁵⁰² There are isolated cases of contamination. The areas of higher pollution are downstream from the cities and industrial centers such as Salto-Concórdia, Paysandu-Colón and the mouth of the Gualeguaychú River. Heavy metals and agricultural chemicals of the organophosphate and organochloride groups were not found at elevated levels and are not considered problems for the lower stretch. Nevertheless, in the study by Angelini et al. (1992), which analyzed the presence of organochlorides in various aquatic species of the Lower Uruguay, high concentrations of organochlorides were found in a single individual from the Paysandu region. Metabolytes of DDT in this individual reached 11.1 mg/g muscle tissue, heptachlor 2.2 mg/g, chlordane 2.8 mg/g, and dieldren 0.2 mg/g. PCBs were found in all of the samples but not quantified.

The average total phosphorus concentration in the water varies between 93 mg/l and 130 mg/l (maximum values of 720 mg/l), indicating eutrophication. Total nitrogen follows this trend, averaging 336 mg/l and 941 mg/l (maximum of 5,430 mg/l)⁵⁰³. Pintos et al. (1992) detected problems related to organic pollution in the region of Paysandu (204 km from the mouth), where the release of domestic and industrial sewage causes high concentrations of phosphorous and nitrogen in the water. Dissolved oxygen here is only 14% of saturation, while solids have modified the granularity of the sediment and influenced the zoobenthic community.

MANAGEMENT AND MITIGATION

Fisheries Legislation

On the Brazilian section of the river a decree published annually by IBAMA prohibits fishing during the spawning season. The ban applies from November 1 to January 31 and prohibits fishing in the rivers, tributaries and marginal lakes, except for fishing using simple hooks. The regulation limits the catch and transport to 5 kg of a single type of fish and one additional individual of any size.

⁵⁰² CARU, 1993

⁵⁰³ CARU, 1993

Sport fishing in the region is well developed and frequently conducted without compliance with the law. The lack of conservation awareness compounded by the lack of the inspectors in the region has led to great pressure on the fish stocks. The need to improve inspection capacity was made clear in a study by Hahn and Câmara (2000), which describes an inspection operation by IBAMA, SUDEPE and the Army during the ban on fishing in the municipality of Itapiranga, SC in 1988. Ten thousand meters of gillnets were confiscated in a short stretch of 1 km of the river. It was estimated that a single family caught between 3,000 and 4,000 kg of fish per week at a time when fishing was prohibited.

Actions to Reduce Water Pollution

In the 1980s the Upper Uruguay was highly contaminated by industrial and urban effluents. Some of the principal tributaries, such as the Peixe and Canoas rivers, had long anoxic stretches periodically covered by foam.⁵⁰⁴ Paper and cellulose factories, tanneries and other industries release residues without the necessary treatment. The fish community was certainly strongly affected by the contamination, causing fishing to shift from an economic to a recreational activity. Various reports confirm that during the phase of extreme pollution there was a long interruption of recreational fishing, due to the scarcity of fish and the poor appearance of the water. A 1986–1987 survey of the fish fauna between 1,510 and 1,770 km from the mouth, which includes the principal tributaries of the region, was unable to capture any of the principal migratory species of the Uruguay such as dourado, curimbatá and piava.⁵⁰⁵

At the end of the 1980s, a large mobilization of the community and governmental authorities to reverse this situation resulted in a gradual improvement of the water quality of the region’s rivers. In fish surveys in the same region from 1995–1998, Zaniboni Filho et al. (2000), observed S. maxillosus, P. lineatus and L. obtusidens, though this may have also been due to an expanded fishing effort and more collection sites than in the

⁵⁰⁴ Godoy, 1987

⁵⁰⁵ Bertoletti et al., 1989

earlier survey. The improvement in water quality affected the structure of the fish community and there was a large alteration in species composition. Figure 4 shows a comparative analysis between the relative percentage of the 10 species that contributed most in biomass to catches between 1995 and 1998 ⁵⁰⁶ compared with 1986 and 1987.⁵⁰⁷

FIGURE 4. Relative fish biomass harvested in experimental fisheries of the Upper Uruguay in the 1980s⁵⁰⁸ and in the 1990s⁵⁰⁹

Reduced water contamination resulted in increased populations of larger species, which are of greater interest for recreational fishing. The dourado, absent in the catches conducted in the 1980s, accounted for 3% of the biomass collected in the late 1990s. In contrast to the 1980s, where the fish community was based on species with short life cycles, environmental improvements in the 1990s made possible the establishment of a more complex fish community, including larger species, species with protracted life cycles, and migratory species. Nevertheless, despite the difference in composition of the fish communities observed in the two periods of collection, the overall catch per unit of effort were similar (~30 g/m²/day.)

⁵⁰⁶ Zaniboni Filho et al., 2000

⁵⁰⁷ Bertoletti et al., 1989

⁵⁰⁸ Bertoletti et al., 1989

⁵⁰⁹ Zaniboni Filho et al., 2000

The (EPISCar) Fish Hatchery

Despite the partial recovery of stocks, the migratory fish population of the Upper Uruguay continues to deteriorate. To aid in their recovery, the “Preservation of the Migratory Species of the Uruguay River” Project was implemented. Financed by the Brazilian Environment Ministry, construction began in 1995 on the São Carlos (EPISCar) fish hatchery in the Santa Catarina municipality of São Carlos, 1,380 km from the mouth. It is the only fish hatchery in the river basin that works exclusively with species of fish native to the Uruguay. Brood stocks were assembled for various species such as dourado, piava, curimbatá, mandi-pintado, piracanjuba and surubi. In addition to conservation in vivo, the genetic diversity of the fish stocks also began to be preserved in vitro through the storage of cryogenically preserved semen. The fish culture station aims to study and produce these species, both to support future programs for re-populating the upper stretches of the river basin and to promote the cultivation of these species instead of the exotic species presently cultivated.

Environmental education is another important activity at the fish hatchery. Their plans include aquaria and awareness materials on fish fauna and on the importance of preserving the Uruguay and address the varying interests of visiting groups. This activity has had enormous importance in the region, considering that a large part of the population is more familiar with the exotic species used in fish culture than with the species native to the basin.

Minimum Flow Requirements for the Itá Dam

A sluice valve or “bottom outlet” was installed in the bottom of the Itá Dam. It was designed to regulate downstream flow during the filling phase, which began in December 1999, and minimize the environmental impact of the dam closure. It guaranteed a water flow in the old river bed equivalent to 80% of the minimum historic flow for the region. This inaugural use of the valve by the Itá Hydroelectric Project has proven efficient in reducing the impact on the downstream fish community.⁵¹⁰ Without the valve, a 20 km stretch of the Uruguay River would have dried out completely. This type of valve has recently become a requirement by Brazilian legislation.

⁵¹⁰ Zaniboni Filho et al., 2000

Establishment of the Turvo Reserve

Future mitigation measures must also focus on the conservation of nonimpacted areas. The preservation of the Turvo Reserve is therefore a high priority. Along the entire Uruguay River the 17,000 hectare Turvo State Park, nearly 1,200 km from the mouth, remains the only large reserve on the margins of the river. On the Argentine side there are five forest reserves totalling 84,000 hectares. The principal tourist attraction of the park is the Yucumã Falls, a longitudinal waterfall of about 1,800 m and up to 12 m high in the dry season, below which a crevice 90 m deep has formed.⁵¹¹ The river crosses the park for approximately 40 km.

The Yucumã Falls are only a temporary obstacle to migrating fish. All evidence indicates that for various populations of migratory fish the area acts as a reserve from which fish move upstream and downstream. The excellent condition of the vegetation along the river banks appears to be the key. Because of the ecological importance of the park for the fish community, the preservation of the region and the protection of the fish stocks within the park are high priorities.

RECOMMENDATIONS FOR CONSERVATION AND RESEARCH

In August 1999 the Administrative Commission for the Uruguay River, or Comissão Administradora do Rio Uruguai (CARU), held an international workshop to prepare a project that sought the conservation of biodiversity and the prevention and control of pollution in the river basin. The principal problems were identified as the need for information, loss of biodiversity, water quality, and deficiencies in institutions and management.

It was noted that the need for information is divided into two parts: the lack of systematization of the data already available and the lack of information itself. Data important to the management of the river basin exist in all three countries that encompass the Uruguay. This information, however, is spread through various federal, state and private institutions. It is extremely important to gather this information in a central database.

⁵¹¹ Gonçalves, 1999

Information is lacking on knowledge of organisms, geological history and the various environments. There is still no complete information on the composition of the native and exotic flora and fauna, the levels of endemism, life cycles of the migratory species and genetic diversity of the populations, making comparisons of the current state of the fauna and flora with historic conditions impossible. Evaluation of the loss of biodiversity and of human impact is incomplete and based on a short time-frame, and the varying environments of the main river bed, tributaries, islands, marshes, and banks are insufficiently characterized.

The loss of biodiversity, even when restricted to a short-term evaluation, is difficult to quantify. The effects of using the river for navigation, fishing and irrigation are not well known, and activities leading to loss of biodiversity from agricultural, industrial and urban pollution must be identified and quantified.

The importance of biodiversity and its economic and ecological role is still underestimated by politicians. Consequently, there is little political will to apply environmental legislation. Actions for environmental inspection and control must be intensified. A management plan for the river basin is needed which will include the control of management techniques and land use, with permanent control and monitoring of the river and its principal tributaries.

Weak points in existing legislation include overlapping responsibility of authorities in environmental management and control, lack of harmonization of international and national legislation on natural resource use, lack of conservation strategies (such as the formation of an ecological corridor along the river), lack of conservation areas, and lack of policies on ecotourism.

The future management of the Uruguay must be based on factors that encompass the entire river basin. CARU co-ordinates the collection of fisheries data, carries out various studies to improve understanding of the basin’s ictiofauna, and can act as a mechanism to regulate fisheries when stocks are threatened and conservation measures are necessary. The current activities of CARU are concentrated on the Lower and Middle

Uruguay, although in the future they should also take in the Upper Uruguay and intensify activities on the Brazilian side. The hydroelectric projects already begun or planned will completely change the ecosystem. The management of fish stocks should be adapted to this new situation in an effort to mitigate the effects of dams. Research projects that investigate the life cycle of the principal migratory species are therefore a high priority.

Other recommended actions include:

1. Investigation of the importance of the Turvo Reserve for the fish populations of the Middle and Upper Uruguay;
   
2. Developing an international program for tagging on all stretches of the river;
   
3. Broadening studies of the distribution of eggs and larvae, spawning locations and larval growth;
   
4. Researching biotelemetry methods for the mapping of species distribution and evaluating the importance of different environments in the life cycle;
   
5. Investigating “homing” behaviour;
   
6. Monitoring proposed re-population programs and guaranteeing restoration of the habitats required;
   
7. Evaluating methods to transport fish over the dams where possible;
   
8. Establishing in vivo and in vitro genetic banks for conservation of the threatened stocks;
   
9. Developing technology for fish-culture of native species and programs for re-populating critical areas of the river basin;
   
10. Undertaking studies to determine the existence of singular or distinct populations;
    
11. Launching a broad public awareness campaign to publicize the problems of the river basin;
    
12. Studying the influence of instream flow and the water level of hydroelectric reservoirs on the life cycle of the ichthyic community to determine the cycle desired to meet the interests of the fish fauna; and
    
13. Establishing a temporary ban on all net fishing in the region (there are efforts on the Brazilian side to enact this measure).

REFERENCES

Agostinho, A. 1997. Ecologia e manejo pesqueiro em reservatórios. Apostila para o curso de extensão, Universidade Estadual de Maringá.

Amestoy, F., and G. Fabiano. 1992. Distribución espacio-temporal, estrutura poblacional y reproducción del armado común, Pterodoras granulosus (Pisces, Doradidae), em el Rio de la Plata medio e interior y em el río Uruguay inferior. Publicaciones de la Comissión Administradora del Río Uruguay, Serie Técnico-Científica, 1:1–12.

Angelini, N., G. Seigneur, and A. V. Atanasiadis. 1992. Investigación de residuos de substancias organocloradas em peces del rio Uruguai. Publicaciones de la Comissión Administradora del Río Uruguay, Serie Técnico-Científica, 1:36–42.

Atlas Mirador Internacional. 1987. Encyclopédia Britânica do Brasil (Ed.), Rio de Janeiro, 7–21 p.

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