Fish diversity and ecology, habitats and fisheries for the un-dammed riverine axis Paraguay-Parana-Rio de la Plata (Southern South America)

Abstract

The riverine complex Paraguay-middle Parana-Rio de la Plata extends more than 3700 km southwards from its sources in the western hills of the Brazilian Shield to its discharge into the Rio de la Plata River. The high variety of habitats is reflected in the large diversity of its fish communities, which are dominated by characiform and siluriform fish species. The potamic axis is evolutionarily and ecologically open to fish movements and there are more than 400 fish species listed for the whole system, but only 100 species are common to both upper and lower basins. However, data limitations in some portions of the system need to be addressed before creation of an ichthyogeographic classification. The river basins that make up the potamic axis are low to medium developed and environmental pressures are unevenly distributed. Chemical pollution is a concern throughout. In the Pantanal, small hydro-projects and sedimentation from agricultural activities have had adverse effects on fish habitat inducing a loss of fish diversity. Un-dammed but more regulated and developed lowland rivers, may be impacted by upstream dams that may create unsuitable habitats for fish adapted to normal main channel conditions because they increase river flows during periods that were formerly low waters or change flows at random. The fisheries are lightly to moderately exploited compared to other subtropical and tropical riverine fisheries, and retain several of their original characteristics in less developed river reaches, although changes are evident. Large potamodromous fish are usually present in the catch, but the abundance of large piscivores is lower and fish size at catch is smaller. The development of the riverine system is expected to continue throughout the basin. If it is implemented as it was executed in the past, a continual loss of fish habitats and a general decrease in ecosystem health can be predicted.

Current state of knowledge regarding South America wetlands and their future under global climate change

“…Stocks in the Magdalena River of Colombia are heavily over-fished (Galvis and Mojica 2007), and those of the São Francisco and Paraná Rivers in Brazil are depleted (Sato and Godinho 2004;Agostinho et al 2007). By contrast, the fishery potential of the Orinoco and Paraná/Paraguay Rivers is not fully exploited Rodríguez et al 2007;Quirós et al 2007). Only half of the fishery potential of about 900,000 t year -1 of the Amazon River is used (Bayley and Petrere 1989).…”

Geographic distribution of the short-tailed river stingray (Potamotrygon brachyura): assessing habitat loss and fishing as threats to the world’s largest obligate freshwater elasmobranch

“…In addition, these dams interrupted the long-distance movements of migratory fish species, which are also the largest and most valuable for fisheries (Quirós 2004;Quirós et al 2007;Baigún et al 2011). These effects are rather indirect as compared to the direct effect that planned infrastructure projects will have on the Paraguay-middle Paraná axis if completed.…”

“…With some exceptions, the region still conserves most of its original wetland-dependent fauna, including large vertebrates, such as two species of caimans (Caiman latirostris and C. yacare), yellow anacondas (Eunectes notaeus), and the mammalian capybaras (Hydrochoerus hydrochaeris) and swamp deer (Blastocerus dichotomus) (Bó 2006;Giraudo and Moggia 2008). Among fishes, although the mean size and abundance has decreased in many areas of the riverine axis (Quirós et al 2007), there are still areas within the region where the largest, most sensitive species persist, such as, among catfish, two species of sorubim (P. corruscans and P. fasciatum), and the manguruyú or jau (Zungaro jahu), and, among characoids, dorados (S. brasiliensis) and pacú (P. mesopotamicus) (Quirós et al 2007). Our finding for P. brachyura therefore appears to be congruent with trends for other large fishes and vertebrates of the Paraguay-middle Paraná axis.…”

“…This is because of the fairly well conserved and mostly unmodified Paraguay-middle Paraná River axis. This riverine axis contains one of the largest continuous wetland areas in the world (from the Pantanal, in the upper Paraguay River, to the huge floodplains of the middle and lower Paraná River), has a rather low human density through most of its area, and has no dams along its 3700 km (de Resende 2003;Quirós et al 2007). In fact, it has been proposed as a baseline for the restoration of other large disturbed floodplain rivers (Nestler et al 2007).…”

“…The hydrological cycle of floods, a process of capital importance in the life cycle of most of the region's biota (Junk et al 1989), has been affected by the cascade of dams built in the upper Paraná River (Quirós et al 2007 effect has been one of modulation of the flood pulse for the middle Paraná River because dams retain water during highwater seasons and release it slowly during low-water periods; this results in a decrease in the variability of the flood pulse, both spatially and temporally (Quirós 2004;Quirós et al 2007). …”