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Oldani, N. O. (1996). [Environmental impact assessment of the Yacyreta Dam on the fish community. The transference system]. Doctoral Dissertation. Facultad de Formacion Docente en Ciencias66 pp.

The increasing interest and need for taking advantage of the Parana River (South America) resources, together with the building of hydroelectric dams, produces a high impact on the fish community due to the interruption of the migratory processes and to the loss of reproductive areas. The construction of the Yacyreta Dam (Argentina-Paraguay) started in December, 1983, level with km 1460 of the Parana River, in one of the richest regions of Argentina as regards to the size of fish populations, the number of species and the specimens length. The aims of the present thesis are: 1) to establish the variations in the fish community structure downstream Yacyreta Dam; and 2) to establish the structure of fish community in the left and right elevators. To propose handling rules in order to improve the fishway efficiency. Fieldworks were carried out monthly, from October 3, 1994, until July 17, 1996, and consisted in: 1) acoustic evaluation of fish abundance and control captures downstream the dam; and 2) fish elevatoris census. The studies allowed to establish that, the total number of fish tranferred in 1995 reached 1,766,924, with a monthly average of 176,692 fish (n=10). The total biomass transferred in 1995 reached 982 metric tons (mt), with a monthly average of 98.6 mt. The right elevator was the most efficient, as regards to the number and weight of fish transferred, due to its nearness to the bank and because the right turbines were not in operation. During the whole period studied, this fish elevator transferred 29% more fish and 42% more biomass. A seasonal variation in the abundance of fish was noticed in the elevators, showing maxima in spring and summer which coincided with the results of fishing control. Pimelodus clarias was the most abundant species in the fish elevator, reaching 76.6% followed by Pterodoras granulosus with 11.8%. P. clarias was the only species having almost constant presence in the fish elevator during the whole year; Megalancistrus gigas was also present but to a lesser degree. The species considered as migratory are predominant in the system from October until January, and they are the following ones: Pseudoplatystoma coruscans, P.fasciatum, Paulicea lutkeni, Pseudopimelodus zungaro, Oxydoras kneri, Ptedoras granulosus, Rhinodoras d'orbignyi, Prochilodus lineatus, Salminus maxillosus, Leporinus obtusidens and Raphiodon vulpinus. Fish elevators were highly selective for small and very big sizes, favouring the transfer of fish whose length vary between 35 and 55 mm. Fish elevators allowed the passing of 5 cohorts for L.obtusidens, O.kneri, P.clarias; of 4 for P.granulosus, P.lineatus; of 3 for M.gigas, R.d'orbingyi, S.maxillosus, P.labrosus, of 2 for Sorubim lima and of 1 for Schizodon borelli. The size and average of transferred fish increases in spring and summer, associated to migratory species. The average annual weight (in 1995) reached 0.66 kg. The highest specific abundance was noticed in the early morning hours and in the late afternoon hours. Considering the daily cycles of system use, 3 groups of species were identified: diurnal species (M.gigas, L.obtusidens, R.d'orbignyi); nocturnal species (P.clarias, P.coruscans, P.granulosus) and diurnal-nocturnal species (S.borelli, O.kneri, S.lima, S.maxillosus, P.lineatus). The highest estimates of the total number of fish obtained by means of acoustic evaluations in the area under consideration took place between March and April. As an average, they represent 1,014 and 1,207 f/ha with 107 and 214 FPEU (Fish Per Effort Unit) values. The highest abundance of fish and species takes place between October and November (in spring), while the lowest, in both cases, happens mainly in autumn. In the periods of highest abundance, the total number of fish would reach 2,000,000.