| DOWNSTREAM PASSAGE OF FISH LARVAE AT SALTO GRANDE DAM, URUGUAY RIVER |
| FUENTES, C. M. Direccin de Pesca Continental. MAGyP. Paseo Colon 982 (1063). CABA. Argentina. carlosmarianofuentes@gmail.com. Gomez, M. I. Direccin de Pesca Continental. MAGyP. ine_gomez@yahoo.com.ar, Salva, J. L.. Direccion de Pesca Continental. MAGyP. Llamazares, S, Direccion de Pesca Continental. MAGyP. Domanico, A. A. Direccin de Pesca Continental. MAGyP. Arcelus, A., Comision Administradora del Rio Uruguay. (CARU). Brown, D. R Instituto Nacional de Investigacion y Desarrollo Pesquero. Espinach Ros, A. Direccin de Pesca Continental. MAGyP. alespin2@yahoo.com.ar |
| Since 1993 the River Uruguay Executive Commission (CARU) supports investigations on ichthyoplankton in the Middle and Lower sections of the river. The presence of fish eggs and larvae up and downriver Salto Grande Dam indicated that populations of migratory species have the necessary habitats to complete their life cycle at both sections of the river. However, the extent at which early stages of fishes are transferred from the lake to the Lower section has not been exhaustively addressed up to the present. With that purpose, ichthyoplankton samples from a location 24 km downriver and from three experiments involving samples taken immediately above and below the dam, were analyzed. During four years of sampling 24 km downriver the dam, both fish eggs and larvae of migratory fish were captured at high waters, usually when spillways were opened. Early stages of migratory fishes captured were frequently un-yolked, corresponding to individuals of age 5+ days, which represent significantly more time than that required for the displacement of the water mass from the dam to the sampling location. In sampling experiments during periods without spillways discharge, fish larvae of the same species and degree of development were captured in the lake near the turbines influence area and below the dam at no more than 1000 m, against a water current of 2-4 knots. Similar composition of ichthyoplankton was observed up to 40 km downriver. When short time and low speed tows were conducted, an average sixty percent of Pimelodinae larvae were observed alive up and downriver the dam, even at 24 km downriver, giving indication that great part of larval mortality was due to sampling and not to turbine passage. The results show that larvae of fish that spawn in the Middle section are, in part, transferred to the Lower section and suggest that both spillways and turbines discharge should be implicated in the passage. The experiments conducted under null spillways discharges, show that the turbines contribute to the transference. We also found evidence that the impact of passage through Salto Grandes Kaplan type turbines on the survival of the small and fragile Pimelodinae larvae is not significant. Our results are coherent with those reported for such low-head turbines and would explain the presence of larvae and post-larvae of other migratory fishes, as Prochilodus lineatus, Salminus brasiliensis and Pseudoplatystoma spp., in small brooks a few kilometers downriver the dam. |
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