Interspecific variation in migratory fish recruitment in the Upper Parana River: effects of the duration and timing of floods

This study evaluated the relationship between the young of year (YOY) abundance of migratory fish species and the interannual variations in the duration and delay of floods in the Upper Parana River floodplain. YOY of the most abundant migratory species Brycon orbignyanus, Hemisorubim platyrhynchos, Leporinus elongatus, Leporinus macrocephalus, Leporinus obtusidens, Pseudoplatystoma corruscans, Pterodoras granulosus, Prochilodus lineatus, Salminus brasiliensis and Sorubim lima were studied in a stretch of the Parana river influenced by dams. Multiple regression analyses were performed with flood duration and delay as predictors for YOY abundances. There were differences among the species in the responses to flood duration and delay. The species B. orbignyanus, L. elongatus, L. macrocephalus, L. obtusidens, and P. lineatus exhibited an exponential increase in YOY abundance with flood duration. An exponential decrease in YOY abundance with flood delay was found for most of the studied species. Nevertheless, flood delay and flood duration were significant predictors of YOY abundance for L. elongatus, L. obtusidens, P. corruscans and P. lineatus. For these species, flood delays may result in failure of recruitment because these species migrate early in the season to adjust their energy reserves and their swimming patterns. Therefore, any efforts to preserve migratory fish should consider discharge manipulation from reservoirs upstream of the study areas. It is crucial that reservoirs are operated in such a way that the quantity, duration and timing of water release are compatible with the viability of spawning and the survival of early life stages of migratory fish.