Altitudinal distribution and body condition of migratory fish juveniles Megaleporinus obtusidens (Valenciennes) and Salminus brasiliensis (Cuvier) in the main channel of the Uruguay River, Southern Brazil.

In this study, we analyzed the altitudinal distribution and body condition of the juvenile migratory fish Megaleporinus obtusidens and Salminus brasiliensis in the Uruguay River, a South American subtropical river. We used the presence/absence data and condition factor (K) of juveniles as indicators to try to characterize some recruitment patterns of migratory fish in the main channel of the Uruguay River. Gillnet sampling was conducted during two reproductive years, 2015-2016 and 2016-2017 (November and March of each reproductive year), in three Uruguay River stretches at different levels of altitude. The nets remained in the water for 10-12 hours per night or day for 10 days during each field trip (NOV-15, MAR-16, NOV-16, and MAR-17). The abundance of M. obtusidens juveniles were similar between the two reproductive years, whereas S. brasiliensis capture was higher during the 2015-2016 reproductive year. The probability of finding M. obtusidens juveniles in the Uruguay River was negatively correlated (p < 0.05) with the altitudinal gradient of the sampling points, as shown by the GLM model. For S. brasiliensis, the logit model indicated a non-significant probabilistic relationship (p > 0.05) between the presence of juveniles of this fish species and the altitude gradient. Regarding the body condition of juveniles, the water quality of the sampling points did not affect the juvenile condition factor for either species; however, a positive relationship (p < 0.05) was found with the water level for both species, even with an adjusted R2 of 0.13 for M. obtusidens and 0.48 to S. brasiliensis. Altitude gradient and water level were factors that may affect the juvenile distribution and body condition of these species, respectively, in the Uruguay River. Therefore, this information can be used in hydropower generation policies to conserve migratory fish populations in the Uruguay River Basin.