TEMPORAL CHANGES IN MIGRATORY FISH BODY SIZE IN A NEOTROPICAL FLOODPLAIN

Body size is a trait that results from evolutionary and ecological processes. Body size not only provides information on the physical condition of individuals but also on the shift in community structure across space and time. Here, we aimed to investigate whether the body size of the migratory fishes changed over time and to ascertain the abiotic variables explaining these changes; and whether the effect on body size was different between fish in dammed (Parana River) and undammed (Ivinhema River) rivers. Using a temporal series (1986 - 2018) and linear mixed effects models, we evaluated the effects of the abiotic variables on the standard length of the fish species from a floodplain system. We found that fish body size tended to decrease; the mean body length had decreased by 25 % since 2000. Abiotic variables related to seasonal variability (water level) and those affected by reservoir operation (water transparency and nutrients) explained the decrease in body size. Our results highlighted that larger migratory fish were found in the undammed river in lower transparency habitats.