Stable isotopes and stomach content analyses indicate omnivorous habits and opportunistic feeding behavior of an invasive fish

The aim of this study was to test the hypothesis that a widespread non-native fish species in Brazil displays opportunistic feeding behavior and changes its diet according to environmental conditions. We compared the diet, feeding selectivity, carbon assimilation, trophic niche, and trophic level of Knodus moenkhausii (a small non-native characid fish species of Upper Parana River) in streams surrounded by natural riparian vegetation (natural cover streams) and in streams impacted by pasture. We analyzed stomach contents and stable isotopes (carbon and nitrogen), simultaneously. Overall, insects were the most common food items (> 65%). In natural cover streams, K. moenkhausii showed higher selectivity among aquatic macroinvertebrates consumed, while in pasture streams, they fed on the most abundant groups. The proportion of feeding groups assimilated by K. moenkhausii and the proportion of primary sources consumed by each feeding group of macroinvertebrates also varied between natural cover and pasture streams, as indicated by stable isotopes. In natural cover streams, fine and coarse particulate organic matter accounted for approximately 80% of K. moenkhausii's diet, while in pasture streams, algae and periphyton also contributed greatly. As a result, K. moenkhausii occupied a higher trophic level and exhibited a broader niche width in pasture streams. We conclude that K. moenkhausii presents feeding selectivity with capacity to alter the trophic niche depending on environmental conditions. Such opportunism could be one of the reasons underpinning the abundance and wide distribution of this invasive species.