Juveniles of the endemic catfish Ictalurus balsanus are less nocturnal in the presence of juvenile Ictalurus punctatus

Abstract Interspecific competition is crucial in shaping biological communities within ecosystems. Local populations of the native Balsas catfish, Ictalurus balsanus , are thought to be negatively affected by the introduction of its congener, I. punctatus . Here, we investigate changes in the daily activity pattern of juvenile Balsas catfish at different densities of juvenile I. punctatus . We exposed Balsas catfish individuals to each of three competitor density treatments (zero, two, and four I. punctatus ) and quantified their hourly activity over three consecutive days. Under all treatments, the Balsas catfish was more active at night. However, the presence of I. punctatus weakened the distinction between daytime and nighttime activity; activity was 69% higher by night than by day with no competitors, but only ~ 30% higher in the two competitor treatments. There was also an interruption of the progression to maximal activity at night, and at the higher competitor density the overall activity during the 24-h period was significantly lower than when no competitors were present. Individual (adjusted) repeatability of hourly activity was low (~ 6%), suggesting that it is a highly flexible behavioral trait. These findings show that the diel behavior pattern of juvenile Balsas catfish is influenced by the presence of I. punctatus. Temporal niche partitioning could be beneficial if it minimizes interactions with an invasive competitor, but it could also result in increased predation or decreased food availability, so it is unclear how these shifts affect fitness. Understanding these dynamics is crucial for the effective management and conservation of native fish populations in aquatic ecosystems.