The effect of flow on the competition between the alien racer goby and native European bullhead

The racer goby is an invasive Ponto‐Caspian fish spreading throughout Europe. They threaten a native species of similar biology, the European bullhead, by displacing them from shelters. These shelters are necessary for reproduction as well as for protection against predators and hydrodynamic forces. However, abiotic conditions may strongly modify the outcome of an interspecific competition in the wild. Nevertheless, little is known of the effect of flow velocity on the competition between these rheophilic species, although this factor is crucial for their distribution in the field. We video‐recorded fish behaviour for 2 h in single‐species and mixed‐species pairs in the presence of single shelters at three flow velocities: 0, 10 (a velocity preferred by the racer goby) and 30 cm·s^?1 (a velocity greater than preferred by the racer goby) to determine whether the invader can deprive the native species of its shelter. At the flow of 0 and 10 cm·s^?1, the racer goby exhibited aggressive behaviour towards bullhead, and this restricted the time spent by the bullhead in the shelter. Moreover, although the flow of 30 cm·s^?1 inhibited racer goby aggression, the time spent by the bullhead in the shelter in interspecific competition was still reduced when compared to intraspecific controls. Our results suggest that under natural conditions, the racer goby displace bullheads from their shelters even at flow velocities greater than optimal for the racer goby.