The effect of flow on the competition between the alien racer goby and native European bullhead Lukasz Jermacz 1 , Jaroslaw Kobak 1 , Anna Dzier _ zy  nska 1 , Tomasz Kakareko 2 1 Department of Invertebrate Zoology, Nicolaus Copernicus University, Toru n, Poland 2 Department of Hydrobiology, Nicolaus Copernicus University, Toru n, Poland Accepted for publication May 27, 2014 Abstract – 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 cmÁs À1 (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 cmÁs À1 , 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 cmÁs À1 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. Key words: interspecific aggression; flow velocity; biological invasion; Babka gymnotrachelus; Cottus gobio; shelter Introduction Biological invasions are regarded as one of the major (Allendorf & Lundquist 2003; Thiele & Otte 2008), if not the greatest (Lambertini et al. 2011) threats to the world biodiversity. The appearance of alien spe- cies in the environment may lead to a significant reduction of the occurrence of native species or even their extinction due to the interference competition of a more aggressive invader (Barbaresi & Gherardi 1997; Martin et al. 2010). One of the fundamental factors determining the result of interspecific interfer- ence competition is abiotic conditions. In aquatic ecosystems, such factors as salinity (Wijnhoven et al. 2003), hydrodynamics (Genin & Karp 1994; Krassoi et al. 2008), temperature (Taniguchi & Nakano 2000) and type of substrate (Palmer & Ricciardi 2004) may influence the outcomes of direct fights. Consequently, abiotic conditions influence displacement of one species by another, their coexistence in the same habi- tat or their separation in space. Hence, there is a need for a thorough examination of the potential results of behavioural interactions between native and invasive organisms under a wide range of abiotic conditions. This would allow for the more precise predictions of ecological consequences of biological invasions in specific environments. Ponto-Caspian neogobiid fish (Gobiidae, Percifor- mes) belong to the most impressive vertebrate invad- ers in freshwater aquatic ecosystems (Copp et al. 2005; Roche et al. 2013). They are bottom-dwelling benthivorous fish with a developed system of pater- nal care of eggs, aggressively competing for territo- ries and breeding sites (crevices in the bottom) with conspecifics and congener species (Dubs & Corkum 1996; Van Kessel et al. 2011; Copp et al. 2013; Kakareko et al. 2013a). Their opportunistic foraging strategy (Kostrzewa & Grabowski 2003; Grabowska Correspondence: Lukasz Jermacz, Lwowska 1, 87-100 Toru  n, Poland. E-mail: lukasjermacz@gmail.com doi: 10.1111/eff.12162 467 Ecology of Freshwater Fish 2015: 24: 467–477 Ó 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd ECOLOGY OF FRESHWATER FISH