Invasion biology research has identified two juxtaposing behavioural traits, aggressiveness and sociality, that may both increase the success of species invasions. Highly aggressive invaders can out-compete native species for resources, while social gregarious invaders can tolerate high conspecific density. In order to tease apart the effects of aggressive versus social tendencies on the success of invasive species, we studied round goby (
Neogobius melanostomus), a highly successful invasive fish species now common in the Laurentian Great Lakes. While round goby are well known for being aggressive, much less is known about their tendency to affiliate with conspecifics, in spite of the fact that they thrive in extremely high densities in many of their invaded habitats. We collected round goby from Hamilton Harbour, ON, Canada and conducted three separate experiments to explore group-forming behaviour by measuring preference for conspecifics. We found that round goby have a strong preference to associate with a single conspecific, and that both males and females showed this preference. No overall preference was detected for large versus small groups of conspecifics. Females chose the safety of a shelter over associating with a conspecific but males were equally attracted to conspecifics as shelter. Our results provide new insight into how interactions between aggressive and social behaviours play a role in the rapid spread of invasive round goby.