Dominance hierarchy establishment in the invasive round goby, Neogobius melanostomus.

Organisms living at high densities may be forced to engage in conflict for access to resources such as food or shelter. When these resources are limited, the outcome of interactions may have important fitness implications. We investigated the behavioural interactions of the invasive Round Goby (Neogobius melanostomus) in a shelter-limited environment. Round Goby are benthic fish that utilize rocky shelters for predator avoidance and as reproductive sites in which territorial males defend clutches of eggs. Previous work on this and other species has shown that larger individuals have greater resource holding potential in dyadic interactions. In order to understand the outcome of agonistic interactions in more complex social environments, we observed groups of three goby of the same sex which varied in relative size in an aquarium in which individuals had the opportunity to compete for access to shelters. We predicted that larger goby would behave aggressively towards smaller goby, and outcompete smaller goby for access to shelters. Because males defend shelters while breeding, we also predicted that male goby would compete more aggressively than females over dominance status. We found that larger goby in groups were socially dominant to smaller goby, regardless of sex. Additionally, we found that the largest goby in each group was involved in more aggressive interactions than the second or third largest goby in each group. We found no effect of relative size or sex on aggressive interaction or the emergent dominance relationships. Our findings highlight that aspects of the social environment may limit the opportunity for individuals to establish dominance or establish ownership of resources.