Assessment of fish size on shelter choice and intraspecific interactions by round gobies <Emphasis Type="Italic">Neogobius melanostomus</Emphasis>

Synopsis We examined shelter occupancy and behavioural interactions in non-reproductive male round gobies, Neogobius melanostomus,an invasive fish, to determine if gobies can assess one another’s prowess effectively. Results of laboratory experiments revealed a significant, positive relationship between body size (total mass) and time for individual fish to occupy shelters. Shelter size selection did not vary with body size, but males that abandoned shelters were smaller than those that remained. Overall, the number of interactions between residents and intruders were low with most interactions occurring between large residents and large intruders. A size differential between residents and intruders of 3% was sufficient to predict the proportion of victories in conflicts between round gobies. This ability of the round goby to perceive size differences between themselves and potential opponents reduces the number of interactions and may account for the occurrence of high densities of round gobies observed in the field.