ORIGINAL PAPER Spatio-temporal patterns of occupation and density by an invasive fish in streams Chelsea May . Gary Burness . Brian Morrison . Michael G. Fox Received: 8 May 2019 / Accepted: 10 March 2020 Ó Springer Nature Switzerland AG 2020 Abstract Since its introduction to North America in the 1990s, the round goby has spread throughout the Great Lakes and inland movement into small tribu- taries, a new environment for this species in both its native and invaded ranges, is becoming more preva- lent. We assessed spatial and temporal occupation of round gobies in four small streams in two systems in south-central Ontario, Canada to determine which habitat variables are the best predictors of round goby density. Two streams are tributaries of Lake Ontario and two are tributaries of the Otonabee River; all of these streams have barriers preventing upstream migration. Occupation and density patterns differed between the systems. In the Lake Ontario system, density was highest near the longitudinal centre of the stream between the mouth and first barrier, and round goby appeared to occupy these streams mainly from spring to fall. In the Otonabee River system, round goby occupied the streams year-round, and the most important factor determining adult density was dis- tance from a barrier to upstream movement, with density highest next to the barriers. Adult density in Lake Ontario tributaries was highest in sites with a high percentage of cobble/boulder and a low percent- age of gravel substrate, while substrate was less important in Otonabee River tributaries. Date was the strongest predictor of juvenile density, with density increasing through the sampling period in both systems. Occupation and density patterns may differ due to contrasting environmental conditions in the source environments and distance to the first barrier preventing upstream movement. This study shows diversity in invasion strategies, and provides insight into the ability of invasive species to change their occupation strategies based on the different environ- ments they encounter. Keywords Generalized additive mixed model Á Invasion front Á Round goby Á Spatio-temporal gradient Á Stream habitat C. May Environmental and Life Sciences Graduate Program, Trent University, 2089 East Bank Drive, Peterborough, ON K9L 1Z8, Canada G. Burness Á M. G. Fox (&) Department of Biology, Trent University, 2089 East Bank Drive, Peterborough, ON K9L 1Z8, Canada e-mail: mfox@trentu.ca B. Morrison 2620 Farmcrest Ave, Peterborough, ON K9L 1H7, Canada M. G. Fox School of Environment, Trent University, 2089 East Bank Drive, Peterborough, ON K9L 1Z8, Canada 123 Biol Invasions https://doi.org/10.1007/s10530-020-02243-3