A study was conducted in 1996 to compare densities of the round goby (Neogobius
melanostomus) in rock and sand habitats in day and night at three sites (Sarnia, St. Clair River; town of
Belle River, Lake St. Clair; Peche Island, Detroit River). The sites represent an expansion in round goby
distribution from Sarnia where gobies were first reported in 1990 downstream to Peche Island where
gobies appeared in 1993. Site affinity at Peche Island was estimated using mark-recapture and observations
of individual fish. Mean density of round gobies differed among sites (p < 0.001), light regime
(p < 0.001), habitat (p < 0.001), and the interaction of light regime and site (p = 0.007). Mean densities
of the round goby were 0.3 to 3 fish/m2 (Peche Island), 0.5 to 3 fish/m2 (town of Belle River), and 5 to 9
fish/m2 (Sarnia). Overall, highest densities of the round goby occurred in the day and on rocks. Small
(≤ 5 cm) round gobies were most common at Peche Island, the most recently colonized site, indicating
that juveniles may disperse more rapidly than adults. Of 200 fish that were marked, 58% were recaptured,
indicating high site fidelity among round gobies. The estimated mean (± SE) home range of the
round goby determined using SCUBA (5 ± 1.2 m2) was likely underestimated because few fish were
observed. Larger round gobies may induce smaller fish to leave preferred rock habitats and move to sand
habitats from which they disperse.