Low‐Head Barrier Dams Restrict the Movements of Fishes in Two Lake Ontario Streams

The Great Lakes Fishery Commission (GLFC) is considering greater use of low‐head barrier dams on stream tributaries of the Laurentian Great Lakes to control populations of sea lampreys Petromyzon marinus. The impact of these barriers on nontarget fishes is not known. A mark–recapture study on four Lake Ontario streams examined movements of fishes in streams with (barrier) and without (reference) low‐head barriers. A significantly lower proportion of fishes moved across a real barrier on barrier streams than across a hypothetical barrier on reference streams (0.15 versus 0.50, respectively). The impact of the barriers on movement was more pronounced in spring and fall than in summer. However, the likelihood of fishes moving versus not moving between sample segments on either side of a barrier location (but not across the barrier) did not differ significantly between barrier and reference streams. The upstream (longitudinal) decline in species richness was greater for barrier streams than for reference streams in each season. At both interspecific and intraspecific levels, mean total lengths of fish traversing real barriers were significantly greater than the mean total lengths of fish traversing hypothetical barriers. Our findings demonstrate that low‐head barriers restrict the movements of some fishes and suggest this restriction affects assemblage structure above the barrier.