High rate of redd superimposition by brook trout (Salvelinus fontinalis) and brown trout (Salmo trutta) in a Minnesota stream cannot be explained by habitat availability alone

This study was conducted to test the hypothesis that redd superimposition b y salmonine fishes is a consequence of Limited habitat availability. We mon itored redd site selection by brook trout (Salvelinus fontinalis) and brown trout (Salmo trutta) for two spawning seasons in Valley Creek, Minnesota, Redd superimposition rates were high; over one half of the brook trout and one third of the brown trout superimposed redds. We tested the role of habi tat availability in this process by characterizing microhabitat at sites wi th and without redds in four small sections of this stream and then determi ned whether superimposition could be explained by random dispersal of fish over available habitat. Brown trout preferred spawning sites with high flow s whereas brook trout strongly preferred deep sites with upwelling groundwa ter. No relationship was observed between fish density and superimposition. Additionally, the observed frequency of superimposition was greater than e xpected by chance in six of eight instances far brown trout and in one of t hree instances for brook trout. Finally a behavioral experiment provided di rect evidence that females have a behavioral preference to spawn on existin g redd sites, suggesting that factors other than habitat may determine redd site selection and hence superimposition.