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Entire year classes of smallmouth bass are frequently lost in midwestern streams during floods that occur when the fry are less than 25 mm long. Since changes in air and water temperatures often accompany these floods, the effects of temperature on swimming ability of fry 20–25 mm in length were measured in the laboratory and related to floodwater conditions. Groups of fry were acclimated to temperatures of 5, 10, 15, 20, 25, 30, and 35 C and tested for swimming ability at each of these temperatures. The maximum swimming speed for fish acclimated to a particular temperature increased with successively higher test temperatures, to a level above which swimming ability declined rapidly. Swimming speeds attained were progressively higher for each higher level of acclimation with the exception of those from the upper extreme of 35 C. The best swimming performance was at a temperature above the acclimation temperature for all fry except those acclimated to 30 and 35 C and these fry failed to swim above their acclimation temperatures. Temperature changes in streams may not directly cause the displacement of smallmouth bass fry but may reduce their swimming ability and thereby contribute to their displacement when exposed to turbid and turbulent floodwaters.

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