Temperature has been shown to have a significant effect on swimming performance in teleost fish. This thermal dependence has usually been attributed to correlated changes in muscle contractile physiology. However, the physical properties of all materials, including both water and living tissues, are profoundly affected by changes in temperature. In particular, both the kinematic viscosity of water and the flexible body dynamics of the fish (independent of muscle contractile activity) are substantially higher at lower temperatures. In this study, we investigated the extent to which the observed thermal dependence of locomotor performance of fish simply reflects changes in the biophysical properties of the aqueous medium independent of the changing physiology of the animals. C-starts (escape swimming) of adult goldfish (Carassius auratus, length approximately 8 cm) were video-taped (400 frames s-1) at 5 degreesC and 20 degreesC in fresh water and at 20 degreesC in fresh water containing 1.2 % dextran (kinematic viscosity equivalent to that of water at 5 degreesC). Temperature had a significant positive effect on maximum forward velocity (m s-1), angular velocity (degrees s-1) and total distance moved, but viscosity had no detectable effect on any kinematic parameters at 20 degreesC. Since viscous forces may have more pronounced effects on smaller fish, C-starts of adult guppies (Poecilia reticulata, length approximately 2 cm) were video-taped in fresh water and in fresh water containing 1.2 % and 3. 6 % dextran at 20 degreesC. Viscosity had a significant effect on kinematic variables only at 3.6 % dextran; at 1.2 % dextran, the effects were marginal. It was concluded that most of the observed thermal dependence of C-starts at temperatures normally experienced by these fish was due to changes in physiological processes as opposed to physical changes in the environment.

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