1. 1. During swimming at constant speed the frequency (f), amplitude (a) and depth (d) of the tail trailing edge, and the length of the propulsive wave (λ) were measured for rainbow trout ranging in total length (L) from 5.5 to 56.0 cm. Fish were tested in a water flume using increasing velocity tests to sample a range of swimming speeds, V.

  2. 2. λ was independent of V and related to size by:

    λ=1.43L0.83

    so that wavelength was relatively larger in smaller fish.

  3. 3. f was related to L and V according to:

    f=3.19L−⅓+1.29V/L.

  4. 4. a was independent of V but was relatively smaller in larger fish:

    a=0.36L0.74

  5. 5. d was also independent of V but relatively larger in larger fish:

    d=0.18L1.05

  6. 6. Thrust power (=drag power) calculated using Lighthill's small amplitude bulk momentum model was two to three times the theoretical minimum of a flat plate of equivalent length and area moving parallel to the flow with a presumed turbulent boundary layer.

  7. 7. Froude efficiency increased with swimming speed, and it is shown that this is the usual relationship for fish studied so far. Froude efficiency was essentially independent of size at the critical swimming speed.

  8. 8. Estimated aerobic efficiency increased with size at the critical swimming speed, implying that muscle efficiency also increases with size.

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