The maximum sustained swimming speeds (Ums) for large (0.45 m long) and small (0.15 m) Atlantic salmon were respectively 0.91ms−1 and 0.54ms−1. Video and cin6 films of fish swimming close to Ums were analysed to obtain variables required for the application of two hydrodynamic models, those of Lighthill and Yates, to determine the mean thrust (T) and mean power output (P) at these swimming speeds (U) close to Ums. A large fish (‘Salmon’) and a small fish (‘Smolt’) were selected for analysis. For salmon using Lighthill's model, T=0.30N and P=0.26W, and using Yates' model, T=0.28N and P=0.25W (U/=0.87ms−1=0.96Ums). For smolt using Lighthill's model, T=0.0052N and P=0.0019W, and using Yates' model, T=0.0065 N and P=0.0024W (U=0.37ms−1=0.69Ums). The power output for smolt swimming at 0.69Ums was corrected to that required to swim at Ums, giving P=0.0059W (Lighthill's model) and P=0.0074W (Yates' model).

At Ums it was assumed that all the red muscle was used. Two fish were selected from each size group and cross-sectioned to estimate their red muscle masses. Using a maximum mass-specific power output of 5–8 W kg for slow red muscle fibres allowed us to calculate that the large and small fish have a power output capacity of 0.125–0.3 W and 0.007–0.019 W, respectively.

The power output values at Ums derived from the different approaches for the large (0.25–0.26 W) and small (0.0059–0.0074 W) salmon agree closely. Effects of scaling are discussed.

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