| Summary: | The maximum sustained swimming speeds ( U ms ) 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 U ms 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 U ms . 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=0.96 U ms ). 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=0.69 U ms ). The power output for smolt swimming at 0.69 U ms was corrected to that required to swim at U ms , giving P =0.0059W (Lighthill's model) and P =0.0074W (Yates' model). At U ms 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 U ms 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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