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Using Transonic® flow probes and a uniquely designed swimming flume, we directly measured cardiac parameters ( Q , cardiac output; S V , stroke volume; and f H , heart rate) in winter flounder ( Pleuronectes americanus) before and during critical swim speed ( U crit ) tests at 4 and 10 °C. Resting Q , S V and f H averaged 9.8 ml min −1 kg −1 , 0.5 ml kg −1 (1.0 ml g ventricle −1 ) and 21 beats min −1 at 4 °C and 15.5 ml min −1 kg −1 , 0.5 ml kg −1 (0.95 ml g ventricle −1 ) and 34 beats min −1 at 10 °C ( Q 10 values of 2.13, 0.91 and 2.35, for Q , S V and f H , respectively). Cardiac output, S V and f H increased by approx. 170%, 70% and 60% at both temperatures during the U crit test. However, cardiac parameters generally reached near maximal levels almost immediately upon swimming and remained at these levels until U crit (0.65±0.06 bl s −1 at 4 °C and 0.73±0.07 bl s −1 at 10 °C). This rapid rise in cardiac function to near maximal levels did not appear to be the result of stress alone, as Q only fell slightly when flounder were swum for 75 min at −1 , speeds at which they appeared to swim comfortably. Our results suggest that both Q and U crit have been significantly overestimated in flatfishes, and that “lift-off”/slow swimming is energetically expensive. Furthermore, they show that maximum and resting stroke volume (per g of ventricle) are extremely high in the flounder as compared with other teleosts.

Repeated critical swimming performance trials (Ucrit) were performed on Atlantic salmon (Salmo salar) to test the null hypothesis that the source of dietary lipids (fish-based, poultry-based, and plant-based) does not influence exercise and recovery performance. Four diets were prepared by extensively replacing supplemental lipid from anchovy oil (AO; 100% AO at 150 g/kg) with cold pressed flaxseed oil (FO; 25% AO, 75% FO), sunflower oil (SO; 25% AO, 75% SO), or poultry fat (PF; 25% AO, 75% PF). These diets had equivalent protein and energy concentrations, but due to the different supplemental lipid sources, varied widely in their fatty acid composition. Fish fed AO had a significantly higher (P0.05) first Ucrit (2.62+/-0.07 body lenght s(-1)) than those fed PF (2.22+/-0.12 body lenght s(-1)) that had low muscle ratios of n-3 highly unsaturated fatty acids (n-3 HUFA) to saturated fatty acids (SFA) and arachidonic acid (AA), and high levels of oleic acid. Fish in the FO and SO diet groups swam as well as AO-fed fish in both swimming trials. The performance of fish fed AO decreased significantly (P0.05) during the second swimming trial (i.e. Ucrit2/Ucrit1=0.92+/-0.02). No significant differences occurred between diet groups for the second swim trial. There was a positive correlation between both n-3 HUFA/SFA and n-3 HUFA/AA ratios, and Ucrit1. A negative correlation was found between dietary AA and oleic acids, and Ucrit1. The present study suggests that low dietary n-3 HUFA/ SFA and n-3 HUFA/AA ratios may negatively affect swimming performance. The former possibly can be offset by increasing linoleic acid in the presence of nutritionally adequate n-3 HUFA (e.g. SO diet). Lipid supplements consisting largely of vegetable oils did not compromise fish cardiorespiratory physiology under the conditions of this study.