Cardiac function and critical swimming speed of the winter flounder (Pleuronectes americanus) at two temperatures

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.