SUMMARYThe mechanism underlying the decrease in aerobic scope in fish at warm temperatures is not fully understood and is the focus of this research. Our study examined oxygen uptake and delivery in resting, swimming and recovering sockeye salmon while water temperature was acutely increased from 15°C to 24°C in 2°C h -1 increments. Fish swam at a constant speed during the temperature change. By simultaneously measuring oxygen consumption (M O2 ), cardiac output (Q) and the blood oxygen status of arterial and venous blood, we were able to determine where in the oxygen cascade a limitation appeared when fish stopped sustained swimming as temperature increased. High temperature fatigue of swimming sockeye salmon was not a result of a failure of either oxygen delivery to the gills or oxygen diffusion at the gills because oxygen partial pressure (P O2 ) and oxygen content (C O2 ) in arterial blood did not decrease with increasing temperature, as would be predicted for such limitations. Instead, arterial oxygen delivery (Ta O2 ) was initially hampered due to a failure to adequately increase Q with increasing temperature. Subsequently, lactate appeared in the blood and venous P O2 remained constant.

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Oxygen consumption (M O2 ) was measured for gilthead seabream (Sparus aurata) during spontaneous and forced activities. During spontaneous activity, the swimming pattern was analysed for the effect on M O2 on the average speed (U), turning rate () and change in speed (DU). All swimming characteristics contributed significantly to the source of spontaneous swimming costs, and the models explained up to 58% of the variation in M O2 : Prediction of M O2 of fish in field studies can thereby be improved if changes in speed and direction are determined in addition to swimming speed. A relationship between swimming speed and M O2 during forced activity was also established. During spontaneous activity, 2.5 times more energy was used than in forced swimming at a speed of 0.5 BL s À1. This indicates that spontaneous swimming costs may be considerably higher compared with those of a fixed swimming speed. However, comparing M O2 at the respective optimum swimming speeds with the lowest cost of transport (U opt ) resulted in similar values independent of swimming mode. This could be an important observation in estimating energetic costs of free-ranging fishes.

Decreased critical swimming speed and increased oxygen consumption (M O2 ) was found for externally tagged Atlantic cod Gadus morhua swimming at a high speed of 0Á9 body length (total length, L T ) s À1 . No difference was found in the standard metabolic rate, indicating that the higher M O2 for tagged cod was due to drag force rather than increased costs to keep buoyancy.