Sustained swimming capacity of Atlantic salmon

Sustained swimming in fish relies on aerobic metabolism alone and has been defined as swimming speeds that can be maintained for 200 min. However, few studies have tested swimming performances at constant velocities for such durations. Instead, the limits of sustainable swimming are often defined indirectly as speeds where anaerobic metabolism is required. Atlantic salmon Salmo salar aquaculture is currently expanding to more exposed locations that experience occasional strong current velocities, and thus a more practical assessment of the sustained swimming capacity is needed to ensure acceptable animal welfare. To this purpose, we performed swim tunnel respirometry experiments with groups of larger post-smolts (similar to 800 g) at 13 degrees C. First, the average critical swimming speed (Ucrit) was determined (97.2 cm s(-1)); then, sustained swimming trials were conducted in which fish were forced to swim up to 4 h at 60, 80 or 100% of the average U-crit. All fish endured 60 and 80% Ucrit for 4 h, while only 2 avoided fatigue at 100% U-crit (n = 24 treatment-1). A decrease in oxygen consumption rate over time at the sub-Ucrit intensities indicated swimming became more efficient and aerobic. To explain variation in swimming performance within the group, Ucrit and fatigue time were correlated with relative ventricular mass, condition factor and weight. However, these parameters were poor predictors of swimming performance. In conclusion, Atlantic salmon possess the aerobic capacity for continuous high-intensity swimming of at least 80% Ucrit for several hours. This adds important temporal insight to welfare guidelines related to current velocities in aquaculture.