Preferred and Optimal Swimming Speeds in Rainbow Trout (Oncorhynchus mykiss) at Three Temperatures

Sustained exercise in aquaculture is known to improve the health and growth of finfish. Implementing exercise regimes has become an increasing focus in aquaculture practice. This study examined the relationship between the preferred swimming speed (Upref) and the optimal swimming speed (Uopt) in rainbow trout (Oncorhynchus mykiss) under non-migratory conditions typical of aquaculture environments. Using a circular raceway, rainbow trout were allowed to swim voluntarily to determine Upref. Uopt was measured using a forced-swimming test in a swim tunnel respirometer. Experiments were conducted at three temperatures (10 °C, 15 °C, and 20 °C). The results revealed a significant difference between Upref (1.18 ± 0.14, 1.17 ± 0.19, and 1.24 ± 0.15 BL s−1, respectively) and Uopt (1.4 ± 0.19, 1.5 ± 0.15, and 1.6 ± 0.24 BL s−1, respectively) across all temperatures. Aerobic scope was greatest at 15 °C (3.8), consistent with the species’ thermal range. Notably, swimming at Upref required 18–22% less energy than Uopt, suggesting that Upref is more suitable for aquaculture systems. This study introduces a minimally invasive and stress-free method for determining Upref and provides insights that can optimize flow regimes in aquaculture tanks, improving both energy efficiency and fish welfare.