Optimising hatchery release strategies: Size-dependent training effects on the swimming performance of juvenile...

The success of fish stock enhancement programmes is often limited by the high mortality of hatchery-reared individuals after their release. Poor swimming performance, a critical factor for survival, is a key reason for this low success rate. While exercise training can improve the physical fitness of fish, it is unclear whether these benefits are consistent across different sizes and developmental stages. Here, we aimed to determine how exercise training affects the aerobic and anaerobic swimming performance of the important economic fish, Schizothorax wangchiachii, across two different body size (age) classes. We subjected two sizes of hatchery-reared juvenile S. wangchiachii (4-month-old and 16-month-old) to 30 days of training. The fish underwent either high-speed aerobic training, low-speed aerobic training, anaerobic training, or were left untrained (control group). Following the training period, we measured their aerobic performance (critical swimming speed) and anaerobic performance (endurance time). We then compared these results with the performance of wild-caught juveniles of a similar age to the smaller cohort to gauge the effectiveness of the training. The training effects were strongly dependent on fish size. Aerobic exercise significantly improved the aerobic and anaerobic swimming performance of the smaller, 4-month-old juveniles. However, the training regimes had a much less pronounced effect on the larger, 16-month-old fish, with only their anaerobic performance showing some improvement. The low-speed aerobic training successfully raised the swimming capacity of the small hatchery-reared fish to a level that was not significantly different from their wild counterparts. Smaller juvenile S. wangchiachii benefit substantially more from aerobic exercise training than larger individuals. Therefore, considering both the training costs and the significant performance gains, implementing a low-speed aerobic exercise programme for smaller juveniles is the most effective and preferable strategy to prepare them for release, enhancing their potential for survival in the natural environment.