Effect of fish size, handling stresses and training procedure on the swimming behavior of hatchery-reared striped jack: implications for stock enhancement

The marine ranching of striped jack (Pseudocaranx dentex) in Japan is a new type of fishery enhancement that releases cultured juveniles and keeps them in coastal waters without cages, utilizing their behaviour of associating with floating objects. To improve the release strategies, the behaviour of released fish under different conditions was observed directly using SCUBA. We found that factors of fish size, the amount of handling stress and trained feeding behaviour had an impact on the potential loss of the juveniles from the release site. Large juveniles (115 mm) showed a greater potential for loss from the release site than smaller juveniles (59 mm) due to diving deeper immediately after release (an average of 5.4 vs. 4.1 m, respectively). Stressed fish dived deeper than fish provided with a vertical underwater structure (average of 8.5 vs. 3.8 m, respectively). Fish trained to respond to sound for feeding dived to a more shallower depth (3.3 m) and stayed near the release site. Handling stress before release may be the main cause of the loss of fish from the release site, while training showed great potential for improving fish retention at the release site.