Exploration of selection methods for optimal juveniles in hatchery-reared masu salmon (Oncorhynchus masou).

To explore selection methods for juveniles with favourable characteristics in masu salmon (Oncorhynchus masou), we tested two approaches: selection based on position in a waterway and selection based on swimming layer in a round tank. After selection, juveniles were reared in identical environments in two tanks until the smoltification season (spring of the following year), and their characteristics - including standard length, weight, condition factor, sex and smoltification rate - were observed. In the waterway selection experiment (Experiment 1), 284 juvenile broodstock fish from the Shiribetsu River strain were placed in an artificial waterway, with 67 individuals selected from both upstream and downstream groups. In the swim layer selection experiment (Experiment 2), 222 juvenile broodstock fish from the Isatomae River strain were used in a round tank, with individuals selected from the surface and bottom groups. The results indicated that in Experiment 1, juveniles positioned downstream exhibited a higher smoltification rate and female ratio compared to those positioned upstream. In Experiment 2, the bottom group had a significantly higher female ratio than the surface group. These findings may be attributed to the territorial behaviour of juveniles in enclosed spaces, where dominance and subordination influence spatial positioning. Dominant individuals likely occupied upstream and surface areas in the artificial waterway and tank, respectively. In masu salmon, male juveniles with high metabolic rates tend to establish dominance in enclosed spaces and exhibit early maturation without undergoing smoltification. Therefore, selection methods based on waterway position and swimming layer may be effective in identifying juvenile groups with a higher smoltification rate prior to the smoltification season. If so, these methods could facilitate the selective identification of smoltifying individuals for release programmes aimed at promoting ocean-migratory forms.