Size-dependent preference for low water temperature in juvenile chum salmon (Oncorhynchus keta) during the migration period.

Recently, sea surface temperature (SST) has increased around Japanese coastal areas, and it has been pointed out that the survival rate of hatchery-released chum salmon (Oncorhynchus nerka) has decreased, partly due to the influence of high SST. To investigate the characteristics of juveniles that prefer low water temperatures, potentially indicating an active northward migration in Japan's coastal waters, we examined the relationship between low water temperature preference and body shape in juvenile chum salmon. Behavioural experiments were conducted between 11 and 18 May using tanks that allowed juveniles to move freely and choose between low- and high water temperature areas. A total of 158 juveniles were tested across five trials, with 35 individuals in four trials and 18 individuals in an additional trial, randomly selected for each experiment. The selection rate for low water temperature areas, as well as the average standard length (Ls) and body mass (M), increased with the number of trials. Juveniles that preferred low water temperatures exhibited greater Ls and M but a lower condition factor (K) compared to those selecting the high water temperature area. Following the behavioural experiments, both groups were reared separately at an intermediate temperature, and their Ls and M were measured every 13-17 days. Growth rates appeared to be maintained in both groups, as Ls and M, but not K, continued to increase between 18 May and 16 June. These findings suggest that well-growing juveniles with lower K values may actively show water temperature preferences. This preference for lower temperatures may drive fish to migrate northward in search of cooler waters in the North Pacific Ocean.