Abstract: In this study, chum salmon (Oncorhynchus keta) were cultured in different salinities for 0, 8, 16 and 24 for 42 days to investigate the effects of salinity level on survival rate, ATP enzyme activity and gill structure. Chum salmon gills were dissected to measure ATP enzyme activity and to observe the gill structure under different salinities using histological photomicrography and scanning electron microscopy technologies for determining the mechanism of catadromous migration. The results showed that the survival rates of the S8 and S16 groups were the highest, reaching 98.89%; the survival rate of the S0 group was 94.45%, and for the S24 group, it was 83.34%. The Na⁺/K⁺-ATP and Ca²⁺/Mg²⁺-ATP enzyme activities first increased and then decreased with increasing salinity, and the highest enzyme activity was found for the S8 group, and the lowest enzyme activity was found for the S24 group. The gill filament width increased with increasing salinity, while the gill lamella length and width decreased, and the differences between these groups were significant (P<0.05). The number of mitochondria-rich cells gradually increased, the apical membrane area decreased and the microvilli disappeared with the increase in salinity, as observed by electron microscopy. The boundaries between the flat epithelial cells on the surface of the gill filament and the circular micro ridges in the S8 group and the S16 group were clearer than those of the S0 group, while the boundaries between the flat epithelial cells on the surface of the gill filament were blurry, and fusion or discontinuity between annular microridges was observed in the S24 group. The surface structures of the flat epithelial cells at the bottom of the gill patches gradually changed from being organized to being disorganized, the number of stomata gradually decreased, and the pore sizes were reduced. According to these results, the suitable salinity range in which juvenile chum salmon survive was suggested to be between 8‰ and 16‰ during the catadromous migration stage, a supposition that needs to be confirmed by further study.