<p>Early life dispersal behaviour in salmonids is a crucial factor because it greatly influences subsequent growth and survival. Although dispersal and growth patterns during early life were previously evaluated, it remains unclear whether these patterns vary across different river environments. While long-distance dispersal may result in high energy expenditure and increased risk of predation, it can also be advantageous by allowing individuals to move to a suitable environment (e.g. optimal temperature for growth and abundant food resources), thereby increasing survival and growth rates. This trade-off likely differs among rivers. Therefore, we clarified dispersal and growth patterns of fry masu salmon (<i>Oncorhynchus masou)</i> in a large lotic ecosystem (&gt; 100 km²) and compared them with those reported by Yamamoto et al. (2021). The results showed that in a large lotic ecosystem, males tended to stay upstream more than those of females, which is similar to those of the patterns observed in other rivers; however, no clear sex-specific dispersal pattern was observed. Furthermore, unlike that in other rivers, individuals staying within 1&#xa0;km of the spawning ground were biased towards offspring from the resident father, suggesting that females and males from an anadromous father could be dispersed beyond the study area. Additionally, females were larger and more obese than those of the males, although this was not observed in the other rivers. It is likely that sex-specific selection pressure is exerted from early life, as predators and competitors are abundant in the mainstream, where females actively migrate more than males.</p>