The strength of density dependence on body size of young-of-the-year masu salmon, Oncorhynchus masou, varies over ontogeny in Horokashubuto stream, Hokkaido, northern Japan

The relationships between body size and density of salmonids in natural streams have been well studied, and density-dependent growth (somatic growth) is interpreted as the principal mechanism responsible for the relationship. Moreover, the body size–density relationship is known to vary over ontogeny. However, the relationship has been studied mostly by experimental procedures (e.g. stocking fish into streams), and knowledge about the relationship in natural populations (e.g. no human-induced input of fish) is still inadequate. In this study, we compared the relationships between body size and density of young-of-the-year (YOY) masu salmon, Oncorhynchus masou, between different ontogenetic stages, that is, fry (spring) and juveniles (autumn). We used the monitoring data of YOY body sizes and densities across three study sites collected from Horokashubuto stream in Hokkaido, northern Japan, from 2014 to 2022. In the juvenile stage, YOY body size correlated negatively with increasing density. This pattern is typical of the density dependence of salmonid growth demonstrated in many previous studies. In the fry stage, however, YOY body size correlated positively with increasing density. This study suggested that the patterns of density dependence vary as functions of the ontogenetic stages of salmonids.