Divergence of sexual size dimorphism between wild and hatchery chum salmon under intensive Japanese hatchery programs

Artificial propagation weakens sexual selection by reducing male-male competition and female choice, which favor larger males in natural reproduction. If homing ability and sexual selection were strong, wild-origin salmon would exhibit more pronounced male-biased sexual size dimorphism (SSD) than their hatchery-origin counterparts. We conducted field sampling of chum salmon (Oncorhynchus keta) in two Japanese rivers, where hatchery fish were marked using otolith thermal marking. The length at maturity of hatchery salmon differed by only 1-2 cm between males and females, whereas the length at maturity of wild-dominated salmon was 5-6 cm larger in males than in females. The mean age at maturity of wild-dominated fish was 0.2-0.3 years higher than that of hatchery fish, but no significant sex differences were observed in either origin. Additionally, macro-level comparisons of SSD, calculated as log10(male length/female length), among 21 populations showed that wilddominated populations had significantly higher SSD than hatchery-dominated populations. Overall, hatchery salmon exhibited weaker SSD, consistent with reduced sexual selection in the hatchery.