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 wild-dominated populations ha..., , , # Divergence of sexual size dimorphism between wild and hatchery chum salmon under intensive Japanese hatchery programs

[https://doi.org/10.5061/dryad.4xgxd25m7](https://doi.org/10.5061/dryad.4xgxd25m7)

## Description of the data and file structure

The field observation was conducted in the Shari River (43°55′N, 144°40′E) and the Yurappu River (42°16′N, 140°16′E), Hokkaido, Japan. Adult chum salmon from the Shari River were collected over two breeding seasons (from October to January in 2014/2015 and 2015/2016). Those from the Yurappu River were also collected over two breeding seasons (from October to January in 2015/2016 and 2016/2017). Both rivers were surveyed once per month during each breeding season. Hatchery and natural sites in the two rivers were visited, and dead and post-spawning salmon were collected. Each collected fish was sexed, aged using scale analysis, and measured to determine fork length. Aging using scales for this species is highly accurate compared with aging ...