DOI: 10.5061/dryad.4xgxd25m7
title: Data from: Divergence of sexual size dimorphism between wild and hatchery chum salmon under intensive Japanese hatchery programs

description: 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 had significantly higher SSD than hatchery-dominated
 populations. Overall, hatchery salmon exhibited weaker SSD, consistent
 with reduced sexual selection in the hatchery.
description: # 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 using otoliths (Kaneto et al. 2017). Sagittal otoliths and tissue samples were collected from each fish and preserved in 99% ethanol for the analysis of otolith thermal marks and genetic analysis. A total of 1,250 adult chum salmon were collected from the study area. Of these, five did not obtain readable scales and their ages were unknown. Otolith thermal marks were determined for all fish. All except one otolith-marked fish had otolith thermal marks specific to each collected river. One otolith-marked fish collected in January 2016 from the Yurappu River had an otolith thermal mark from the Gakko River hatchery (39°04′N, 139°53′E) and was omitted as a stray from the analysis. In this study, fish produced from hatcheries are defined as hatchery fish, and fish born from natural spawning are defined as wild fish. Otolith-marked fish are from hatcheries, whereas unmarked fish may be of wild or private hatchery origin. In this study, unmarked fish caught at natural sites were treated as wild-dominated fish. However, it should be noted that the differences between hatchery and wild-dominated fish identified in this study are conservative because specimens considered wild-dominated might include some unmarked hatchery fish. ### Files and variables #### File: Morita\_and\_Sato\_data.xlsx **Description:** Biological data of chum salmon in two Japanese rivers ##### Variables * "Fiscal Year": Fiscal year in which samples were collected * "Date":Date the sample was collected * "River":Rivers from which samples were collected * "Month":Month in which samples were collected * "Stream":Name of tributary from which samples were collected * "Site":Site type from which samples were collected * "FL": Fork length (cm) * "SEX": Male or Female * "AGE":Age estimated from scales(years) * "Brood Year":Year of birth calculated from age and year the sample was taken * "Otolith":Presence or absence of otolith thermal marking * "Otolith mark":Otolith thermal marking code