In some anadromous (sea-run) salmonids, male adult body size or secondary sexual traits vary greatly among populations. Do such interpopulation variation certainly result from differences in sexual selection on males? Or do other causes shape such variation? A study is ongoing to address to these questions analyzing field data from anadromous populations of masu salmon, Oncorhynchus masou. Here, I would like to talk about preliminary results of the study.

Anadromous male adult body size (body length) and two representative secondary sexual traits (hump depth and snout length) after adjusting for body size were compared among three populations (abbreviated name, Oku, Betsu and Mosho) of masu salmon in Hokkaido, Japan using data obtained from 2009 to 2011. For the body length, there was a marginally significant difference among populations. Betsu tended to be larger than the other populations. For the hump depth, Betsu was higher than Mosho, but no differences were recognized among the remaining combinations. For the snout length, within-population (interannual) difference was found in Betsu, so it was unclear whether this trait varies among populations. I plan to refer briefly to factors shaping interpopuation variation in body size and hump depth, and to interannual difference in snout length found in Betsu.