Invasion through the sea of a brown trout Salmo trutta in a small river, southern Hokkaido

Abstract

A non-native brown trout Salmo trutta (407 mm in fork length, 717 g in body weight) was collected from the Otsukushinai River, southern Hokkaido, Japan, in 2019. This specimen was an immature male (0.4 g in gonad weight). The otolith Sr:Ca ratio profile and annual rings suggested that the specimen had migrated to the sea at the age of 4+ years and ascended the Otsukushinai River in the year of the seaward migration. This study is the first record of invasion of anadromous brown trout through the sea in Japan.