Female-biased migration of stream-dwelling Dolly Varden in the Shiisorapuchi River, Hokkaido, Japan

The first evidence for female-biased migration in a partially migratory stream-dwelling salmonid the Dolly Varden Salvelinus malma, a phenomenon well known in sea-run and lake-run populations, is presented. Dolly Varden in the Shiisorapuchi River in central Hokkaido, Japan, used both tributaries, of which there are many, and the main stem, but spawned only in tributaries. The size structures of Dolly Varden (>= age 1 + years) in tributaries were unimodal (< 100 mm fork length, L-F) during non-spawning seasons but changed to bimodal during spawning seasons (lower mode < 110 mm, upper mode > 120 mm L-F). Mature individuals were observed in both modal groups. From the trapping and census data, the small group appeared to be tributary resident and the large group main stem migrant. Males were common in both resident and migrant components. Most females, however, migrated to the main stem to mature, indicating female-biased migration. (c) 2006 The Authors Journal compilation (c) 2006 The Fisheries Society of the British Isles.