Downstream migration of hatchery-reared smolts of Atlantic salmon ( Salmo salar L.) in the River Imsa, Norway

Abstract

On 16 May 1983 1750 tagged 1+ and 1280 tagged 2+ Atlantic salmon smolts of the River Imsa strain were released into the River Imsa in the evening. In 1984 1409 tagged 2+ smolts were divided into three groups. The first was released at 09.00 h on 21 May, the second group at 21.00 h the same evening, and the third group was released at 21.00 h the next evening. The smolts were recaptured in a Wolf trap, 1000 m downstream. To test effects on smolt migration, the light at the trap (4 lux) was switched on and off successively every 15 min during the first night after the 1983 release. In 1984 the whole width of a fast flowing part of the river was constantly illuminated (88 lux) during the first night after the last release. Most smolts descended during the first 12 h after release, and the smolts released in the morning descended in considerable numbers during daytime. 2+ smolts descended faster than did 1+ smolts. Small 1+ smolts descended faster than larger 1+ smolts whereas large 2+ smolts descended faster than smaller ones. Smolts released in the evening descended faster than smolts released in the morning. Illumination of the river during the night reduced the speed of descent, the effect being most pronounced in the pool just above the trap. Males that were mature in autumn 1983 smoltified and descended when released in 1984, but they were smaller than immature males and females. It is suggested that wild and hatchery smolts staying in the river may be attracted to shoals of released hatchery smolts and join them when migrating downstream.