MIGRATORY PATTERNS AND RETURN TO THE CATCH SITE OF ADULT BROWN TROUT (SALMO TRUTTA L.) IN A REGULATED RIVER

摘要

The spawning migration and local homing of adult brown trout was analysed using radio telemetry in a regulated river in central Norway. Twenty-eight large (37-64 cm) brown trout (Salmo trutta L.) were tracked before, during and after spawning in the River Nea, a watercourse with several obstructions, including an outlet tunnel from a power station and a regulated stretch (26 km) with 45 weirs. Two major patterns of spawning migration were found: (1) about half (n= 16; 57%) of the trout moved very little and remained in the deeper pools of the river from June until November; (2) about half (n =12; 43%) of the trout migrated relatively long distances (12.5-28 km) up the river prior to the spawning period where they stayed in the outlets of small tributaries, or in rapids on the main river during the spawning period. We assume that these trout belong to a population of lake-run migratory trout using the River Nea for spawning. There was no significant difference in body length of migratory and stationary brown trout and no significant difference in total distance moved by migratory males (30.5 km, n = 6) and females (20.5 km, n = 6, p > 0.05). Among migratory trout, we found no correlation between body length and migrated distance. Of the 12 migratory trout, nine undertook fast upward migration in periods of high water flow ( >100m~3/s). They passed the outlet tunnel from the power station and negotiated two to 35 weirs before reaching their main reproduction areas. Three trout crossed several weirs when the discharge was low (10-40m~3/s). When there was low discharge, fish remained at the outlet tunnel for up to four weeks and showed a high level of activity. Postspawning downstream migration started between 25 September and 25 October. Most migratory trout (n = 9) wintered in pools on the lower part of the river or in weir basins; only two trout descended to the lake (Selbusjoen) in late autumn.