Movements of brown trout, Salmo trutta, and juvenile Atlantic salmon, Salmo salar, in a coastal stream in northern Norway

Abstract

Movements of resident brown trout (age 2+ to 9+ years) and young Atlantic salmon (age 1+), stocked as fry, were studied in July, August and September in a coastal stream in northern Norway. Between 85 and 89% of the brown trout were recaptured in the study area (346m, 1326m2) within 45m of their release point throughout the investigation period. Most specimens had moved less than 150m. Trout movements were related to local variation in density. Trout occupying those sections of stream with the lowest fish densities (5.3–10.9 fish 100m−2) had a significantly lower movement rate than fish from sections with densities between 13.7 and 31.5 fish 100m−2. Trout that moved from their marking section were significantly larger than specimens that did not leave their original site. There was a significant correlation between permanence of station each month and the mean water level that month. The majority of the trout (47%) were caught at undercut stream banks or at sites in the proximity of these. A decrease in water level during the study period resulted in a high loss (36%) of such habitat, probably forcing some individuals to move. The recapture data indicate that the trout population consists of one small (c. 15–20%) mobile, and one large sedentary component.Young salmon displayed high station permanence in July and August (93% and 96%). However, in the autumn they exhibited a significant downstream movement, and only 73% were recaptured within their release section. This movement was significantly higher for larger specimens, and is thought to occur because of a pre‐winter change in habitat, initiated by a decline in stream temperature. In contrast to trout, salmon in sections containing the lowest densities (22.0–25.0 fish 100m−2) did not show significantly lower movement rates when compared with salmon at higher densities (32.2–46.3 and 51.8–60.6 fish 100m−2). The spatial distribution of young salmon indicated the formation of territorial mosaics over the stream bed, which are thought to reduce intraspecific competition.