Return migration of Atlantic salmon in the River Tana: the role of environmental factors

Multi-sea-winter Atlantic salmon (75-115 cm fork length L-F, 2-4-winter fis h) were radio-tagged in the Tanafjord (70 degrees N), Norway, in 1992-1993, and 130 fish entered the large subarctic River Tana (Teno). They entered t he fresh water at any time of the tidal cycle but more so during the high a nd ebbing tides. No diel rhythm was detected in river entry under polar day conditions. There were no differences in the change of flow between days w hen salmon moved and when they did not, but during active migration increas ing discharge was associated with increased swimming activity of salmon, es pecially later in the summer. Increasing air temperature was also associate d with enhanced migration activity. Low river flow was associated with incr easing delay in salmon passing the first riffle area of the river, 35 km fr om the sea. (C) 1999 The Fisheries Society of the British Isles.