Early-season river entry of adult Atlantic salmon: its dependency on environmental factors

River entry of adult Atlantic salmon Salmo salar into the River Tornionjoki, monitored during three migration seasons (1997-1999) by horizontal split-beam hydroacoustics, started early in June when water temperature was c. 9degrees C and when the discharge varied between 1700 and 2000 m(3)s(-1). In 1997 and 1999, migration peaked during the latter half of June, 17 days after the peak flood, at water temperatures ranging from 11.5 to 18.2degrees C. Few statistically significant correlations were observed between river entry and six measured environmental factors and those that were significant were not persistent over the years. The strongest correlation (r = -0.60) wits between the number of upstream migrants and seawater level, with a time lag of I day in 1998. In 1998 and 1999, no clear diurnal migration pattern was observed, although in 1997 the intensity of midday migration was higher than that of the midnight migration. It is concluded that environmental factors have little effect on river entry of Atlantic salmon in a large pristine river located at high latitude. (C) 2003 The Fisheries Society of the British Isles.