Management and Ecological Note Variations in Atlantic salmon, Salmo salar L., smolt age in tributaries of the River Teno, Finland V. ENGLUND, E. NIEMELA È & M. LA È NSMAN Finnish Game and Fisheries Research Institute, River Tenojoki Fisheries Research Station, Utsjoti, Finland M. HEINO Division of Population Biology, Department of Ecology and Systematics, University of Helsinki, Helsinki, Finland The smolt age of Atlantic salmon, Salmo salar L., varies greatly over its wide geographical distribution, and is 1+ or 2+ years at southern latitudes under optimal growing conditions (Metcalfe, Huntingford, Thorpe & Adams 1990) but can sometimes be as high as 7±8+ years in the northern-most rivers (Power 1969; Kuzmin & Smirnov 1982; Jensen & Johnsen 1986). Although altitude-dependent variations in smolt age have been studied intensively, interannual differences, especially fluctuations in age-structure, have attracted only minor attention. The present paper describes interannual variations in smolt age in six tributaries of the River Teno, Finland, in the extreme north of Scandinavia (70° N, 28° E). The present analysis was based on material collected between 1989 and 1995 from the six tributaries of the River Teno, i.e. the rivers Tsarsjoki, Pulmankijoki, Kuoppilasjoki, Kevojoki, Kalddasjoki and Karigasjoki. The drainage area of the River Teno is 16 386 km 2 and more than 1000 km of the river system is accessible to salmon. It is the most productive salmon river in either Finland or Norway, and its population is based on natural reproduction. Fishing was carried out in early summer (between June and July) using traps placed near the river mouths with wing nets stretching to either bank. Despite the wing nets and traps effectively blocking the river channel, it was evident that the trapping system was incapable of capturing all of the smolts. However, it is reasonable to assume that the number and age structure of the smolts captured were representative of the population in the river. The traps were emptied several times a day, scale samples were taken for age determination and the smolts were released into the river below the trapping site. The ages were determined using scales from near the lateral line, between the adipose and dorsal fins. Six smolt age groups (from 2+ to 7+ years of age) were identified in the River Teno watercourse, although the 2- and 7-year-old specimens were not numerous. In 1989±1995, the mean smolt age (Ô SE) in each of the six rivers studied was 4.1 years (4.4 years Ô 0.18, n = 6424; 4.3 Ô 0.07, n = 5528; 4.1 Ô 0.18, n = 1698; 4.0 Ô 0.08, n = 5073; 4.1 Ô 0.24, n = 1122; 3.9 Ô 0.09, n = 6341; in the rivers Tsarsejoki, Pulmankijoki, Kalddasjoki, Karigasjoki, Kuoppilasjoki and Kevojoki, respectively), which agrees with the general geographical variation in smolt ages in Europe (Metcalfe & Thorpe 1990). The annual Fisheries Management and Ecology, 1999, 6, 83±86 Correspondence: Eero Niemela È, Finnish Game and Fisheries Research Institute, Pukinma Èenaukio 4, P.O. Box 6, FIN- 00721 Helsinki, Finland (e-mail: eero.niemela@rktl.helsinki.fi). ã 1999 Blackwell Science Ltd 83