Aquaculture and Fisheries Management 1991, 22, 357-362 Effect of smolt age on migratory behaviour of Baltic salmon, Salmo salar L., transplanted to the east Atlantic L. P. HANSEN & B. JONSSON Norwegian Institute for Nature Research, Trondheim, Norway Abstract. This paper describes tests which were made to determine if smolt age at release influences the migratory pattern of three different stocks of salmon, Salmo salar L. The fish were hatchery-reared and released in two different rivers, the River Imsa and the River Akerselv. Based on the tag returns we found that Baltic salmon from the River Neva, USSR differed in migratory pattern from two Norwegian stocks from the River Lone and the Riv^r Imsa. A large proportion of the 2+ River Neva smolts stayed in the fjord during the summer and autumn after release. On the other hand, 2+ smolts of the Norwegian stocks left the fjord and migrated to the feeding areas in the Norwegian Sea within a short time after release. The 1+ smolts of all stocks showed the same migratory pattern as the 2+ smolts of Norwegian origin. We propose that the observed differences in migratory pattern are influenced by the developmental rate of the smolts. The effect of developmental rate on the migration may differ among stocks. Our results show that it is possible to develop a fjord fishery by releases of 2+ smolts of Neva salmon. However, such releases must be carried out with the utmost caution, preferably in fjords with no salmon rivers, so that the possibility of gene flow between populations is minimized. Introduction Typically, migratory behaviour varies among stocks within species of anadromous salmonids. For instance, Svardson & Fagerstrom (1982) showed different patterns of migration among stocks of brown trout, Salmo trutta L. In salmon, inherited variation in the timing of the seasonal return migration has been revealed both in the Pacific (Rich & Homes 1929; Foerster 1946; Cope & Slater 1957; Blair 1968; Bams 1976) and the Atlantic Ocean (Hansen & Jonsson 1991a). However, few studies have focused on revealing whether developmental conditions influence migratory patterns of fish. Recently, it was demonstrated that the migratory pattern of salmon was altered by changes in the seasonal timing of the smolt migration. When retaining smolts of Baltic, Salmo .ya/arL., Chinook, Oncorhynchus tshawytscha (Walbaum), and coho, O. kisutch (Walbaum), salmon in sea cages and subsequently releasing them as post smolts in the autumn, the fish migrated shorter distances (Novotny 1980; Eriksson 1988). Atlantic salmon, Salmo salar L., originating from the southernmost part of Norway leave the rivers as smolts in May. They can be caught along the coast until July, and they appear in the high seas salmon fisheries at the Faroes in November-December, when some of them have reached a size that makes them vulnerable to long-lines. Some fish also appear in the west Greenland fishery. When mature, the adults return along the west coast of Norway towards their home rivers. Correspondence: Dr L. P. Hansen, Norwegian Institute for Nature Research, Tungasletta 2, N-7004 Trondheim, Norway. 357