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The Effect of Release Time On the Migratory Behaviour of Baltic Salmon (Salmo Salar): Influence of an Annual Time Program

[1. This study evaluates a field experiment testing a hypothesis that migration of Baltic salmon (Salmo salar L.) is influenced by an annual time program, affecting the migratory pattern during the sea phase. According to the hypothesis, migratory distance in the Baltic should be a result of a migratory activity sequence rather than a definite goal orientation. 2. During three years 1980-82 a total of about 18.000 individually tagged two year old hatchery-reared Baltic salmon smolts were used. At time of normal smoltrun smolts of the Angerman river stock were transferred to the sea and kept in netpens located north of the river mouth. Each year there were two groups of delayed releases. These groups of fish were released 2-6 months after transfer to the sea site. Fish released in the river and in the sea at normal time of smoltrun served as controls. 3. A seasonal difference in migratory propensity (i. e. readiness for migratory activity) was apparent in the experiment. Nettings carried out 2-3 weeks after release showed that a considerable fraction of the released salmon, after a delay of 4-6 months, lose their migratory propensity and stay during the following winter in the vicinity of the release area. This conclusion was supported by a high rate of recaptures reported by local fishermen. Delayed release postsmolts were caught throughout autumn and winter in the release area. 4. A generally shorter distance between release point and area of recapture was showed among fish that were detained before release compared to the fish released at normal time of smoltrun. The migratory distance appeared to be inversely related to period of delay before release. During all three years, salmon released with a 4-6 months delay were recaptured significantly closer to the release point than fish released in June (student's t-test, p < 0.05). 5. The results from the experiments give no reason to reject the hypothesis that the migratory behaviour of Baltic salmon is under the influence of an annual time program affecting the propensity for migration, and thereby the travelled distance. The function of an annual time program and the relevance of a precise timing of migration is discussed., 1. This study evaluates a field experiment testing a hypothesis that migration of Baltic salmon (Salmo salar L.) is influenced by an annual time program, affecting the migratory pattern during the sea phase. According to the hypothesis, migratory distance in the Baltic should be a result of a migratory activity sequence rather than a definite goal orientation. 2. During three years 1980-82 a total of about 18.000 individually tagged two year old hatchery-reared Baltic salmon smolts were used. At time of normal smoltrun smolts of the Angerman river stock were transferred to the sea and kept in netpens located north of the river mouth. Each year there were two groups of delayed releases. These groups of fish were released 2-6 months after transfer to the sea site. Fish released in the river and in the sea at normal time of smoltrun served as controls. 3. A seasonal difference in migratory propensity (i. e. readiness for migratory activity) was apparent in the experiment. Nettings carried out 2-3 weeks after release showed that a considerable fraction of the released salmon, after a delay of 4-6 months, lose their migratory propensity and stay during the following winter in the vicinity of the release area. This conclusion was supported by a high rate of recaptures reported by local fishermen. Delayed release postsmolts were caught throughout autumn and winter in the release area. 4. A generally shorter distance between release point and area of recapture was showed among fish that were detained before release compared to the fish released at normal time of smoltrun. The migratory distance appeared to be inversely related to period of delay before release. During all three years, salmon released with a 4-6 months delay were recaptured significantly closer to the release point than fish released in June (student's t-test, p < 0.05). 5. The results from the experiments give no reason to reject the hypothesis that the migratory behaviour of Baltic salmon is under the influence of an annual time program affecting the propensity for migration, and thereby the travelled distance. The function of an annual time program and the relevance of a precise timing of migration is discussed.]