Isojoen taimenten radiotelemetriaseuranta 2019–2021: Taimenten vaelluskäyttäytyminen, levittäytyminen ja kutualueiden sijainti

River Isojoki sea trout stock has suffered from a number of human activities, such as migration barriers, deteriorating water quality and high fishing pressure. Measures have been undertaken in recent years to reduce the impact of
these problems. The opening of migration connections and the removal of migration barriers has become a central part of these measures. During 2019–2021, an extensive radio telemetry survey was carried out in River Isojoki to estimate the impact of decommissioning the dam in Villamo and the opening of the migration connections. The survey was part of the Freshhabit Life project. The main objective of the telemetry survey was to investigate the migration behavior and distribution of adult sea trout, as well as investigating the location of spawning areas in the River Isojoki watercourse. A total of 108 trout were marked during the survey. The results of the radio telemetry survey show that trout can now migrate to areas above the removed dam in Villamo and thus have open connections to the upper reaches of the river system. Even if the connection has been opened, there are still other migration barriers that delay the migration. The dam at Peru's rapids is the main barrier, even if the dam has two fishways. The dam area forms a migration barrier under low flow conditions, which usually occur in the middle of the migration period. A number of migration patterns were distinguished in the migration behavior of tagged trout. Some trout migrated rapidly to spawning grounds, while others spent months in other parts of the watercourse prior to migrating to spawning habitats in time for spawning. Increased flow seemed to activate the migration. Trout that spawned in tributaries tended to only spend a short period of time at the spawning habitat and migrate back to the main stem post spawning. Trout that survived the spawning migrated downstream to the sea either in autumn (1/3) or after overwintering in the following spring (2/3). The most important spawning areas for sea trout stock were located in the rapids of the main channel in the Vanhakylä area, as well as in Karijoki and Heikkilänjoki tributaries. Metsäjoki, a tributary of Karijoki, turned out to be an important spawning area where several radio-tagged trout spawned during both survey years. The survey results indicate that sea trout migrate and spawn in a more extensive area than previously appreciated. Based on the results of this study we conclude that the measures taken to improve the condition of the sea trout stocks seem to have had a positive impact. Trout have now migration connections to extensive spawning and rearing habitats in the watercourse. Other monitoring results indicate that the trout stocks have reacted positively to the changes and the stock has gradually started to recover. However, there is still a need to improve the quality of key life stage habitats for trout, as well as implementing fishing regulations both in river and at sea, to ensure that this positive development continues in the future.