First-ever count of Ivalojoki trout population underway

Researchers are conducting the first systematic survey of trout numbers in Finland’s Ivalojoki river, using sonar and underwater cameras to determine how many fish migrate upstream from Lake Inari, state broadcaster Yle reports.

The population could range from 500 to 5,000 trout, according to Panu Orell, a researcher at the Natural Resources Institute Finland (Luke). The exact figure remains unknown until the survey results are processed.

“No one has ever done this kind of count on the Ivalojoki to find out how many trout actually ascend from Lake Inari into such a large river system,” Orell said.

The survey, which runs until mid-September, will also track the timing of the trout migration and their size distribution. Data collection relies on a single sonar device and four underwater cameras positioned along the lower river. The sonar measures fish length but not species, while the cameras confirm which fish pass through—allowing researchers to distinguish trout from other species.

This method has been used for over 15 years to count salmon in the Tornionjoki and Teno rivers. In the Ivalojoki, researchers selected a site where the river deepens gradually toward the center, with a sandy bank on the opposite side to minimize shadows. While the river’s brown water poses minor challenges for video clarity, the sonar functions effectively in turbid conditions.

The findings will inform fishing regulations and help assess the success of trout stocking programs in Lake Inari. By knowing how many trout enter the river and their sizes, authorities can determine whether the migrating population is robust or insufficient.

“This gives us the first data point,” Orell noted, “but to get a reliable overall picture, the count must be repeated over several years.”

Preliminary results are expected in early winter 2026.