As a basis for salmon population management and fishing regulation, information is needed on the numbers of salmon ascending to rivers to spawn and the timing of their spawning run. The Natural Resources Institute of Finland monitors the salmon runs on River Tornionjoki and Simojoki every year from the end of May until the end of August, during which period the graphs below are kept as up to date as possible.
The monitoring sites are located at the Kattilakoski rapids on the Arctic Circle on River Tornionjoki, and on River Simojoki at the bridge where National Road 4 crosses the river. Echo sounders generate a kind of video of the underwater sites which enables the counting of fish on the echo sounder display. The echo sounder enables the observation of salmon-sized fish up to the distance of almost one hundred metres.
In summer months, the monitoring data in the graphs is updated at least once a week. Salmon are identified from other fish on a provisional basis. At the deepest point of the river bed, there is a 15-20-metre stretch of water where the echo sounder cannot see all the way to the bottom, so the fish running in the deepest waters are not included in the graphs.
In summer months, the monitoring data in the graphs is updated almost on a daily basis. Salmon are identified from other fish on a provisional basis. At Simojoki, the small salmon (‘kossi’, typically between 50 to 65 cm in length) are difficult to identify, and their annual numbers are not estimated before the autumn, when the whole echo sounder material is available for examination. To make year-to-year comparison easier, the numbers of small salmon are not included in the graphs.