Migratory behaviour and holding habitats of adult river lampreys (Lampetra fluviatilis) in two Finnish rivers

The effect of environmental factors on migratory activity of adult river lampreys entering the Kalajoki and Perhonjoki, rivers in Finland, for spawning was studied using correlation and regression analyses. Telemetric tracking of 60 individuals was utilized to study the migratory patterns and holding habitat requirements of adult river lampreys. The increases in the river discharge, wind forcing towards the river mouth and speed and magnitude of river water cooling had positive effects on the numbers of lampreys entering the rivers whereas the increase in the illumination intensity of the moon had a negative effect on their migration activity. Radio-tagged lampreys typically passed slow-flowing river sections as well as steep riffles during one night, and were holding in runs, glides and the lowermost section of riffles. Substratum dominated by large boulders (> 256 mm) was preferred during winter holding behaviour. The migratory activity of lampreys released in low (< 2 °C) and high (> 16 °C) river water temperatures was low. Lampreys tended to halt next to illuminated bridges, and we suggest that this behaviour markedly shortened migration distances. The passage efficiency through a natural-like fish ramp in the low-head barrier was 100%. The results can be utilized in mitigation actions like river restoration, transplanting of adults, and improving passage and water flow regulation.