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
( 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.peerReviewe
