Relative Importance of Water Temperature, Water Level, and Lunar Cycle to Migratory Activity in Spawning-Phase Sea Lampreys in Lake Ontario

We analyzed historical trapping records from six Lake Ontario tributaries to (1) compare the relative importance of water temperature, water level, and lunar cycle to migratory activity in upstream-migrating sea lampreys Petromyzon marinus and (2) determine whether the relative importance of these variables differs among streams. We found significant stream-dependent differences in the relative importance of the environmental variables. Water temperature was the best predictor of migratory activity in all six streams. The seasonal distribution of migratory activity was related to mean stream temperature, with an estimated peak migration temperature of approximately 15 degrees C. Changes in stream temperature were equally as important. Migratory activity was stimulated when mean stream temperature increased between consecutive days and was suppressed when mean stream temperature decreased between consecutive days. Water level was a reliable predictor of migratory activity only in the two smallest streams. We hypothesize that high water levels may increase migratory activity in small streams by making them more noticeable. There was no evidence to support either a circumlunar rhythm or gravitational influence on migratory activity. However, because we did not account for cloud cover or the presence of artificial light sources, we cannot exclude the possibility that nighttime light levels influence migratory activity. Our findings provide a set of rules for coarsely projecting migratory activity in upstream-migrating sea lampreys.