Downstream movement of recently transformed sea lampreys, Petromyzon marinus, in Carp Lake River, Michigan

In order to obtain more precise information concerning the downstream movement of recently transformed sea lampreys, a trapping device was operated in the Carp Lake River, Emmet County, Michigan, from October, 1948, to July, 1951.

The period of downstream migration typically extends from the latter part of October to the middle of April. It varies, however, from year to year with climatic conditions. A few migrants usually appear every day during this period but a sudden increase in water level is characteristically accompanied by a surge in downstream movement of newly transformed sea lampreys. This phenomenon is realized principally during the spring floods and often concurrently with autumn rains and mid-winter thaws.

A total of 7,969 downstream migrants were taken in the 1948–49 season; 16,235 in 1949–50, and 15,103 in 1950–51. Measurements of representative samples from the total catch of sea lamprey migrants of the 1948–49 and 1949–50 seasons revealed a range in length of 95 to 190 millimeters and an average length of 145 millimeters (3.7 to 7.5 inches; mean –5.7 inches).