The sea lamprey (Petromyzon marinus) gained entrance into Lake Superior in the early 1940's, and began making drastic inroads on the fish stocks by the early 1950's. Serious efforts to control the parasite began in 1953 with the installation of electrical barriers in streams to block spawning runs. Control measures became much more effective after 1958, when a selective toxicant, the lampricide 3-trifluoromethyl-4-nitrophenol (TFM), was used to destroy larval lampreys in streams.
A unique methodology was developed for stream treatments which included surveys to find sea lamprey larvae, bioassays to determine effective lampricide concentrations, analytical techniques to monitor concentrations of lampricide throughout the treatment, and feeder systems to apply the toxicant in controlled amounts. Evidence of successful control was indicated first by reduced sea lamprey spawning runs, as measured by the numbers of adults taken at electrical barriers. The runs declined in 1962 by about 86%; periodic re-treatments of lamprey-infested streams held the population at a low level in 1963-70. Other indicators of success were decreases in the incidence of sea lamprey wounds on lake trout (Salvelinus namaycush), in the numbers of sea lamprey larvae in streams, and in the number of streams regularly used by sea lampreys for spawning.
Although sea lamprey control and heavy plantings of hatchery-reared stock had restored lake trout abundance to prelamprey levels in many areas by 1970, the trout had not yet become self-sustaining. Additional effort will be required to further reduce the effects of lamprey predation.