Extinction of a Population of Anadromous Parasitic Lamprey, Lampetra tridentata, Upstream of an Impassable Dam

Abstract

The construction of dams on the outlet of Elsie Lake, in British Columbia, prevented young adult Pacific lamprey (Lampetra tridentata) from going to sea and adult lamprey from spawning above the dams. This resulted in a high incidence of lamprey attacks on resident trout that was initially interpreted as indicating that the lamprey population was landlocked. This study showed that a landlocked population of lamprey was not established. The attacks on fish occurred for only a brief period each year. Attacks on trout stopped approximately 7 yr after dam construction. Because this was the average estimated age when ammocoetes metamorphosed and scarred fish or ammocoetes were no longer found, we concluded that metamorphosed lamprey were unable to survive to maturity. The inability to establish a landlocked population indicated that the transition from an anadromous parasitic life history type to a freshwater parasitic life history type did not occur as quickly or as easily as previously suggested. The study also provided evidence that the statolith method of age determination may provide accurate ages.