Pacific lampreys (Lampetra tridentata) have declined in abundance in the Columbia River Basin. Although, the reasons for the decline are unclear, we suggest that development of hydroelectric dams and habitat alterations in tributaries as the main causes. The available knowledge of life history of Pacific lampreys and status from dam counts (trend data) in the Columbia River Basin and the Umpqua River along the Oregon Coast shows that populations have been declining over the last 30 years. Even though Pacific lampreys have been shown to have ecological importance both as predator and prey, the declines in their populations have been largely ignored by fisheries agencies and the public. Recently, the National Marine Fisheries Service initiated studies on using radio-telemetry of Pacific lampreys in order to study the impact of hydroelectric dams on migration behavior. To address one of the fundamental assumptions of radio-telemetry, namely, that tagged fish are "normal," one must be able to measure whether or not an animal is stressed. We identified clinical indicators of stress in adult Pacific lampreys. Plasma glucose became elevated soon after acute stress and remained elevated for one week. Plasma lactate also became elevated by 30 minutes; however, it decreased to resting levels by one hour after stessor. Muscle lactate was shown to have an inverse relationship with glucose. Muscle lactate levels decreased by 4 hours and remained depressed for two days. Plasma chloride ions decreased by one hour, then returned to resting levels by 8 hours; by 24 hours, levels were again decreased with recovery occurring by 48 hours. The steroid cortisol was not found in the plasma of Pacific lampreys. The swimming performance and physiological effects of surgical implantation of three different sized dummy radio transmitters in Pacific lampreys were assessed. Intraperitoneal implantations of 3.4 g transmitters had no significant effect on circulating levels of glucose (an indicator of stress) 4 months after surgery, while 10 gram transmitters showed a significant increase in plasma glucose. Lampreys implanted with 7.4 g transmitters recovered from surgery by day 4 based on levels of plasma glucose. Lampreys implanted intraperitoneally with 7.4 g dummy transmitters showed no significant differences in circulating glucose 30, 60, 90, and 180 days after surgery in comparison to sham-implant controls. Ventilation rate decreased significantly by 30 minutes after surgery and was stable by 60 minutes; suggesting initial recovery from surgery is rapid. Swimming performance was impaired immediately after surgery; however, swimming was not compromised at 1 and 7 days after surgery. Tagged fish showed a significant difference in oxygen consumption when tested immediately after surgery; however, oxygen consumption was at control levels at 1 and 7 days after surgery. / Graduation date: 2001