Movement and Survival of Juvenile Lost River Suckers in a Remotely Monitored Mesocosm

Rapidly declining catch rates of endangered juvenile Lost River suckers between late July and mid-September each year indicate poor survival in Upper Klamath Lake, Oregon. Poor water quality, inadequate habitat, non-native species, predation, algal toxins, disease, and parasites have all been suggested as causes of mortality. Studies directed at these hypotheses have not identified a primary cause of mortality. To study summer survival and movement, we introduced 395 hatchery-raised PIT-tagged suckers into a mesocosm in Upper Klamath Lake. Daily survival and diurnal movements were monitored using vertically stratified remote antennas. Water quality was monitored using vertically stratified sondes. Moribund suckers were sampled weekly for histopathology.  Survival was lowest from late July to mid-August, when water quality conditions were relatively benign. Vertical movements into the upper water column were associated with darkness rather than extreme water quality conditions. Most (10 of 12) moribund suckers had gill infestations of the protozoan parasite Icthyobodo, but it is not clear if this parasite originated in the hatchery or was acquired in the mesocosm. Integration of passive PIT tag detection within mesocosms can be a robust tool for understanding movement and survival of rare or cryptic life stages or species.