Juvenile Sucker Cohort Tracking Data Summary and Assessment of Monitoring Program, 2015

Populations of federally endangered Lost River (Deltistes luxatus) and shortnose suckers (Chasmistes brevirostris) in Upper Klamath Lake, Oregon, are experiencing long-term declines in abundance. Upper Klamath Lake populations are decreasing because adult mortality, which is relatively low, is not being balanced by recruitment of young adult suckers into known adult spawning aggregations. Previous sampling for juvenile suckers indicated that most juvenile sucker mortality in Upper Klamath Lake likely occurs within the first year of life. The importance of juvenile sucker mortality to the dynamics of Clear Lake Reservoir populations is less clear, and factors other than juvenile mortality (such as access to spawning habitat) play a substantial role. For example, production of age-0 juvenile suckers, as determined by fin ray annuli and fin development, has not been detected since 2013 in Clear Lake Reservoir, whereas it is detected annually in Upper Klamath Lake.

We initiated a long-term juvenile sucker monitoring program in 2015 designed to track cohorts through seasons and among years in both Upper Klamath Lake and Clear Lake Reservoir. Specifically, our goals are to track annual variability in age-0 sucker production, juvenile sucker survival, growth, and condition. In this first year of the monitoring program, we assessed assumptions that sampled fish were representative of populations of suckers in each lake. The size, age, and species composition of suckers were similar between randomly determined sites and fixed sites in each lake. We captured a wide size and age range of suckers using similar gear, indicating our gear did not exclude older and larger fish. We identified improvements that could be made in the monitoring program including increasing the number of randomly determined sample sites in both lakes, evaluation of gear-size selectivity, and validation of aging methods for juvenile Lost River and shortnose suckers.