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.

Differing age composition of juvenile suckers between lakes in our 2015 catches and as reported in previous studies indicate that juvenile suckers are produced in relatively larger numbers each year in Upper Klamath Lake than in Clear Lake Reservoir. Most (96.6 percent) of suckers captured in Upper Klamath Lake in 2015 were age-0, whereas age-0 or age-1 suckers were not captured in Clear Lake Reservoir. Despite ample effort, age-0 suckers have not been captured in Clear Lake Reservoir since 2013. Estimated ages of suckers captured in 2015 in Clear Lake Reservoir ranged from 2 to 6 years. Low flow during spawning seasons in the only known spawning tributary to Clear Lake Reservoir (Willow Creek) appears to explain the lack of age-0 sucker production in recent years.

Juvenile sucker mortality is relatively higher in Upper Klamath Lake than in Clear Lake Reservoir. We compared data collected in 2015 to previously published catch rates to produce an index of annual juvenile sucker survival for these species. We calculated indices of annual apparent survival of juvenile sucker ages 0–5 years old in Clear Lake Reservoir to be between 0.37 (±0.86 standard error [SE]) and 0.44 (±0.84 SE). This is the first time indices of annual apparent survival for Lost River and shortnose suckers have been calculated. This estimate has the limitation of being non-species specific because not all individuals were identified to species in previous years, and suckers that were identified included both taxa. In contrast, catch rates decreased by 89 percent for juvenile Lost River suckers and decreased 50 percent for juvenile shortnose suckers in Upper Klamath Lake between August and September 2015. Very low catch rates of age-1 and older suckers in Upper Klamath Lake indicate that annual juvenile sucker survival rates are near zero.

Condition of suckers was assessed in 2015 based on age-0 sucker growth rates in Upper Klamath Lake and the prevalence of externally observable afflictions on suckers from both lakes. Age-0 Lost River suckers grew an average (± standard deviation [SD]) of 0.72 (±0.01) millimeters [mm] standard length [SL] per day, and age-0 shortnose suckers grew an average of 0.57 (±0.04) mm SL per day in 2015. This growth rate was similar to growth rates reported for these species in Upper Klamath Lake in previous years. Opercular deformities, skin hemorrhages, black-spot causing parasites, and Lernaea spp. parasitism were the most common afflictions observed on suckers. Observed afflictions were primarily on suckers from Upper Klamath Lake, with the exception of Lernaea spp., which occurred more frequently on suckers from Clear Lake Reservoir. Opercular deformities and black-spot causing parasites were each observed on 5 percent of age-0 suckers from Upper Klamath Lake. Petechial hemorrhaging of the skin was observed on 43 percent of age-0 Lost River suckers, 38 percent of age-0 suckers of undetermined taxa, and only 24 percent of age-0 shortnose suckers from Upper Klamath Lake. Petechial hemorrhaging of the skin was only observed on a single shortnose sucker from Clear Lake Reservoir. Within Upper Klamath Lake, the prevalence of these hemorrhages was exactly twice as high as was reported in 2014.