Associations between Water Quality and Daily Growth of Juvenile Shortnose and Lost River Suckers in Upper Klamath Lake, Oregon

Poor water quality from hypereutrophic Upper Klamath Lake in south central Oregon has been suspected of contributing to the recruitment failure of two endangered endemic fish species. The Lost River sucker (Deltistes luxatus)and the shortnose sucker (Chasmistes brevirostris). We used otolith daily increment widths as a proxy for juvenile somatic growth to construct two growth models: (I) a linear mixed-effects (LME) model examining the lifetime effects of lakewide averages of potentially stressful daytime water temperature, pH. and nighttime dissolved oxygen (DO). and (2) a simple linear regression model examining the effects of locally measured water temperature. pH. and daytime DO on growth or fish over 3 d before the fish 's capture. Graphical relationships between daily growth and biweekly un-ionized ammonia failed to show a sublethal effect on the growth of suckers captured in areas where un-ionized ammonia surpassed levels lethal to both species. For both species , our LME models indicated that at temperatures greater than approximately 22°C, low nighttime DO (less than 4 mg/L for Lost River suckers and less than 1 mg/L for shortnose suckers) caused enough stress to reduce growth. whereas at temperatures less than approximately 22 C, any stress from low nighttime DO was not reflected in reduced growth. We attribute the pattern to the species tolerance of low D O. the short duration of nighttime events, the fish's increased oxygen demand at higher temperatures, and growth compensation due to increased food resources associated with low DO. The combination of low DO and high temperature has also been implicated in adult fish kills in Upper Klamath Lake. Because 34% of the time lakewide August average temperatures exceeded 22°C, extended periods of warm temperatures and high primary production could affect the sizes or recruits surviving into fall. Both growth models suggested that shortnose might be more tolerant of poor water quality than Lost River suckers.