Effects of ambient water quality on the endangered Lost River sucker in Upper Klamath Lake, Oregon

Populations of the Lost River sucker Deltistes luxatus have declined so pre cipitously in the Upper Klamath Basin of Oregon and California that this fi sh was recently listed for federal protection as an endangered species. Alt hough Upper Klamath Lake is a major refuge for this species, fish in the la ke occasionally experience mass mortalities during summer and early fall. T his field study was implemented to determine if fish mortalities resulted f rom degraded water quality conditions associated with seasonal blooms of ph ytoplankton, especially Aphanizomenon flos-aquae. Our results indicated tha t fish mortality did not always increase as water temperature, pH, and un-i onized ammonia concentration increased in Upper Klamath Lake. Little or no mortality occurred when these water quality variables attained their maximu m values. On the other hand, an inverse relation existed between fish morta lity and dissolved oxygen concentration. High mortality (>90%) occurred whe never dissolved oxygen concentrations decreased to 1.05 mg/L, whereas morta lity was usually low (<10%) when dissolved oxygen concentrations equaled or exceeded 1.58 mg/L. Stepwise logistic regression also indicated that the m inimum concentration of dissolved oxygen measured was the single most impor tant determinant of fish mortality.