The Role of Poor Water Quality and Fish Kills in the Decline of Endangered Lost River and Shortnose Suckers in Upper Klamath Lake

Lost River (Deltistes luxatus) and shortnose (Chasmistes brevirostris) suckers are federally endangered species endemic to shallow lakes of the Upper Klamath River Basin in Oregon and California. Upper Klamath Lake represents the majority of the remaining habitat of these suckers, but has been a site of intermittent fish kills. We studied fish kills and associated water quality dynamics in the lake in 1995, 1996, and 1997 to determine factors responsible for dieoffs. Over 7,000 dead suckers were collected in the three years, and 85% of annual collections occurred during a 15-20 day period that began between mid August and late September. Suckers collected during the fish kills, as well as live fish captured the following spring, had a high incidence of afflictions such as parasitic and bacterial infections, cysts, and ulcers. The 1995 and 1996 fish kills were biased toward larger species (suckers), and larger individuals within species. Water quality in the lake was largely influenced by the dynamics of the bluegreen algae Aphanizomenon flos-aquae, which comprised over 90% of algal biomass. Associated with each fish kill was an extended period of water column stability and high algal biomass (>150 μg L-1 chlorophyll a) before the kills, followed by a well-mixed water column and algal collapse with little residual algae. Before the kills, algal photosynthesis caused high pH (9-10) for 30-90 days, which maintained a large proportion of the total ammonia in the toxic, unionized form (200-2000 μg L-1 NH3). Algal collapse decreased photosynthesis and increased biological oxygen demand, leading to dissolved oxygen levels less than 4.0 mg/l throughout the water column for 10-24 hours a day, for up to several days. Fish mortality coincided with algal bloom collapse and continued for 20-30 days after the period of low dissolved oxygen. We concluded that hypoxia, caused by the collapse of A. flos-aquae blooms, was the primary mechanism that triggered the 1995-97 fish kills.