Subchronic Toxicity of Low Dissolved Oxygen Concentrations, Elevated pH, and Elevated Ammonia Concentrations to Lost River Suckers

Abstract

The Lost River sucker Deltistes luxatus is a federally listed endangered fish species that inhabits hypereutrophic Upper Klamath Lake in southern Oregon. We conducted four subchronic toxicity tests with Lost River suckers by continuously exposing them to low dissolved oxygen (DO) concentrations for 14 d (late juveniles); elevated pH for 30 d (late larvae); elevated ammonia concentrations for 30 d (late larvae); and sublethal ammonia concentrations for 14 d followed by sublethal low DO concentrations for 14 d (early juveniles). Mortality thresholds were approximately 1.4 mg/L in the DO test, more than 10.0 in the pH test (the highest pH tested), and between 0.37 and 0.69 mg NH₃-N/L in the ammonia test. Contrary to the common expectation for fish exposed to toxicants, Lost River suckers generally did not display sublethal responses (i.e., reduced growth, altered whole-body ion content, and reduced swimming performance) to low DO concentrations, elevated pH, or elevated ammonia concentrations. In the 14-d exposure to low DO concentrations and the 30-d exposure to elevated ammonia concentrations, the traditional sublethal endpoints were no more sensitive than survival (P > 0.05); only in the 30-d exposure to elevated pH was a sublethal endpoint (whole-body Na content) more sensitive than survival (P < 0.05). In the test involving a sublethal exposure to ammonia followed by a sublethal exposure to low DO, survival did not decrease significantly and no sublethal effects were observed (P > 0.05). Therefore, it appears that, within the resolution of the traditional sublethal endpoints we used, a Lost River sucker essentially had to be dying before an adverse functional effect of the toxicant could be identified.