Lethal levels of selected water quality variables to larval and juvenile Lost River and shortnose suckers

Resource managers hypothesize that occasional fish kills during summer-earl y fall in Upper Klamath Lake, Oregon, may be linked to unfavorable water qu ality conditions created by massive algal blooms. In a preliminary effort t o address this concern, short-term (96-h-long) laboratory tests were conduc ted with larval and juvenile Lost River (Deltistes luxatus) and shortnose ( Chasmistes brevirostris) suckers to determine the upper median lethal conce ntrations (LC(50)s; also referred to as median tolerance limits) for pH, un -ionized ammonia, and water temperature, and the lower LC(50)s for dissolve d oxygen. The mean LC(50)s varied among species and life stages as follows: for PH, 10.30-10.39; for un-ionized ammonia, 0.48-1.06 mg litre(-1); for t emperature, 30.35-31.82 degrees C; and for dissolved oxygen, 1.34-2.10 mg l itre(-1). Comparisons of 95% confidence limits indicated that, on average, the 96-h LC(50)s were not significantly different from those computed for s horter exposure times (i.e., 24 h, 48 h, and 72 h). According to two-way an alysis of variance, LC(50)s for the four water quality variables did not va ry significantly (p > 0.05) between fish species. However, LC(50)s for PH ( exposure times of 24 h and 48 h) and dissolved oxygen (exposure times of 48 h, 72 h, and 96 h) differed significantly (p less than or equal to 0.05) b etween life stages, whereas LC(50)s for un-ionized ammonia and water temper ature did not exhibit significant differences. In general, larvae were more sensitive than juveniles to high PH and low dissolved oxygen concentration s. When compared to ambient water quality conditions in Upper Klamath Lake, our results strongly suggest that near-anoxic conditions associated with t he senescence phase of algal blooms are most likely to cause high mortaliti es of larval and juvenile suckers. Published by Elsevier Science Ltd.