Lethal Effects of Elevated pH and Ammonia on Early Life Stages of Walleye

Abstract

Elevated pH may be lethal to larval fish and can affect the toxicity of ammonia. Fry of walleye Stizostedion vitreum propagated in a controlled hatchery environment are often stocked, despite little knowledge of their tolerance to high pH or ammonia. A series of 6-h static bioassays was performed with walleye fry of various ages to determine their tolerance to elevated pH and the effect of ammonia on pH tolerance, The 6-h mortality threshold pH (the lowest pH at which significant mortality occurs) was between 10.0 and 10.3 for 3-d-old walleyes, and between 9.8 and 10.0 for both 8- and 12-d-old walleyes, when no measurable ammonia was present. Sublethal concentrations of un-ionized ammonia increased the toxicity of elevated pH, indicating an interaction between ammonia and pH. Values of pH that caused statistically significant mortality were well below values measured in some fish culture ponds and nutrient-rich impoundments. Elevated pH at the time of stocking walleye fry may result in extensive mortality.