Use of Bluegill Toxicity Indexes in Illinois

Toxicity indexes that relate concentrations of up to 20 toxicants to their acute lethal effects on bluegill, Lepomis macrochirus (Rafinesque), have been compiled during three water research projects in Illinois. Indexes were compiled for individual stream stations by expressing each toxicant concentration as a fraction of its 96-h median lethal concentration (LC₅₀) to bluegill (adjusted when possible for varying conditions of pH, temperature, dissolved oxygen, and hardness). The resulting component toxicities in bluegill toxic units (BGTUs) were added to yield a toxicity index for each station. Computer programs were developed to perform these calculations using data generated routinely by established water quality monitoring programs or by short-term spot surveys.

The objectives of the projects for which toxicity indexes were compiled included: (1) assessment of Illinois River water quality for fish life, (2) identification of the relationships between fish populations and ammonia and chlorine in sewage discharges, and (3) development of a continuous stress function to evaluate the responses of aquatic communities to environmental and pollutional factors. The projects have resulted in useful comparisons of stream toxicity values to fish numbers and diversity. Although the question of whether the acute effects of combinations of toxicants are additive may never be resolved, toxicity indexes have provided valuable comparisons of water quality at different stations, and at the same station through time, and have identified potentially dangerous toxicant problems.