Measurement of pollutant toxicity to fish I. Bioassay methods for acute toxicity

The review describes profitable methods for measuring lethal levels of pollutants for aquatic organisms. Methods for research in the laboratory are emphasized but the same principles could be applied in field work. Greater use of standard toxicological methods and terminology is urged. For 211 out of 375 toxicity tests reviewed, acute lethal action apparently ceased within 4 days, although this tabulation may have been biased towards short times by a large number of static tests. The incipient LC50 (lethal concentration for 50 per cent of individuals on long exposure) is recommended as the most useful single criterion of toxicity. If this cannot be estimated, the 4-day LC50 is a useful substitute, and often its equivalent. A desirable first step in toxicity tests is to estimate median lethal time for each of a series of concentrations. A toxicity curve should be drawn by plotting median survival times against concentrations on logarithmic paper. The curve helps to reveal any unusual features of toxicity. Whenever possible, tests should be prolonged until the toxicity curve becomes parallel to the time axis, indicating a lethal threshold concentration. The incipient LC50 is then estimated by selecting an exposure time from the asymptotic part of the toxicity curve; for this exposure time, observed mortality is plotted against concentration on log-probit paper, and the LC50 is read from an eye-fitted line. Confidence limits of the LC50 may also be estimated by simplified methods. These should be given in published work along with a value for slope of the probit line. Alternative approaches use reciprocal transformations or estimate LC50's for a series of exposure times. A graph is given for estimating partial replacement times of water in tanks of continuous-flow tests. Rate of flow should give a short partial replacement time, such as a flow equal to volume in 3–5 hr, and also adequate water for respiration of fish, usually 2 or 3 l/g of fish/day, or more.