Reference Toxicants in Quality Control of Aquatic Bioassays

Use of a reference toxicant promises to improve the usefulness of toxicity tests. Criteria such as <10 percent mortality during a pretest acclimation period indicate only that the test organisms can survive the holding facility. A more reliable and logical method is to use a reference toxicant to estimate the fitness of organisms. Coupled with other standard bioassay procedures, a reference toxicant enables interlaboratory and intralaboratory comparisons of toxicity information to be made. In the context of invertebrate toxicity tests, where the available space and numbers of organisms are not so limiting as in fish bioassays, routine testing of a reference toxicant, conducted in conjunction with routine bioassays, is a realistic expectation. One problem with reference toxicant information is the question of what can be done when the test organisms are inexplicably abnormal in their resistance. A proposed solution is to use a “sensitivity factor” to adjust the LC50 values obtained with the abnormal test group. Sodium pentachlorophenate is recommended as a reference toxicant. It has a convincing history of use in toxicity testing, it is fast acting and nonselective in its toxicity to both invertebrates and fish, and it is recognized as an environmental contaminant. Other potential reference toxicants are also considered.