Sporadic mortality in chronic toxicity tests using Pimephales promelas (Rafinesque): Cases of characterization and control

In whole effluent toxicity tests, organisms are exposed to various effluent concentrations for a specific time period to estimate the potential effect s of an effluent on a receiving stream. Laboratories typically have good su ccess performing valid chronic toxicity tests. However. some difficulty in conducting valid chronic whole effluent toxicity tests with Pimephales prom elos (fathead minnow) has been encountered as a result of sporadic control mortality. Some investigators report an interference that causes anomalous patterns of survival in chronic fathead minnow tests. This interference has been termed sporadic mortality phenomenon. Characteristics of sporadic mor tality phenomenon include high variability among replicates, nonmonotonic d ose responses, mortality beginning on or about day 4 of the test, and funga l growths often observed on the larvae. Histopathologic examinations often indicate bacterial and/or fungal infections on fish exhibiting symptoms of sporadic mortality phenomenon. The most plausible explanation of sporadic m ortality phenomenon is a naturally occurring pathogen or pathogens that int erfere with the test method. This interference may invalidate tests or fals ely indicate toxicity. Sporadic mortality phenomenon can be reduced or elim inated by sample treatments intended to inactivate (heating, antibiotics, o r ultraviolet light) or remove (filtration) pathogenic microorganisms. Thes e methods must be used with forethought because of their potential to alter the toxicity of a sample.