An optimised experimental test procedure for measuring chemical effects on reproduction in the fathead minnow, Pimephales promelas

The production of viable offspring is fundamental to the survival of any population. Tests that quantify effects on reproduction can, therefore, inform on the potential for long-term health effects of exposure to endocrine active chemicals. Surprisingly little is known, however, about the reproductive capacity of laboratory fish species used for chemical testing. As an example, the fathead minnow, Pimephales promelas, is widely used in chronic assessments of reproductive toxicology, and is readily induced to reproduce in captivity, yet there is little agreement on the reproductive capacity (egg number) of this species. For this species, the notable variation in reported estimates of egg number might relate to differences in the methods of egg collection adopted by many laboratories. To investigate this hypothesis, reproduction was assessed in a total of 200 pair-breeding fathead minnow, using egg collection methods that included the addition of trays placed beneath an inverted U-shaped PVC tile that is conventionally used alone for egg collection. The results demonstrated that the placement of a mesh-screened egg collection tray, beneath the spawning tile, increased estimates of the egg number by 25–67%. In addition, adopting the mesh-screened tray reduced variation in egg number between pairs, within an experiment, from >50% to <30% and variation between experiments was reduced from 53% to 7%. Adoption of the revised system for egg collection shows that egg number in the fathead minnow is considerably more consistent than frequently reported and is a highly robust endpoint against which chemical effects can be challenged effectively.