USE OF CODED WIRE TAGS TO IDENTIFY FATHEAD MINNOW (PIMEPHALES-PROMELAS RAFINESQUE) ADULTS IN AN OUTDOOR MICROCOSM STUDY DESIGNED TO EVALUATE CONSISTENCY IN REPRODUCTION

Fathead minnows (Pimephales promelas) were evaluated for use as a test species in ecosystem-level studies investigating pesticide effects on fish reproduction. Twenty adult fathead minnows (8 females:12 males) were stocked in four, 17-m(3) outdoor microcosms and collected 98 d la ter. Each fish was identified with a coded wire tag (CWT). In a prelim inary indoor experiment, survival was 100% and tag retention was 93%, 33 d after tagging. At the end of the field study, 100% of surviving s tocked adults had tags in three of the four replicates. In these micro cosms, stocked adults could be differentiated from young-of-the-year ( YOY) because there was no size overlap of YOY with the larger stocked adults, whereas in the fourth replicate the YOY grew to maturity. Fath ead minnow reproduction was compared to a study conducted the precedin g season utilizing the same experimental systems and incorporating the same 8F:12M sex ratio at stocking. Reproduction was consistent for id entical sex ratios in the different seasons despite differences in wat er conditions. In both years, fathead minnow fecundity was similar, an d fish size distributions had equivalent attributes. Furthermore, the relationship between fish number and biomass was comparable, and regre ssions of weight on length were equal.