Effects of Azinphos-methyl on the Reproductive Success of the Bluegill Sunfish, Lepomis macrochirus, in Littoral Enclosures

Adult bluegills were exposed to a single application of azinphos-methyl in 12 littoral enclosures in a northern Minnesota pond. Responses measured were adult behavior and spawning, embryo hatchability, larval survival until swim-up, young-of-year (Y-O-Y) growth, and total biomass. Four enclosures each were treated at 1.0 and 4.0 μg/liter and four remained untreated. The half-life of azinphos-methyl was 2.3 and 2.4 days at each of the two treatment levels, respectively. Quantifiable residues remained in the water for 8 days. Concentrations of 4.0 or 1.0 μg/liter did not cause any significant long-term (63 day) effects on bluegill reproduction, embryo hatchability, larval survival, growth, or biomass. Although important bluegill prey such as copepod nauplii and cladocerans were significantly or greatly reduced by Day 7 following treatment, they recovered to levels equal to or greater than some of the control enclosures by Day 35. The apparent lack of significant long-term effects on reproductive success can be partially explained by the relatively short half-life of azinphos-methyl in littoral enclosures.