A 12-year whole-lake experiment found male fathead minnows exposed to synthetic estrogen (5 ng/L) in 2001–03 spent less time tending nests and chased other fish more often. Egg counts fell 38% initially and 67% late in exposure.
Researchers added EE2 to one lake three times weekly during the open-water seasons of 2001–03 and compared minnows with fish in nearby reference lakes. The effects persisted for two years after treatment stopped; by 2011, nest attendance and aggression had returned to levels in the reference lakes.
Near the end of exposure, males also had fewer nuptial tubercles and smaller dorsal pads, which they use to clean eggs. The pads contain chemosensory cells that may help with offspring recognition. Researchers also found some evidence that exposed females produced smaller broods.
