The effects of endocrine disruptors on the reproductive biology of resident fish in the Sacramento-San Joaquin Delta have been studied at BML. Endocrine disruption in the inland silverside (Menidia spp.) has been the focus for a number or years since populations of native fish are so low and several are listed as endangered, Menidia is an ideal indicator species as they have a limited home range, are found across broad salinity gradients, and are reproductive for about 4-6 months. With support from both graduate and postdoctoral Delta Science Fellowships, investigations of endocrine disruption in different regions of the Delta using plasma egg chorion (egg coat) protein presence in males and immature fish as an indicator has been extremely useful. The focus has been on the common pyrethroid pesticides in the laboratory but a more multidisciplinary approach has been taken in the field where numerous stressors may contribute to endocrine disruption. The most recent publication in PLoS ONE shows endocrine disrupting chemicals have very different impacts at urban and ranch wastewater runoff sites. Changes in sex ratios and other population parameters such as growth and time to reproduction are affected by environmental estrogens and androgens.