Prior to 1981, the Kesterson Reservoir supported a wide variety of life, including several species of fish. After 1981, the reservoir only supported the most saline tolerant mosquito fish. In 1982, the United States Fish and Wildlife Service began a study to determine the cause for declining wildlife use at the reservoir. Selenium concentrations at these locations were found to be greater than 1400 micrograms per liter. Mosquito fish samples from 1982 showed that fish from the Kesterson Reservoir contained selenium concentrations more than 70 times higher than fish sampled from a nearby reference site.