During the period 1934–1942, 32 lakes in Michigan were treated with rotenone to remove or reduce unwanted fish populations, and an attempt to recover the entire fish population was made on 18 of them. The majority of these lakes were not supporting a desirable or normally growing fish population at the time they were treated. They were mostly small, all less than 22 acres, and included lakes whose waters ranged from very soft to very hard, from acid to alkaline, and from shallow to very deep.