This chapter presents the study of fish populations, placed by nature in the unique conditions of existence, and allows one to obtain rich information on the peculiarities of structure of populations, their dynamics and adaptational capacity. The structure of roach populations was investigated in 13 lakes, of which were treated with ichthyocides. Bleak is often found in small lakes but acquires commercial importance in a small number of water bodies. White bream is rare in small lakes. The material on the structure of fish populations from small lakes discussed above was obtained in the absence of any regular fishery in these lakes. Such material is available for Lake Pelyuga in which continuous intensive fishing and observations on the structure of populations of ruff, roach, bleak, perch and pike were conducted for a period of 11 years. The data obtained on the dynamics of the age of ichthyomass culmination are presented.