evidence: The nature of secondary immunodeficiency has been studied on the example of fish inhabiting in the northern and southern parts of the Russian sector of the North Caspian. The study was conducted on the basis of a modified approach to the study of the immune system, consisting in the identification of a natural inhibitory factor in biological fluids. It is shown that the level of secondary immunodeficiency is an integral indicator that expresses not only the state of the immune system, but also the physiological status of the studied fish and their adaptive capabilities. In the studied areas of the North Caspian, the worst indicators of secondary immunodeficiency were found in gobies (family Gobiidae, genus Proterorhinus, Neogobius, Mesogobius) caught in the areas of Maly Zhemchuzhny Island. The formation of immunodeficiency is associated with prolonged exposure to gobies of flood waters contaminated with toxic substances. The most sedentary representatives of the aquatic fish community, which fully experience the physical and chemical effects associated with large-scale oil production in the North Caspian Sea, were specially selected as objects of research.