The article is devoted to current trend in fish ecology under long-term chronic toxic impacts. It is shown that fish exposed to sublethal doses of toxic substances for a long time have a smaller body size, the period of their sexual maturation becomes more variable, and the frequency of spawning and life span decrease. Changeability factors for population under toxic impacts are distinguished. Regularities of fish populations associated with selective effect of toxic agents and additional energy supply for detoxification are detailed in this work. The biochemical and bioenergetic mechanisms, responsible for changeability of populations are analyzed. The ecological expediency of change in the life cycle strategy corresponding to r-selection (conversion to a short reproductive cycle) is substantiated. Role of evolutionary pre-adaptation and biological mechanisms in maintenance of population homeostasis are considered. It is shown, that adaptation develops through a number of broods due to selection pressure, eliminating the least resistant individuals. Selection improves population’s resistance to toxic pollution, but at a later stage it may result in reduction of the genetic diversity, decrease of its adaptive capacities to other environmental factors along with all negative consequences, arising under it.