Human-induced environmental changes, especially eutrophication, and fluctuations of natural environmental conditions under the changes in climate and solar activity affect the reproduction and growth of various fish species and communities in the Baltic Sea. The importance of human impacts has increased considerably during the last decades, overshadowing in many cases natural factors. High exploitation rates have depressed some valuable fish species and affected the species dominance hierarchy. Oxygen depletion in deeper layers progressively deteriorates the living conditions of certain marine species. Mass immigration of nonindigenous species with ballast waters can seriously affect ecosystems and fish stocks. The summary effect of these variables upon marine, relict and freshwater species can yield unexpected results. Fish resources should be properly assessed and managed by their natural units (populations). Based on cyclic fluctuations of global climatic processes, composition of long-term forecasts on changes in the structure and abundance of fish fauna should be started.