Identification of periods of high and low cod production, and the reasons for these periods, can increase understanding of variability in populations and ecosystems. In this study we investigate the multi-decadal and multi-century scale variations in the cod population in the eastern Baltic Sea (ICES Subdivisions 25-32). Analytically derived estimates of biomass are available since 1966. These estimates show that biomass increased in the late 1970s-early 1980s, but decreased nearly 10-fold until the early 1990s and is still well below the long-term average. Prior to 1966 the biomass of cod is unknown, as is the relative role of fishing, climate variability/regimes, eutrophication and reduction of marine mammal predator populations. We have begun to investigate whether historical fisheries information (landings, effort, distribution) from before the 1880s is available in Baltic archives and museums, and to what extent this information can be used to interpret variations in this population. We have located fisheries data for different parts of the Baltic for different time periods since the 1550s and have interpreted the findings using current process knowledge of oceanographic mechanisms affecting cod reproduction and ecology in the Baltic Sea. The recovered data show that the Baltic ecosystem was able to support modest-large cod populations even though it was oligotrophic and contained large populations of cod predators (e.g., marine mammals). Current ecosystem management policy in the Baltic as developed and implemented by organisations such as the International Council for the Exploration of the Sea (ICES), the Baltic Marine Environment Protection Commission (HELCOM), the nine coastal countries and the European Union includes recovery of the cod population, a reduction in nutrient loading and measures to promote recovery of seal and harbour porpoise populations. If these policies are successful, the role of predatory fish in the future Baltic could again be substantial and comparable to that which we show existed 450 years ago. However, such a scenario will also require a major reduction in cod fishing mortality and suitable hydrographic conditions which promote successful cod reproduction. Historical ecology investigations in the Baltic can contribute to scientifically based fishery and ecosystem management and recovery plans. (c) 2007 Elsevier B.V. All rights reserved.