The wild populations of salmon in the Baltic Sea suffer from yolk-sac fry mortality (M74). M74 mainly occurs in populations spawning in rivers flowing to the Gulfs of Bothnia and Finland. On the basis of studies with fry, M74 may be caused by oxidative stresses. Because the eggs of M74-offspring-producing females have lower thiamine and astaxanthin levels and more oxidized fatty acids than eggs of females producing healthy offspring, oxidative stresses that adult salmon experience during their feeding migration may be decisive for the development of M74. In this study we have measured several oxidative stress parameters and have evaluated both temporal and regional differences in these parameters in salmon individuals during their feeding migration. At present, salmon feeding in the Gulf of Finland and in the Bothnian Sea are affected by oxidative stress as compared to populations feeding in the Baltic Proper. Moreover, the feeding population of salmon in the central Baltic Proper suffered much more from oxidative stress in 1999 than in 2006-2007. In 1999 the incidence of M74 was higher than that expected in 2007/2008. Oxidative stresses experienced by feeding salmon may thus be behind the development of M74.