In the coastal areas of the Baltic Sea, there are two forms of perch (Perca fluviatilis). One of the forms is migratory and spawns instreams entering the Baltic Sea. The other form is resident and spawns in brackish water. Both forms utilise the coastal habitat for foraging. We examined the spawning success of the two forms in fresh and brackish water (7 parts per thousand, equal to salinity in thesouth Baltic Sea). The experiments showed that hatching success was equally high in freshwater and in brackish water despite female origin. The survival of yellow-sac and free swimming fry was significantly reduced in brackish water, which was independent if the fish wasof migratory or brackish resident origin. Further, growth rate of perch fry was severely reduced in brackish water. The results indicate thatperch has not developed any tolerance to brackish water in the young life stages. The migratory life strategy of perch can thus be explained by higher survival of fry in freshwater.