Sälpopulationerna i Östersjön består av tre arter: gråsäl, vikare och knubbsäl. Sälarna befinner sig som rovdjur högst upp i näringskedjan och livnär sig till största delen på fisk. Detta gör att de i hög grad utsätts för de miljögifter som finns både i vattnet och i fisken. Under 1970-talet såg man en en kraftig nedgång av sälpopulationerna. Detta fanns vara delvis orsakat av reproduktionsstörningar som manifesterades som ocklusioner och stenoser av uterus med missade dräktigheter som följd.

During the 1960´s and 1970´s the number of grey seals in the Baltic Sea was decreasing rapidly, mostly due to hunting and toxic substances like DDT and PCB. When hunting became less intense and toxic substances decreased in the environment the grey seal population started to increase. Today grey seals are found common in the Baltic Sea and have started to become a big treat and a problem to the fishing industry. The grey seal destroys and enters fishing traps and consumes large quantities of the fish that have been caught.The knowledge of the grey seal, like abundance and food preferences, is today limited. It is also important to define the position of the grey seal in the ecosystem in the Baltic Sea and to be able to predict changes that could occur if the population would rapidly decrease or increase.