Littoral fish communities in the Archipelago Sea, SW Finland: a preliminary study of changes over 20 years

A collapse of the littoral fish populations was found in the middle archipe lago zone near the Island of Seili (northern Baltic Sea) when fish populati ons were studied in summer 1996 in a comparable way with earlier studies do ne in the early 1970s and 1980s. A similar comparison was made in the outer archipelago (the islands off the island of Uto), but no evident change was found. Reasons for the decline in littoral fish stocks are discussed, but no single-cause effect can be pointed out. Among possible explanations is a substantial increase of eutrophication in the middle archipelago together with increasing ferry traffic which both cause structural changes in the li ttoral environment of the study area.