Hydrobiologia 393: 253–260, 1999. E.M. Blomqvist, E. Bonsdorff & K. Essink (eds), Biological, Physical and Geochemical Features of Enclosed and Semi-enclosed Marine Systems. © 1999 Kluwer Academic Publishers. Printed in the Netherlands. 253 Littoral fish communities in the Archipelago Sea, SW Finland: a preliminary study of changes over 20 years Marjut Rajasilta 1,∗ , Jukka Mankki 1 , Kari Ranta-Aho 2 & Ilppo Vuorinen 1 1 Archipelago Research Institute, University of Turku, FIN-20014 Turku, Finland Fax +358-2-3336592 (winter) or +358-2-4656100 (summer) 2 Turku Fisheries District Office, Tuurintie 3 A 5, FIN-20100 Turku, Finland ( ∗ author for correspondence) Key words: littoral fish, Archipelago Sea, Baltic Sea, eutrophication Abstract A collapse of the littoral fish populations was found in the middle archipelago zone near the Island of Seili (northern Baltic Sea) when fish populations were studied in summer 1996 in a comparable way with earlier studies done in the early 1970s and 1980s. A similar comparison was made in the outer archipelago (the islands off the island of Utö), 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 littoral environment of the study area. Introduction The fish fauna of the littoral zone in the Archipelago Sea consists of a number of small-sized species which have no importance in commercial fisheries but which are central links in food chains of the Baltic ecosys- tem. On shallow bottoms of this sea area, the fish community consists of 15–20 species of either marine or freshwater origin. The number of species found in a locality varies according to exposure, bottom quality, vegetation etc. (Hänninen & Kurkilahti, 1989; Mankki & Vauras, 1974; Rajasilta, 1982; Sundell, 1994). In general, fish communities contribute substantially to species diversity in littoral areas. The wide-reaching ecological changes found in the Baltic Sea pelagic ecosystem since the turn of the cen- tury (discussed by, e.g., Elmgren, 1989) are expected to have consequences also in the littoral ecosystem. Elmgren (1989) has discussed the lost food chains in the ‘largest desert in Europe’, as anoxic bottom water in the Baltic proper has destroyed stocks of the Snake Blenny (Lumpenus lampetraeformis) together with all macrobenthic communities over nearly 100 000 km 2 Several studies of the littoral zone indeed demon- strate the on-going eutrophication to have resulted in disappearance of bladder-wrack (Fucus vesiculosus) (HELCOM, 1993) and a substantial increase in the biomass of filamentous algae. The loose-lying mats of detached filamentous algae cover large areas in the Archipelago Sea and form one mechanism by which shallow water anoxic bottoms may reduce the diversity of littoral fauna (Norkko, 1997). The ecosystem of the Archipelago Sea has changed considerably during the past 30 years (e.g., Bonsdorff et al., 1997; Hänninen et al., 1997; Jumppanen & Mat- tila, 1994). In the early 1970s, only the innermost parts of the archipelago were eutrophicated, but nowadays signs of increased nutrient input can be found also in the middle and outer archipelago (Hänninen et al., 1997). As the fish communities of the littoral areas have not been monitored on a long-term basis in the ar- chipelago, there is no information on the present state of the littoral fish communities. The studies made in the beginning of the 1970s (Mankki, 1975; Mankki & Vauras, 1974) and in 1980 (Rajasilta, 1982) in differ- ent parts of the Archipelago Sea (see Figure 1) give some information on individual numbers and distribu- tion of the littoral fish species in the middle and outer archipelago at that time. Some of the locations stud-