Rapport no 168 : Förändringar i fisksamhället längs en skärgårdsgradient på NV Åland - en uppföljningsstudie

Abstract

The structure of the fish communities along the archipelago gradient and three shallow bays in NW Åland were studied in the summer of 2023 between July and September as a follow-up study of earlier studies conducted in 1997 and 2003, respectively. Ten locations along the gradient and in the shallow bays were fished at a depth of 3-5 meters and at five occasions using standardized coastal survey nets. The depth gradient (down to 10-15 meters) was studied at four of these locations, one in each archipelago zone and one in the shallow bays. Hydrography parameters were measured at each fishing occasion, and nutrient samples (nitrogen and phosphorus) were taken only at three occasions. Focus was mainly put on pikeperch (Sander lucioperca L.). The population of pikeperch has not significantly changed since the last survey. The species was most abundant in the shallow bays, and the inner- and middle archipelago. Only one individual was caught in the outer archipelago. The population was smaller in individual size than before, and their stomach content consisted mainly of fish. The results indicate that some changes in the structure of these fish communities have taken place since the last survey 20 years ago. The number of fish and the overall biomass has decreased significantly. The patterns of the dominating fresh- and cold-water species has stayed the same, but perch (Perca fluviatilis L) and cyprinids has increased in number and rate of dominance. These species have also increased in average size. Contents of phosphorus had stayed stable, but nitrogen contents have increased. This, together with an increased occurrence of cyprinids and shallower Secchi depth, indicates increased eutrophication in the inner archipelago and the shallow bays, which has been noted in most previous studies in NW Åland.