Spring migration of some anadromous freshwater fish species in the northern Bothnian Sea

Quantitative estimations of spring migrating fish have been made in the mouth part of the small coastal river AngerAn which flows into the northern Bothnian Sea (63° 35'N, 19°50'E) . In 1981 nearly 3 000 fish were counted ascending to the spawning grounds in the lower reaches of the Angeran . These species, such as pike, perch, roach and ide, adapted to the oligohaline environment in the Bothnian Sea for most of the year, migrate to spawn in the coastal stream . The reason for these migrations can be interpreted to indicate that the Angeran offers more favourable water temperature conditions at spawning time compared with the Bothnian Sea, which is ice-covered up to the beginning of May . The most important result of the investigation in the AngerAn is that these fish species, in the same way as the salmonids, return to their'home-stream' every year as adults . Andreasson & Petersson (1982) listed 69 species of fish in the oligohaline Gulf of Bothnia (Table 1) where salinity varies from 2% o near the mouth of the River Tornealy to 6% o in the vicinity of the Aland Islands (Fig. 1). The fish fauna comprises freshwater and marine species, fish migrating between brackish and freshwater rivers and streams, and recently introduced non-endogenous species . Andreasson & Petersson (1982) designated only five species as anadromous migrators, whereas our studies show that 11 species migrate from the sea to spawn in the Angeran, a small river discharging into the northern Bothnian Sea (Fig . 2) . Earlier reports on these migrations have been given by Berglund (1978) and Johnson (1978, 1982) and for another small stream in the area by Berg (1982) . The present paper describes the annual spring migrations of pike, perch, roach and ide between the northern Bothnian Sea and the AngerAn, for the year 1981 . Table l . Number of fish species in the Gulf of Bothnia (after Andreasson & Petersson 1982) . Hydrobiologia 96, 161-168 (1982) . 0018-8158/82/0962-0161/$01 .60. © Dr W . Junk Publishers, The Hague . Printed in The Netherlands . Freshwater species Marine species Migrating species Non-endogenous Common Rare Common with Without Rare Anadromous Katadromous reproduction reproduction in the area in the area