Establishment of the exotic species Fucus evanescens C. Ag. (Phaeophyceae) in Öresund, Southern Sweden

• The spread and establishment of the exotic species Fucus evanescens in öresund, Southern Sweden was documented through inventories in 1966–72 and 2000. The species spread fast from the first known introduction in 1955, colonising harbours along the coast. After this expansion the further spread has been limited and the species is still largely confined to harbour areas, where it occurs in the same depth zone as its native relatives, F. vesiculosus, F. serratus and Ascophyllum nodosum. It is more common in the northern part of the area and has not expanded into the Baltic Sea proper. Tests of the attachment strength of F. evanescens plants suggest that the restriction to harbours is not due to the direct effects of wave exposure on adults. In the laboratory, reproductive success of F. evanescens decreased from 99% in 24 psu to 12% in 10 psu and at lower salinity reproduction failed. Growth of embryonic recruits was similarly affected by salinity. Hence, low salinity explains the limited success of F. evanescens in southern Öresund, where the salinity is low and fluctuating, and its failure to colonise the Baltic Sea proper.

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