Vertical patterns in biomass, size structure, growth and recruitment of Mytilus edulis in an Archipelago area in the northern Baltic Sea proper

Although dwarfed by the low saline conditions, the blue mussel (Mytilus edu lis L.) is a biomass dominant in coastal areas of the Baltic Sea proper, ow ing to the lack of invertebrate predators and competitors for space. In the present study patterns of distribution of blue mussels on relatively smoot h, moderately sloping rocky surfaces in the Asko area, northern Baltic prop er, are described and interpreted. Biomass, growth rate, maximum shell leng th and recruitment of mussels decreased with water depths (from 3 to 15 m). No consistent differences in total abundance were, however, detected among depths. Shell growth, as measured by annual growth rings, was linear betwe en about 2-8 yr. of age. Within this linear interval the estimated rate of shell growth was significantly faster (3.4 mm yr.(-1)) at 5 m than at 10 m depth (2.7 mm yr.(-1)) Although the observed vertical patterns in biomass d istribution correlated both with recruitment and growth, the year round pre sence of large numbers of small mussel individuals (less than or equal to 2 mm) at all depths indicates that differences in somatic growth may explain the observed variability in biomass among depths. Potentially important pr ocesses such as intraspecific competition, food availability and food quali ty, which may generate the observed patterns in growth and biomass are disc ussed.