Spatial patterns of abundance and growth of Mytilus edulis on boulders in the Northern Baltic Sea proper

Small scale spatial patterns of abundance, growth and condition of the mussel Mytilus edulis on sub-littoral boulders (approx. 1–2 m high) were investigated at the island of Askö in the northern Baltic proper. The effect of side (exposed/sheltered with respect to wave action and sunlight) of boulder and position (up/down) on boulder was investigated. A large spatial variability in abundance of M. edulis between boulders and between various sites within boulders were found. The highest numbers of mussels were found on the wave exposed side, near the top of boulders. Shell growth was favoured by a sheltered side and a down position. The body condition (meat weight/shell weight) of mussels was, on the other hand, affected only by position, the condition of mussels being better at the up position. Consequently, there seems to be temporal differences in the condition for growth within a spatial position. The body condition of the mussels was best near the top of boulders in the spring, but long-term shell growth was favoured by a sheltered side and a down position. This may reflect changes in the composition and availability of food during the year with phytoplankton as the major food source during the spring bloom and resuspension of benthic production and detritus as relatively more important during the rest of the year.