SPATIAL AND TEMPORAL VARIATIONS IN THE POPULATION-STRUCTURE OF THE SOFT-SHELL CLAM MYA-ARENARIA IN THE POMERANIAN BAY (SOUTHERN BALTIC SEA)

The population structure of Mya arenaria has been investigated in the sublittoral zone of the Pomeranian Bay (southern Baltic Sea). Box-core r samples were collected during a 1.5-year period in 1993/94 to follow changes in size and age structure of the clam populations in differen t parts of the study area. Large spatial differences in population str ucture were found between the sheltered southwest of the Bay and the s hallow and exposed Oder Bank in the centre. The stock of the Oder Bank consisted of two different clam types. A slow-growing cohort was assu med to be autochtonous on the Oder Bank, whereas a fast-growing one wa s assumed to have immigrated from the surrounding area. The contributi on of the two cohorts to the total density varied seasonally Because o f bedload transport of clams, the contribution to the reduction of the clam stock by wintering sea ducks could not be quantified. In the sou thwest of the Pomeranian Bay erosion was of minor importance. High mor tality rates during the first two years of life were assumed to be cau sed by predation. Mortality rates of older cohorts remained stable unt il old age. Variations in cohort strength were related to interannual differences in the reproductive success. A mild winter presumably lowe rs the reproductive success in the subsequent summer. Severe oxygen de pletion in summer 1994 caused a strong reduction in the cram stock at stations deeper than 10 m.