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-corer 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 different parts of the study area. Large spatial differences in population structure were found between the sheltered southwest of the Bay and the shallow and exposed Oder Bank in the centre. The stock of the Oder Bank consisted of two different clam types. A slow-growing cohort was assumed to be autochtonous on the Oder Bank, whereas a fast-growing one was assumed to have immigrated from the surrounding area. The contribution of the two cohorts to the total density varied seasonally. Because of bedload transport of clams, the contribution to the reduction of the clam stock by wintering sea ducks could not be quantified. In the southwest of the Pomeranian Bay erosion was of minor importance. High mortality rates during the first two years of life were assumed to be caused by predation. Mortality rates of older cohorts remained stable until old age. Variations in cohort strength were related to interannual differences in the reproductive success. A mild winter presumably lowers the reproductive success in the subsequent summer. Severe oxygen depletion in summer 1994 caused a strong reduction in the clam stock at stations deeper than 10 m.