Dynamics of Mya arenaria beds in two bights of the Chupa Inlet (Kandalaksha Bay, White Sea) were studied on a long-term basis. Observations were carried out at 1-to 3-year intervals from 1979 up to 1999. The studied soft-shell clam beds were characterised by a substantial instability of age structure. Since 1988, only one year-class has dominated in the beds while other generations have been scarce and recruitment was not observed. This pattern of Mya bed dynamics was related neither to interannual environmental changes nor to differential reproduction success or predation effects in the benthic assemblages. Favourable conditions for spat formation in 1988 (low abundance of other M. arenaria generations), as well as for juvenile survival during the following winter, resulted in high abundance of juveniles in both investigated locations in 1989. The mortality rate (m) in this 1988 generation varied throughout the period of investigation and was related to age. The mortality level decreased for the first 2-4 years of the life cycle, then stabilised for the next 3-4 years, and eventually increased in subsequent years. Overall m values ranged from 0 to 1.68 year -1 . The oldest specimens observed were 17 years old and had a maximum shell length of 79 mm. Significant differences in average growth rates were observed between molluscs of different locations.

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The growth of mussels, Mytilus edulis (L.), was studied in most of the northeastern part of their distribution. The growth, longevity and maximal size of molluscs from 24 wild populations and one cultured population located in the White Sea and the southeast part of the Barents Sea were compared. The 25 studied populations were combined in six clusters. The maximal longevity and the size of the mussels varied between 7 and 18 years and 25.5 and 77.7 mm, respectively. The geographical location of the population within the studied region did not affect either maximal longevity or maximal size, or the growth rate of mussels. However, these parameters were influenced by local habitat conditions, primarily connected with the position within intertidal and subtidal zones. Animals inhabiting the intertidal zone were characterized by relatively low growth performance, a short life span and a small size. The longest life span was typical for deep subtidal mussels, whereas the highest growth rate was recorded in the cultured population and in the upper subtidal habitats. Growth patterns of Mytilus edulis in the subarctic White and the Arctic southeast Barents seas are similar to those reported from other parts of the area of distribution. Therefore, growth was mostly determined by local environmental factors, including those related to vertical zonation, rather than by latitude/longitude and related temperature effects.

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Linear growth of soft-shell clams, Mya arenaria L., was studied at six locations in the White Sea situated in the middle and low tidal zones. The main aim was to analyse the growth heterogeneity both within beds (with respect to individual growth) and between beds (with respect to group growth). Mya population was characterized by a slow growth and a long lifespan. Maximum shell size was 70-80 mm, and longevity was at least 25 years. In addition, growth rates were highly variable, statistically significant differences being present both within and between the beds. No differences were found in group growth between different tidal horizons of the same site. The slowest growth of soft-shell clams was observed at sites with the greatest content of fine particles (silt and clay) in sediments. Differences between beds in Mya group growth could reflect variability of feeding conditions, which were probably determined by some local features (e.g. hydrodynamic regime). However, the variation of the individual growth rate within beds was comparable to that of the group growth rate between studied locations. To a great extent it seemed to be determined by the characteristics of the initial period of soft-shell clam growth.