Cohort life tables for a population of the soft-shell clam, Mya arenaria L., in the White Sea

Abstract

The Mya arenaria generation in the White Sea was observed for almost the whole life cycle (around 25 years). Using the data on this generation dynamics, the cohort life table was built. The main purpose of the research is analysis of age-specific mortality in this softshell clam population. The mortality rate was found to change more than tenfold throughout the study period. No regular changes in its value were observed. Periods of low mortality alternated with periods of much higher mortality. Several reasons for the increase in M. arenaria mortality rate during its life cycle are proposed: (a) living in the surface sediment layer at early stages of life cycle (unstable environment, high mortality of non-viable clams, predator impact); (b) intense intraspecific relationships in dense aggregations of young clams; (c) increased intraspecific competition during periods of the rapid individual growth; (d) ageing (clam mortality increases with older ageachievement of an average and a maximum lifespan).

Growth of Mya arenaria L. at the northern edge of the range: heterogeneity of soft-shell clam growth characteristics in the White Sea

“…First, the comparison of age series using the proposed algorithm [35] was possible only in molluscs older than 5 years. According to this algorithm, each age row should comprise at least 4 measurements, but the first growth mark (corresponding to the growth delay during the first winter after settlement) was not visible in specimens older than 2 years [27]. So the youngest specimens suitable for the age row comparison were 5-year-old clams.…”

“…Observations were carried out at 1-to 3-year intervals mostly between late June and late July. Detailed description of sampling methods is provided in earlier publications [26,37,27]. At sites 1 and 2, molluscs were collected in the middle and low tidal zones and, at sites 3 and 4, only in the middle tidal zone (there were no soft-shell clams in the low tidal zone).…”

“…All study sites were located in semi-closed areas and differed slightly in respect of salinity and sediment type. The surface salinity in the Lebyazhya bight (site 1) was relatively low, no more than 14-17 ‰ in summer, because of a nearby river estuary [37,27]. The surface salinity at other sites ranged from 24 to 26 ‰ in summer [5].…”

“…Age of molluscs was determined by counting annual growth marks on shells [37,27]. An average shell length of the dominant generation (1988 or 1999) was determined at each sampling.…”

“…Long-term (since 1980) observations of the structure of soft-shell clam beds have been performed at four typical tidal beaches of the Keret Archipelago in the White Sea [26,37,27]. The most prominent trait of these Mya beds was a pronounced dominance of a few generations over a considerable period of time (up to 15 years).…”