Long-term changes in cohort structure of the soft-shell clam Mya arenaria in the White Sea: growth rate affects lifespan and mortality

The population dynamics of a Mya arenaria cohort in the White Sea was studied throughout almost its entire duration (ca. 25 years). We aimed to study changes in the growth heterogeneity of the cohort in terms of individual growth rate and to find out whether there was a connection between the growth rate and both lifespan and mortality rate. Significant variation of the individual growth rate within the cohort of the soft-shell clams was detected. To a great extent it seemed to be determined by the characteristics of the growth achieved in the initial growth period (shell size by the start of the second growing season). Most of the cohort individuals (about 60%) had a similar growth rate during the study period. They belonged to the group of medium-growing clams and with a few exceptions represented the cohort at later stages of development (at the age of 16–24 years). In the most slow-growing group no individuals older than 15 years were found. Clams of this group had the highest mortality rate and the shortest lifespan. In all other groups, periods of low mortality alternated with periods of a much higher mortality. The survival of Mya arenaria seems to be affected mostly by the degree of protection (largely determined by burial depth), intraspecific relationships and ageing.