Mortality and Survival in Marine Bivalve Beds under Undisturbed Environmental Conditions (in the White Sea)

Spatial and temporal variabilities in the mortality of nine bivalve species widely distributed in the littoral and sublittoral zones of the White Sea have been studied in 19 mollusk beds in the Keret Archipelago (Kandalaksha Bay). Life tables are constructed based on long-term observations (1989–2014) of the dynamics of the size and age distribution in the beds. Using these tables, one may study the age-specific mortality and, accordingly, its influence on different stages of the life cycle. In some cases, it is possible to follow the dynamics of individual generations throughout their entire life cycle and construct cohort life tables, which is almost unprecedented for free-living unattached marine bottom invertebrates. The mortality characteristics of most of the analyzed bivalve species are obtained for the first time. Significant fluctuations in the elimination level at different ages and their main causes are identified. The average (regardless of age) annual mortality occurs to be sensitive to environmental conditions, and thus can be used for an integral assessment of mollusk life conditions. For White Sea bivalves, the mortality varies 1.5–2 times in different beds of the same species. At the same time, the mortality rate was not constant throughout the life cycle of Bivalvia. The estimate varied more than ten-fold in different age groups, with a maximum from 0.04 to 0.92 year–1. The main influence on mollusk survival was provided by the degree of protection, intraspecific relationships, and aging.