Migration of the bivalve Macoma balthica on a highly dynamic tidal flat in the Westerschelde estuary, The Netherlands

Population dynamics of the tellinid clam Macoma balthica (L.) were studied at a highly dynamic intertidal sandflat in the Westerschelde estuary, south-western Netherlands. In order to study temporal development of density and population structure (12 size classes from 0.3 to 20 mm), 3 point sampling stations were sampled fortnightly from March 1992 to March 1998. Within the same period, spatial population dynamics was studied seasonally on a spatial grid (700 x 800 m, 43 plots, distance between the plots 100 m), which covered a range in bedlevel height from-50 to +140 cm relative to mean tide level. Quantitative estimations of early recruitment, growth, and survival plus migration were calculated from the temporal and spatial population data. Early recruitment was highest at the higher tidal levels, where the sediment contained the smallest sand grains. In that same area, the highest disappearance of the juvenile M, balthica, caused by emigration and/or mortality, was observed. In the lower intertidal area, where the sediment contained larger sand grains, the number of recruits in the successive classes increased after Size Class 2 to 3 mm. Based on further analysis of the population data, this increase in the number of recruits is concluded to have been mainly caused by immigration of M. balthica to the lower tidal levels. Since there seemed to be no important immigration into the entire spatial grid population, the migration may have been directed from the higher intertidal levels towards the lower. The strong initial early-recruitment pattern, supposed to be comprised of passive processes, became weaker for the successive size classes after Size Class 2 to 3 mm, which resulted in a spatial distribution of Size Class 7 to 8 mm that was neither related to bedlevel height nor to sand grain size. Therefore, active migration processes are suggested to have been of higher importance than passive migration processes.