Redistribution of spat-sized Macoma balthica in the Wadden Sea in cold and mild winters

During the first winter of their life, large numbers of the tellinid bivalve Macoma balthica (L.) migrate from their first-summer coastal nurseries on high tidal flats in the Wadden Sea to areas further offshore, including low tidal flats and subtidal areas in the Wadden Sea and adjacent parts of the North Sea. In the western part of the Dutch Wadden Sea, we compared M balthica redistribution in cold and mild winters during the 29 years from 1973 to 2002 to find out whether migration activity was related to winter character. Migration occurred on a larger scale in cold winters than in mild winters, with larger decreases in density of coastal populations and larger increases in density of offshore populations following a cold winter than following a mild winter. In subtidal areas, such secondary recruitment, at an age of almost 1 yr, was higher than primary recruitment through post-larval settlement. In subtidal areas, M balthica year-classes were particularly strong when their first winter had been cold and when primary first-summer recruitment had been abundant in nearby coastal tidal-flat areas.