Changes in distribution and decrease in numbers during migration of the bivalve Macoma balthica

The population development of the 1998 year class of the bivalve Macoma balthica was studied by repeated sampling of a tidal flat area in the eastern Dutch Wadden Sea from May 1998 to August 2000. The juveniles migrated twice, once in mid-1998 from their primary settlement locations in the low sandy intertidal to the nursery in the high intertidal (spring migration, 25% of all juvenile M. balthica relocated from low to high intertidal) and once in late 1998/early 1999 from the nursery to the low intertidal and the subtidal (winter migration, between 8 and 15% of all 0-group (less than 1 yr old) M. balthica relocated from high to low intertidal). During winter, M. balthica migration was most intensive at the lowest temperatures. Relative to the abundance on the tidal flats, the 1+ group (older than 1 yr) M. balthica (i.e. from the second summer onwards) were rare in the tidal channels of the Wadden Sea and were only slightly more common in the North Sea adjacent to the tidal inlet. During both the spring and winter migration, many bivalves disappeared from the tidal flat population. This could partly be explained by normal mortality and by emigration to the subtidal channels and the North Sea. The remaining mortality was probably due to the risks inherent in migration: predation during pelagic floating or not reaching the right locations. The number of bivalves that disappeared was very high. In spring, slightly more disappeared than relocated. In winter, > 4 times more bivalves disappeared than relocated. Migration is obviously a very dangerous activity, therefore great advantages must be associated with the nursery use that makes such migrations necessary. An analysis of literature data on the density of M. balthica shows that in the eastern Dutch Wadden Sea, 90% of the population lives in the intertidal, about 10% in the adjacent North Sea, and a negligible fraction in the subtidal channels of the Wadden Sea.