This thesis is about the movements of the coastal marine bivalve Macoma balthica. M. balthica migrates over several kilometres between nurseries at high tidal flats, where juveniles are found in high numbers, and the adult habitat on low-lying tidal flats. Most benthic species in the Wadden Sea have been caught in the water column; also, Macoma balthica has been frequently caught. Although M. balthica normally lives buried in the sediment, it can migrate over large distances (kilometres) via a long byssus thread, which is secreted into the water column. This thin mucous thread increases drag force on the animal and allows it to be transported over large distances by the current. In the Wadden Sea, juvenile M. balthica is normally mainly found in the high intertidal (the nursery). Juveniles settle in the low intertidal in May at a size of 300 µm.Subsequently, they migrate to the high intertidal in June, where they stay until winter. In winter, juvenile M. balthica (5 mm) migrate back to the low intertidal and to the North Sea. Adults are much more widespread in distribution, occurring both in the low and high intertidal, as well as the subtidal of the Wadden Sea and adjacent North Sea. Since the locations where adults and juveniles live are spatially separated, M. balthica has to undertake migrations between these locations. Migration may be profitable if another habitat has a higher quality than the current one. However, migration takes time, uses energy and the journey may be dangerous. Mortality and fecundity (and thus fitness) are likely to be affected by migration: by its energy cost, by its effect on food supply and predation rates and by its other dangers. Therefore, migration costs must be balanced by the benefits of living in a more favourable environment. Thus, the decision of whether or not to migrate is a major component of a mobile organisms' life history strategy. The quality of a location for an animal depends on the developmental stage of an animal. Due to ecological and physiological differences between juveniles and adults, they may prefer different habitats. Spatial variation in the environment may make costly migration worthwhile because of these different preferences of juveniles and adults. The aim of this thesis is to determine why M. balthica migrates in the Wadden Sea; do these migration increase the fitness of Macoma balthica? To answer this question the costs and benefits of migrations and nursery use were assessed using laboratory and field experiments. The costs were sought in increased mortality during migrations, benefits were sought in differences in predation pressure and growth between the low and high tidat flats. Eventually, the costs and benefits of migration and nursery use were weighed in a model that calculates fitness of M. balthica as a function of migration strategy. The next paragraphs present the results of the different studies. First, I describe the results of a study in which I estimated the costs of migration for M. balthica. Migration may increase mortality rates among Macoma balthica populations, e.g. because migrating M. balthica run a greater risk to be eaten by predators than buried M. balthica, or may end up at unsuitable locations. I examined if mortality rates of the M. balthica population were higher during migration periods than outside these periods. Hence, population development of the 1998-year-class of the bivalve Macoma balthica was studied by repeated sampling of 57 stations at a 7 km² tidal flat (Groninger Wad) area in the Lauwers tidal basin in the eastern Dutch Wadden Sea from May 1998 to August 2000. Additional data was collected by sampling a tidal channel close to the study area, and by collecting M. balthica-densities for the Lauwers tidal basin and adjacent North Sea from the literature. During both spring and winter migration, many animal |
