Modelling the adaptive value of intertidal migration and nursery use in the bivalve Macoma balthica

Ontogenetic niche changes (migrations) are worthwhile when spatial differences that are age or size selective exist. In the Wadden Sea, a Macoma balthica individual migrates twice; from the primary settlement locations at the low tidal flats to the high level nursery (spring migration) and back to the low intertidal at an age of 9 mo (winter migration). Benefits of migration may be found in differences in predation pressure between the low and high tidal flats. This study evaluates, by means of a model based on empirical data, under what conditions the costs of migration (increased mortality) are traded-off against an increased reproductive output (RO) due to avoiding size-selective predation (by shrimps, crabs, polychaetes and birds) and size-selective infection by the parasitic trematode Parvatrema affinis. Density was modelled as a function of predation and migration mortality. The RO was compared for M balthica that did and did not migrate, and for different times of the spring and winter migration. RO was maximised for M balthica that settle in the high intertidal and migrate to low tidal flats at an age of approximately 9 mo. Shrimp predation makes living on the low tidal flats unfavourable for small M balthica. Parasitation by P. affinis makes it beneficial for M. balthica to leave the high tidal flats around the age of 1 yr. In conclusion, migrations of M. balthica to and from nurseries on high tidal flats of the Wadden Sea may be seen as an adaptation to avoid shrimp predation on the juveniles and parasite infection of the adults. Although the costs of migration are large, fitness is increased due to the migration because it is traded off by an increased RO.