Shallow areas, such as coastal zones, estuaries, intertidal wetlands and mangrove swamps are extensively used by many marine and freshwater fish species. For temperate estuaries, this pattern of movements is resulting in consecutive migration waves of juveniles of marine fish, crabs and shrimps. The major objec- tive of this research proposal is to develop a qualitative ecological model that explains the observed migration sequence of fish in the Zeeschelde estuary. The model quantifies the ecological benefits and costs of reamining in the native habitat versus moving into the estuary or out to the sea. Benefits are growth, costs are mortality and unsuitable salinities and tempe- ratures. A general calibration will compare the model predictions with an extensive data set containing fish abundance of the Zeeschelde.