NURSERY FUNCTION OF AN ESTUARINE TIDAL MARSH FOR THE BROWN SHRIMP CRANGON-CRANGON

The brown shrimp Crangon crangon migrates into the brackish part of th e Westerschelde estuary (southwest Netherlands) shortly after metamorp hosis and uses the tidal marsh habitat as a nursery until reaching a t otal length of about 15 mm. The importance of the marsh as a nursery w as evaluated by estimating foraging activity, predation mortality and residence time. In early postlarval stages, C. crangon utilised the in tertidal creeks of an estuarine tidal marsh from (March-April) until l ate autumn (October-November). Postlarval shrimp leaving the marsh wit h the ebb tide always had significantly more food in their stomachs th an shrimp entering the marsh with the incoming flood water. Predation upon the shrimp population was relatively low during most months, but it increased between August and October when common gobies, Pomatoschi stus microps, were present in high densities. There was also predation by the small seabass Dicentrarchus labrax. The marsh creeks function both as foraging areas and as predation refuge. Depending on temperatu re, postlarval shrimp stayed in the marsh for a period of two to three weeks. Quantitatively, the value of the marsh as a nursery area had c hanged drastically during a second year of sampling, illustrating high natural year-to-year variability. However, the seasonal pattern remai ned. Recruitment to the subtidal adult population represents an export of animals from the marsh to the estuary. This export is negligible i n terms of biomass (as compared to the total biomass of the estuarine population) but it may be important in terms of numbers of individuals .