André Cattrijsse

Nursery function of an estuarine tidal marsh for the brown shrimp Crangon crangon

Abstract The brown shrimp Crangon crangon migrates into the brackish part of the Westerschelde estuary (southwest Netherlands) shortly after metamorphosis and uses the tidal marsh habitat as a nursery until reaching a total length of about 15 mm. The importance of the marsh as a nursery was evaluated by estimating foraging activity, predation mortality and residence time. In early postlarval stages, C. crangon utilised the intertidal creeks of an estuarine tidal marsh from early spring (March–April) until late autumn (October–November). Postlarval shrimp leaving the marsh with the ebb tide always had significantly more food in their stomachs than shrimp entering the marsh with the incoming flood water. Predation upon the shrimp population was relatively low during most months, but in increased between August and October when common gobies, Pomatoschistus 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 temperature, 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 changed drastically during a second year of sampling, illustrating high natural year-to-year variability. However, the seasonal pattern remained. Recruitment to the subtidal adult population represents an export of animals from the marsh to the estuary. This export is negligible in terms of biomass (as compared to the total biomass of the estuarine population) but it may be important in terms of numbers of individuals.

Early postlarval fish in the hyperbenthos of the Dutch Delta (south-west Netherlands)

Early (post)larval fish constitute a significant part of the temporary hyperbenthos, i.e. of the fauna living in the lower reaches of the water column close to the substratum. Information on the densities and spatial and temporal variations of these stages, as well as on their lengths at the moment of occurrence, can be an important contribution to the knowledge of their dispersion, migrations and to the identification of possible bottlenecks in their recruitment. Therefore the hyperbenthos of the Voordelta, a shallow coastal zone in front of the Dutch Delta, as well as that of the subtidal channels and the intertidal brackish marsh creeks of the Westerschelde Estuary, was sampled monthly during one year by means of a sledge and a stow net. A total of 37 taxa were recorded from 410 samples. Densities often exceeded 400 ind 1000 m −2 in the Voordelta and 300 ind 1000 m −2 in the Westerschelde, including the tidal marshes of Waarde and Saeftinghe. The dominant taxa were clupeid larvae (predominantly herring Clupea harengus and sprat Sprattus sprattus ), Pomatoschistus spp. (a mixture of sand goby P. minutus and Lozanos goby P. lozanoi ), common goby Pomatoschistus microps , Syngnathus spp. (probably almost exclusively Nilssons pipefish S. rostellatus ), plaice Pleuronectes platessa , sole Solea solea and flounder Pleuronectes flesus . Most species were found at a larger size in the Westerschelde (and in the tidal marshes) suggesting migration from the shallow coastal area into the estuary. Several species utilize the tidal marshes in the brackish reaches of the Westerschelde. The nursery value of these areas however, is restricted to specific early life history stages of a few species, especially flounder, bass Dicentrarchus labrax and the common goby. The estuary itself seems to function as a nursery for other species like sole and plaice. The hyperbenthic sledge was found to sample specific life history stages of postlarval flatfish (early settlement stages at the onset of asymmetry), which are not easy to sample with other types of sampling gear.

The EBB tidal delta of the grevelingen: A man-made nursery for flatfish?☆

In order to investigate the impact of the closure of the Grevelingen estuary on the benthic life and the nursery function of its ebb tidal delta, a comparison was made between the present ebb tidal deltas of the Grevelingen and the Eastern Scheldt. Three groups of benthic animals were compared: macrobenthos, epibenthic animals (including demersal fish) and hyperbenthos. The ebb tidal delta of the Grevelingen is much richer for almost all animal groups discussed. The data suggest that this richness is a consequence of the changes that have occurred in the hydrodynamics since the closure of the Grevelingen estuary in 1971. The area now traps and retains mud, detritus, larvae of different groups of infaunal animals, larvae of epibenthic crustaceans, and eggs and larvae of fish. This change is reflected in the increased importance of the area as a nursery for flatfish, especially dab and sole. However, the increase in the nursery function of the ebb tidal delta does not make up for the loss of nursery areas in the estuary itself.