K. Hostens

Spatial patterns and seasonality in the epibenthic communities of the Westerschelde (Southern Bight of the North Sea)

Beamtrawl samples were taken monthly between January 1988 and December 1989 in 14 subtidal stations along the longitudinal axis of the Westerschelde Estuary. The estuary harboured high densities of epibenthic species (total average of 2250 ind 1000 m -2 ). Only 37 fish species were recorded and three mobile macroinvertebrate species were quantified. Seventeen epibenthic species were common, only eight of which were very abundant. An overall dominance of the brown shrimp Crangon crangon was noted. The near absence of anadromous and freshwater species was correlated with the low oxygen concentrations upstream the Dutch Belgian border. The epibenthic species could be divided into a polyhaline and a mesohaline community along the main salinity-oxygen- turbidity gradients of the estuary The mesohaline zone was characterized by higher densities and biomasses for shrimps, gobies and flatfish. The spatial structure in the epibenthic community of the polyhaline zone was more influenced by the water-masses from the sea and inflowing channels, and was correlated with the degree of exposure (current velocities and tides). A classification is proposed, based on the season in which the epibenthic species occur in the estuary, and on their life stages and abundances during that period. The monthly data demonstrate that while the eight abundant epibenthic species were generally present throughout the year (summer or winter resident), the majority tended to appear in the Westerschelde at varying times (summer, winter or bimodal) and often remained in large numbers (nine common species) for only a short time. The fact that mainly juveniles were recorded in the higher density-classes, suggests that the Westerschelde Estuary still acts as a nursery area for many epibenthic species.

The mobile epibenthic fauna of soft bottoms in the Dutch Delta (south-west Netherlands): spatial structure

During 1989 monthly beam trawl samples were collected at 48 stations in the Dutch Delta. The annual mean densities of the demersal fishes and mobile epibenthic invertebrates at these stations were calculated. These data were then subjected to multivariate statistical techniques for an analysis of the spatial structure of the communities found and to study the relationship between these communities and their environment. The present study confirms the results of HENDERSON (1989) that, given a fairly limited number of environmental variables, mainly salinity and exposure/substratum type, quite accurate predictions of the type of community expected at a certain site can be made. The explicit inclusion of epibenthic invertebrates, a dominant group in most assemblages, in studies mainly targeted at demersal fishes, is strongly recommended. In the Dutch Delta rich and varied communities exist in the Voordelta. A number of groups, such as salmonids, anadromous species and starfish, expected in the westerschelde are absent or extremely rare probably as a consequence of pollution stress. The Oosterschelde is relatively poor in density terms but has a highly diverse epibenthic fauna dominated by fishes.

Changes in the ratio of the sulcus acusticus area to the sagitta area of Pomatoschistus minutus and P. lozanoi (Pisces, Gobiidae)

The Pomatoschistus minutus (Pallas) and P. lozanoi (de Buen) stock of the ebb-tidal delta of the former Grevelingen estuary was sampled monthly from September 1992 to October 1993. Sagittae were extracted and the areas of the sagitta and sulcus acusticus were measured with a digitizer. The frequency response and auditory threshold of these two sympatric goby species were determined from the ratio of the sulcus acusticus area to the sagitta area (S:O ratio). The S:O ratio of P. minutus increased while in P. lozanoi the S:O ratio was constant with increasing standard length. The average S:O ratio of P. lozanoi was significantly higher than that of P. minutus of the same mean length, which makes the former more sensitive to sound frequency. The S:O ratios of these two sympatric fishes were lower than the S:O ratios of demersal and pelagic fishes available from the literature. An interspecific growth difference of sagitta and sulcus acusticus was observed. The sagitta and sulcus acusticus of P. minutus are larger than those of P. lozanoi of the same length.

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.