This study focused on the spatial structure along the salinity gradient and the seasonal distribution of the epibenthos on the diet of the less abundant fish species as a reflection of the existing of two food chains in the Westerschelde area. The two food chains were influenced by the hydrodynamics through the season with higher concentration of dissolved nutrients in the brackish part and the natural phytoplankton sources in the marine part. The abundance of epibenthic fishes as top consumers can be expected to be high due to the functioning of estuarine ecosystems as nursery grounds. The ecological importance of the two food chains in the Westerschelde area, can be seen through the diet of the epibenthic fish species. The epibenthic fauna was collected with the R. V. 'Luctor' with a 3 metre beam trawl every month during 1990. Forty-five epibenthic species were found in the study area and dominated by brown shrimp Crangon crangon, lozano's goby Pomatoschistus lozanoi, sand goby P. minutus, dab Limanda limanda and shore crab Carcinus maenas. The spatial and temporal structures have been analysed by multivariate statistical techniques: a classification technique TWINSPAN based on fourth root transformed biomass data, and the ordination techniques Correspondence Analysis (CA) based on density and Canonical Correspondence Analysis (CCA) correlated with the three environmental variables. The TWINSPAN analysis divided the marine and the brackish stations into two major clusters of sampling sites. This was also reflected in CCA analysis, where the marine part correlated with high salinity and oxygen saturation and the brackish part with high turbidity. The CA analysis obtained 4 major temporal segregations in the marine and brackish areas. The marine part was dominated by Z. viviparous (winter months), P. lozanoi (spring months), T. luscus (summer months) and P. minutus (autumn). While in brackish area dominated by: T. luscus (summer months), S. sprattus (winter) and M. merlangus (spring). Forty-three prey items were determined from the stomach analysis. Generally, the changes in the diet was dependent on the stage development of the predators with exception of the plankton feeders such as herring, sprat and sandeel. In the global analysis Crangon crangon is an important prey both numerically and gravimetrically for predators such as bib, five-bearded rockling, hooknose, dragonet, sea scorpion and bull rout. Shrimp an only gravimetrically important for species like whiting, nillson's pipefish and hooknose. Only for Trisopterus luscus a comparison in stomach contents could be given between the marine and the brackish area. By using CA analysis the seasonal pattern in the diet of bib can be described as: from January till May they mainly preyed upon small prey items (i.e. calanoida copepods), from June till July they shown a high diversity in the diet but still mysid Neomysis integer and Crangon crangon are important prey, from Augustus till October they mainly feed on mysids and from November till December (again) the diversity of diet is high but amphipods and small fish are important prey. From the CCA analysis the segregation between two different area becomes clear with the typical prey Gastrosaccus spinifer in the marine side and Neomysis integer in brackish side. Crangon crangon is found in both area but it is mainly preyed by Trisopterus luscus in the brackish area. |
