Henrietta Hampel

Feeding habits of young predatory fishes in marsh creeks situated along the salinity gradient of the Schelde estuary, Belgium and The Netherlands

Fish and macrobenthos were sampled in four different marshes along the salinity gradient of the Schelde estuary, Belgium/Netherlands, to investigate the importance of marsh creeks as foraging grounds for the dominant, larger fish species. The total density and biomass of all the main macrobenthic taxa (Corophium volutator, Nereis diversicolor, Oligochaeta, Macoma baltica and Heteromastus fliliformis) were measured. The feeding habits of the larger predatory fishes (Platichthys flesus, Dicentrarchus labrax) were investigated. Qualitative and quantitative stomach analyses included the calculation of different indices, showing the niche breadth (as diet diversity) and the niche overlap (as similarity between the predators’ diet) for this habitat. These analyses showed that the two most important benthic prey species for P. flesus were C. volutator and N. diversicolor. D. labrax preyed upon a wider range of species, including C. volutator, N. diversicolor, Crangon crangon, Carcinus maenas and Orchestia spp. The stomach diversity of D. labrax and P. flesus showed differences between the marshes although there was no consistent pattern in diet composition, reflecting the opportunistic nature of feeding by these large predators. The fullness indices of both flounder and sea bass did not differ significantly along the salinity gradient and the estimated minimum consumption by these predators did not indicate a top-down control of the macrobenthic community. The salt marsh creeks seem to provide excess food for the visiting fish species. The benthic prey was present in very high abundances, which may suggest that the typical nursery species such as C. crangon and C. maenas, and early juveniles of P. flesus, D. labrax and Pomatoschistus microps were not preyed upon significantly. This supports the hypothesis that salt marsh creeks provide good refuge areas for nursery species against predation by larger fish.

Temporal variation in feeding rhythms in a tidal marsh population of the common goby Pomatoschistus microps (Kroyer, 1838)

Abstract.Pomatoschistus microps (Teleostei, Gobiidae) intensively uses the mesohaline marsh of Westerschelde estuary as a nursery and foraging ground. The sampling campaign covered the semi-lunar, diel and tidal cycles. The density of P. microps and potential hyperbenthic prey species in the marsh creek, fullness index, evacuation rates and daily ration of common goby were calculated. Mesopodopsis slabberi, Neomysis integer and Corophium volutator were the most dominant prey items in terms of biomass. Numerically, copepods dominated the diet. Migrating fish enter the marsh creek with a relatively empty stomach and leave the marsh with a higher stomach content. Pomatoschistus microps seemed to feed more intensively during the day than the night, however the influence of the diel cycle is inferior in comparison with the tidal influence on the feeding behaviour of the common goby. A significant difference in foraging activity occurred between the spring and neap tide. The common goby migrated in lower abundance into the creek during spring tide but foraged more intensively. At both spring and neap tide, a significant difference was found in the fullness index between day and night. At spring tide, gobies feed more during the day, while they forage more intensively at night at neap tide. All the three cycles (tidal, diel and semi-lunar) influenced the feeding rhythm of the common goby. The tidal influence is superior over the diel variation, while the explanation of the combined effect of diel and semi-lunar cycle needs further studies.