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|Onderzoek naar de habitatkwaliteit voor bot (Platichthys flesus) in het Schelde-estuarium op basis van een veldstudie en wiskundige modellen|
Engelstalige titel: Habitat quality of flounder (Platichthys flesus) in the Scheldt estuary: a field and modelling study
Identifier financieringsorganisatie: 3E040975 (Other contract id)
Periode: Oktober 2000 tot December 2006
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|Animals are influenced by both biotic and abiotic factors when selecting a habitat. The final decision of an organism to select a habitat generally depends on the fitness increase that can be expected. Within a selected habitat, interactions with conspecifics and other species also affect its quality. This thesis describes the habitat use of estuarine fishes in the Scheldt estuary. In the first part (Chapter 1 to Chapter 3) we studied the importance of intertidal migration for fishes and discussed the various factors that affect intertidal habitat quality. We also examined to what extent fishes affect their environment by predation on the intertidal infauna. The second part (Chapter 4 and 5) describes the effect of abiotic factors on the use of the Scheldt estuary by European flounder Platichthys flesus (L.).
The fish assemblage on a mudflat in the brackish part of the Scheldt estuary is dominated by juveniles, confirming the nursery status of this part of the estuary. The composition of the intertidal fish community reflects well the cyclicity of seasonal recruitment of species into the estuary. Flatfishes migrate actively onto the mudflat whereas the distribution of (semi-) pelagic species in the fyke nets suggests that they are transported passively by the tidal currents. Most fish species do not show a clear zonation on the mudflat and, if observed, this is mainly the result of species-specific differences in mobility. Zonation of flatfishes tends to be affected by density-dependent processes. When fish abundance in the nets is higher, relatively more flounder are caught on the upper shore. By doing so, they probably avoid competition for food or space with sole, which stays on the middle and lower zones of the mudflat. Because of the low predator abundance and high turbidity in the brackish part of the estuary, predation is probably not an important trigger for intertidal migration.
The most important (and perhaps only?) trigger for intertidal migration in turbid estuaries is the high intertidal food availability. The concentration of large numbers of juvenile fishes in these intertidal areas may give rise to competitive interactions. Diet analysis shows that all fish species on the mudflat, without exception, target the same prey species (Corophium volutator). The relative importance of prey species in the diet of fishes reflects the seasonal prey availability in the field, confirming the generalist and opportunistic feeding nature of estuarine fishes. For all species, the niche width is larger when resources are less available or when the population density is higher. It is suggested that a generalist and opportunistic feeding strategy reduces competition and results in a broader population diet with increasing population density. As most fishes are feeding on a limited number of prey species, niche overlap is generally high between species. A significant niche overlap was found in summer between flounder and sole. Direct competition for food, however, is probably avoided by spatial niche separation and by resource partitioning at the level of prey size.
The intensive use of the intertidal benthic resources raises the question whether or not predators deplete the macrobenthic prey populations. The results of two exclosure experiments on an estuarine mudflat indicate that fishes and birds have only a negligible effect on the abundance of their infaunal prey species. The effects of short-term experiments in these highly productive areas may only be noticeable at high predator densities. Long-term exclosure experiments on the other hand, seem to reflect the indirect effects of predation, as in the absence of predation, infaunal regulation (competition and predation) becomes more important. The lack of clear direct effects of predation on the abundance of organisms on the lower trophic levels, suggests that the interaction strength in the benthic food web is rather weak. Weak links generally support food web stability and decrease the probability of tropic cascades. These weak interactions may be the result of (1) the three-dimensional structure of soft sediments, which reduces the risk of strong competitive and predatory interactions and (2) the prevalence of omnivory in the benthic food web. The fact that benthic prey is superabundant and that fishes are flexible to switch between different prey species (infaunal, epibenthic, hyperbenthic, pelagic), suggests that the carrying capacity of the estuary for benthic fishes is only rarely reached.
If food and predation are not limiting, which is thought to be the rule in estuarine nurseries, then abiotic factors determine the habitat selection of fishes. We were able to accurately describe the growth of flounder in an estuarine environment, solely based on temperature, which demonstrates the predominance of temperature as a regulator of growth and hence habitat quality. A multivariable bioenergetics model, based on temperature, oxygen concentration and salinity, was constructed for European flounder (Platichthys flesus) to generate spatially-explicit estimates of growth in the Scheldt estuary. The model was run for two years (1998 and 2003) and predicts that in March, when 0-group flounder is known to migrate upstream, growth is highest in the upper freshwater reaches of the estuary. This suggests that freshwater migration of flounder may be, at least partly, temperature driven. In July and October, the unfavourable oxygen concentration in the central part of the Zeeschelde reduces growth and probably prevents upstream migration. In summer and autumn, the growth rate in the brackish part of the estuary is higher compared to the marine part. Field data on the distribution of flounder in the estuary shows that the abundance of this species is highest in the brackish part, where it also finds plenty of suitable prey items. Our results indicate that abiotic variables may be useful to predict the habitat use of diadromous species in estuaries. Given the importance of food in determining habitat quality, the model should be further extended with a foraging compartment to account for spatial differences in prey availability.
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