Fish and crustacean response surfaces to environmental gradients in the Westerschelde estuary (poster)

The probability of occurrence of eleven fish and three crustacean species in the Westerschelde as a response to several environmental variables is predicted. Single logistic regression provides good descriptions of the occurrence along one environmental variable, which is related to the maximum likelihood of presence in the field. Multiple (fractional polygonal and factorial) logistic regressions give insight into the relative importance of each environmental variable. For all 14 species the predictions were relatively successful (60 to 90 percent correctly predicted) by combining data on salinity, temperature, turbidity and/or oxygen. However, these response surfaces should not be interpreted as physiological limits but as actual distribution patterns as a function of these abiotic variables. The addition of other variables such as current velocity, chlorophyl a, SPM or mysid density did not improve the predictions. The models are cross-validated internally and evaluated with a limited data set from the adjacent Oosterschelde

Application of the Westerschelde response models to fish and macro-crustacean data from the Oosterschelde

The aim of this study is to test the applicability of the statistical models, which were developed to predict fish and macro-crustacean responses to the environmental conditions in the Westerschelde estuary (Chapter 6). For this, a data set from the Oosterschelde was used. The probability of occurrence and the prediction of abundance of several dermersal fish and macro-crustaceans in reponse to four environmental variables (salinity, temperature, dissolved oxygen concentration and turbidity) are investigated. The present study can only be seen as a preliminary attempt to validate the robustness of the models that were established for another system. The diagnostics (% correctly predicted; sensitivity and specificity) for the 15 presence absence models based on the Oosterschelde data were comparable to the diagnostics based on the Westerschelde data. Only the models that explained >45% of the variance in density for six species in the Westerschelde, were applied to the density data from the Oosterschelde. These were largely successful when temperature was the main forcing variable (e.g. for Sprattus sprattus and Pomatoschistus microps). The constant high salinity in the Oosterschelde is a favorable condition for Carcinus maenas and Liocarcinus holsatus, partially masking the effect of the other environmental variables in these models. Limanda limanda and Pomatoschistus minutus have different seasonal distribution patterns in both systems. In the Westerschelde both species only occurred during a relatively short period; while in the Oosterschelde they were present throughout the year. This shows the difficulty of applying statistical models from a real estuary to a different type of ecosystem, namely a marine bay like the Oosterschelde

Introduction: objectives and outline of the thesis

The thesis focuses on the nursery function of the Westerschelde and Oosterschelde estuaries for demersal fish and macro-invertebrate species. The main aim of the thesis is to provide a baseline about the higher trophic levels for future process studies in the Delta area. More specifically, both structural and functional patterns in several population parameters (density, biomass, growth, biodiversity, Jood and feeding behaviour) are investigated at community and species levels, and at different temporal (short-, mid- and long-term) and spatial (between and within estuaries) scales. This introductory chapter gives a general description of the epibenthic assemblages and a short overview of the factors on which the nursery function of a system depends. The study area is described and the major engineering works in the Delta area are summarized. An overview of the different sampling methodologies and of the available and used datasets is given. This chapter finishes with the main objectives and an outline of the thesis

The demersal fish and macro-invertebrate assemblages of the Westerschelde and Oosterschelde estuaries: overview and final conclusions

An overview on the nursery function of the Westerschelde and the Oosterschelde estuaries for demersal fish and macro-invertebrates is presented; based on all data sets presented in this thesis. The present thesis provides a baseline, and suggests that both the Westerschelde and Oosterschelde estuaries play a significant role as nursery areas for several demersal fish and macro-invertebrate species. The forcing factors for recruitment and the seasonal appearance of (post) larval fish and macro-crustaceans are explained. Biodiversity is related to habitat use and habitat availability. The faunas of the Westerschelde and Oosterschelde are structured by a limited number of environmental variables, and the prediction of species occurrence by means of single- species logistic models is discussed. Spatial, long-term and seasonal patterns in density in relation to the environment are summarized by means of multivariate analyses and graphical representation of trends per species and per taxonomic group. The seasonal appearance of both (post)larval and juvenile fish and macro- crustaceans in the Westerschelde is described. Density of the commonest species is predicted by means of single- species linear regression models. A short description of the human impact on the fish and macro-invertebrate assemblages is given. In the functional part of this overview, a generalized food web for the Westerschelde is presented; and the importance of mysids as prey for several fish and macro-crustaceans in the Westerschelde is discussed. Finally, 10 major conclusions and some remarks are formulated and some recommendations for future research are given

The mysid-feeding guild of demersal fishes in the brackish zone of the Westerschelde estuary

The demersal fish fauna of the mesohaline zone of the Westerschelde estuary (south-west Netherlands) was sampled intensively in the period 1990-1992. Almost 500 beam trawl samples were taken in both subtidal (330 samples) and intertidal (144 samples) habitats. These yielded 44 fish species, mostly as juveniles. The area functioned as a nursery for several demersal fish species, and harboured large populations of hyperbenthic mysids. Three gobies, three flatfish, one clupeoid and one gadoid dominated the fish fauna, while three mysid species were important components of the holohyperbenthos. From c. 1500 stomach contents of 25 fish species, 44 prey species were identified, the most abundant of which were also common in the hyperbenthal. The demersal fish community consisted of a group that foraged subtidally on fast-moving epi- and hyperbenthic prey (for example gadoids, gobies and clupeoids) and a group that foraged on slow-moving or sessile endobenthic organisms, mainly in intertidal areas (for example most flatfish species). Mysidacea occurred in >50% stomachs analysed and were taken as prey by 19 of the 25 fish species. Mysids were most important in the diets of Pomatoschistus minutus, P. lozanoi, Trisopterus luscus and Merlangius merlangus, and were present in appreciable numbers in Pleuronectes flesus, Trigla lucerna, Clupea harengus and Pleuronectes platessa. These species fed mainly on the brackish water endemic Neomysis integer. Mesopodopsis slabberi (present in 35% of the gobiid stomachs) and Gastrosaccus spinifer (present in 25% of the gadoid stomachs) were of secondary importance. P. minutus and T. luscus showed a diet shift from calanoids (Eurytemora affinis and Temora longicornis) to mysids at Ls of 30 and 50 mm, respectively. Only 1% of the standing stocks of the N. integer and M. slabberi populations was removed by the local demersal fish community, so top-down control of mysid populations in estuaries seems unlikely

Assessment of the conservation value of the Vlakte van de Raan Site of Community Interest

Based on the best available knowledge, the current study analysed the habitats’ and species’ conservation value for the specific delineated Vlakte van de Raan Site of Community Interest (SCI) in the Belgian part of the North Sea (BPNS). The habitat types 1110 and 1170 and the species Phocoena phocoena and Alosa fallax were analysed for the 19km² Vlakte van de Raan SCI. These analyses are based on the criteria: representativeness, conservation status, recovery potential, surface area and conservation value. The Habitat type 1110 at the Vlakte van de Raan SCI turns out to be a mosaic of areas of poor and medium ecological value, interspersed with some elements of high ecological value (i.e. part of the Abra alba biotope). The combined surface area is negligible compared to the Vlaamse Banken SCI or the total surface of the BPNS, indicating a low conservation value. The ecological value of the Habitat type 1170 at the Vlakte van de Raan SCI scores very low, indicating no conservation value for this habitat type. For the harbour porpoise Phocoena phocoena, the Vlakte van de Raan area shows the lowest densities found in Belgian waters, leading to the conclusion that the ecological value of the Vlakte van de Raan SCI for harbour porpoise is to be considered negligible.Twait shad Alosa fallax is known to occur within the Vlakte van de Raan SCI (data from 1 beam trawl sample in the area). However, taking into account the different scientific criteria, the ecological value of the Vlakte van de Raan SCI for A. fallax is to be considered anecdotal and its conservation value as negligible. Any conservation measure within the Vlakte van de Raan SCI will be ineffective for this migrating species. In conclusion, based on different criteria, the scientific analysis shows very poor to no nature conservation value for the habitat types and species for which the site was initially proposed, leading to the conclusion that the Vlakte van de Raan SCI has no value as a special area for conservation