Estuarien habitatgebruik door een grondelsoort: een geochemische benadering = Estuarine habitat use by a goby species: a geochemical approach

Many marine fish species are known to seasonally enter estuaries in large numbers during a period of their juvenile life stage. Yet, very little is known about the interaction between the estuarine populations and the population at sea or about the use of estuaries on a spatiotemporal scale by individual fish. Such knowledge is however fundamental to understand population dynamics, life history tactics and behaviour of marine fishes. Moreover, detailed knowledge of habitat use patterns is necessary to comprehend habitat function and forms the basis of efficient conservation and integrated management plans. The functional significance of estuarine visits at the level of the individual, the population and the species is still debatable for most marine fishes. This gap in knowledge can be attributed to the complexity of studying and following marine organisms from one habitat to another. Conventional methodologies often suffer from a constrained spatio-temporal resolution. Furthermore, many of those methods are not applicable for (post)larval and small juvenile fish susceptible to dispersive processes and high mortality rates. The migration dynamics of sand goby Pomatoschistus minutus (Pallas, 1770) between the North Sea and the Scheldt estuary (Belgium and The Netherlands) was investigated in this study by means of two biogeochemical tracers, namely stable carbon isotopes in dorsal muscle tissue (Chapter 4) and otolith [Sr/Ca] (Chapter 6). Prior to the application of both geochemical tracers in our study system, the techniques needed to be calibrated and conditions verified (Chapter 2, 3 and 5). We depended in this study on sand goby gut contents for determining the isotopic difference between food sources in the upper and lower estuary, and for the prediction of an upper estuarine end member signal. Therefore, we first describe an experiment towards the isotopic effects that might occur during digestion and assimilation (Chapter 2a). Gut contents of sand goby showed higher δ13C and δ15N isotope values than the food before ingestion. This enrichment was more pronounced in the hindgut than in the foregut, probably because of preferential assimilation of 12C and 14N along the gastro-intestinal tract. There was however no statistically significant difference for δ13C between the food source and the foregut content. A prerequisite to trace animal movements between two areas using stable isotopes is that food sources of the species under study are isotopically different between both areas. Additionally, for clear interpretation, it is important that the source population is relatively homogeneous in isotopic composition (Chapter 2b). Stable isotope analyses on monthly gut contents demonstrated that the δ13C value of sand goby prey items was on average 6 ‰ higher in the lower estuary than in the upper estuary. From June until November, δ15N was higher in the upper estuary than in the lower estuary, but this pattern reversed during winter and early spring. Sand goby muscle tissue showed no spatial δ13C variability along the Belgian coast. Our data however, revealed that coastal δ13C values were depleted relative to the offshore values. Coastal δ15N values, on the other hand, increased considerably with increasing distance from the estuary during summer and autumn, but an inshore-offshore δ15N gradient was not detected. These results confirm that δ13C, in contrast to δ15N, is an appropriate tracer to study fish migration into the Scheldt estuary. Studies on diet or migration of organisms based on stable isotopes require precise estimates of the rate at which the isotopic composition changes in the investigated tissues. Isotopic turnover rates in fish, unfortunately, are poorly understood. A laboratory diet switch experiment (Chapter 3) was conducted (1) to determine C and N isotopic turnover rates in sand goby muscle, liver and heart tissue and (2) to evaluate the relative contribution of growth and metabolic replacement to the t