Migratie van juveniele haring (Clupea harengus L.) en sprot (Sprattus sprattus (L.)) tussen de Noordzee en het Schelde-estuarium zoals aangetoond met stabiele C en N isotopen
Guelinckx, J. (2002). Migratie van juveniele haring (Clupea harengus L.) en sprot (Sprattus sprattus (L.)) tussen de Noordzee en het Schelde-estuarium zoals aangetoond met stabiele C en N isotopen. MSc Thesis. Katholieke Universiteit Leuven (KUL): Leuven. 109 pp.
The temporal changes in abundance of juvenile herring (Clupea harengus Linnaeus, 1758) and sprat (Sprattus sprattus (Linnaeus, 175)) in the Schelde estuary were examined by means of stable isotopes. Juvenile herring and sprat typically overwinter in the estuary. Herring exhibits a second, smaller density peak in the summer. A similar density peak for sprat is not observed. The temporal use of the estuary by clupeoid fish has previously been attributed to seasonal migrations of juveniles between the North Sea and the estuary. Using stable isotopes of carbon and nitrogen we have tried to elucidate these migration patterns. Herring and sprat were sampled between May 2000 and April 2001. Samples were taken every month in the cooling water of the Doel Nuclear Power Plant (in the brackish part of the estuary) and the Borssele Nuclear Power Plant (at the mouth of the estuary). Using cluster analysis on the deltaC and deltaN values of individual muscle tissue, fish which recently immigrated from the North Sea (marine group with typical marine isotope values) could be distinguished from individuals which had resided in the estuary (estuarine group with typical estuarine isotope values). The analysis showed that herring and sprat had very similar migration dynamics in the Schelde estuary, characterized by immigration and emigration almost throughout the year and an intensive migration activity during the winter. Net upstream immigration (i.e. the majority of fish enter the estuary) started in September and peaked in November. During December immigration remained high but had already decreased, which probably explains lower fish densities recorded at Doel. Although the density of herring and sprat further declined in February and March, net seaward emigration sensu strictu (i.e. the majority of the fish leave the estuary) was not demonstrated using the stable technique. During the winter larger proportions of individuals with a typical marine isotope signature at Doel were not only associated with migration but also with a slower tissue turnover rate. Seasonal variation in the isotope signatures of fish tissue were related to similar patterns in lower trophic levels, faster tissue turnover rates in small fish due to their higher respiration per gram body weight and indirectly to the seasonal variation in temperature. No relationship has been found between the total body length of herring or sprat and delta13C or delta15N values. Finally, the ecological relevance of the observed migration patterns was discussed.
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