Feeding of glass eels (Anguilla anguilla) in the course of their estuarine migration: new insights from stable isotope analysis

This study investigates the potential benefits of analysing stable isotopes in muscle for understanding the feeding behaviour of glass eels (Anguilla anguilla) in estuaries. According to the δ13C values in muscle, most of the glass eels sampled in the estuary keep their 'marine' isotopic signature and therefore do not feed significantly on estuarine organic matter. However, a small proportion (15%) looses its 'marine' isotopic δ13C signature to acquire a 'terrigenous' isotopic signature. The δ13C variation observed in glass eels varies from a 'marine' to a 'terrigenous' signature, indicating the consistent incorporation of organic matter of terrigenous origin (i.e. plankton, terrestrial vegetation) carried by the river into the estuary. The replacement of the 'marine' carbon pool by a 'terrigenous' carbon pool in some glass eels strongly suggests a period of residence in estuarine waters. Hence, glass eels may use the estuary not only as a simple migration route, but also as a feeding habitat.