d¹³C and d¹⁵N in yellow and silver eels (Anguilla anguilla, 1758) from different Mediterranean local stocks and their variation with body size and growth

The European eel, a catadromous endangered species, displays plasticity in phenotypic traits and in trophic behaviour, features that can eventually affect growth. The aim of this study was to compare variation in carbon (d¹³C) and nitrogen (d¹⁵N) stable isotopes in muscle tissue of the adult eel life stages (i.e. yellow and silver eel), as function of growth. Eel samples were obtained from nine sites across the Mediterranean region, that included seven coastal lagoons of different size and ecological structure, and two freshwater habitats corresponding to one river and one lake. A large variability was found in both d¹³C and d¹⁵N values, suggesting that eels fed over a broad range of potential food sources. Eels from each location showed specific range values of d¹³C, in most sites irrespective of stage, reflecting lagoon- or freshwater-specific carbon sources. A positive correlation was found between individual d¹⁵N values and annual growth rates, suggesting that eels generally seem to feed on prey of higher d¹⁵N values among the trophic resources available in each foraging habitat. This allows individuals to grow faster and to attain earlier the pre-spawning phase (i.e. the silver stage). A better understanding of eel trophic ecology across different life stages and different habitats is one of the prerequisites for the effective management of local stocks.