From trophic ecology to fish condition: contrasting pathways for European hake in the western Mediterranean

In marine ecosystems, where variability in the type and availability of food resources is a common property, species must adjust their feeding behaviour in order to fulfil their energetic needs within the requirements of ontogeny. These variations in the quality and quantity of food supply are crucial for the nutritional status of individuals. We investigated these relationships by analysing the isotopic composition and somatic body condition of European hake Merluccius merluccius within 3 areas in the western Mediterranean: the waters off the Ebro River delta and the northeastern coast of the Iberian Peninsula, both off the Spanish mainland, and the Balearic Islands. These locations have contrasting oceanographic conditions and degrees of productivity. Results reveal important ontogenetic and spatial differences in the isotopic composition and body condition of recruits and juveniles, as well as a distinct trophic structure and pathways between areas. Isotopic signatures revealed differences between the waters off the mainland and the Balearic Islands, with an overlap in the isotopic niche of recruits and juveniles in mainland waters that contrasted with a clear separation in the Islands. The linkage between trophic ecology and somatic body condition revealed that this relationship cannot be as straightforward as expected, highlighting the importance of ecological processes such as competition for food resources. This study reveals the small-scale spatial complexity in the trophic ecology of European hake and its relationship with nutritional status. Acknowledging such complexity is of high importance for the implementation of integrative management of marine living resources.