Otolith shape and chemistry reveal life history plasticity of an euryhaline species

Plasticity in fish movement patterns is frequently observed in euryhaline species. Otolith shape and chemical composition are natural recorders of the environmental conditions experienced by individual fish, providing valuable tools to reconstruct habitat use and movement patterns. The European sea bass (Dicentrarchus labrax) is a euryhaline species for which partial migration has been suggested, whereby some individuals remain resident while others migrate in coastal and marine environments, but the use of freshwater habitats by adults has only been reported recently. To assess the existence of distinct contingents, this study analysed the otolith shape and chemical composition of sea bass collected in freshwater, adjacent coastal areas and distant coastal areas using Wavelet reconstructions and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). Both approaches identified a divergent life-history strategy within the sea bass population, characterized by a consistent use of freshwater habitat. Otolith chemistry further suggested that life histories differentiate with age, becoming especially evident from age two onward, when freshwater-associated signatures start to become more noticeable. These findings reveal the ecological plasticity of European sea bass and highlight the importance of sea-land connectivity for the conservation of euryhaline species.