Prolonged use of freshwater habitats by the European sea bass revealed by otolith chemical analysis.

The European sea bass, Dicentrarchus labrax (Linnaeus, 1758), is a highly valuable marine resource for both commercial and sport fishing. Estuaries are well-recognized as critical feeding areas, particularly for juveniles, throughout the species' distribution range. Empirical observations suggest that sea bass can also use freshwater habitats, although the time spent in these habitats remains unclear. This study aimed to investigate the seasonal occurrence of sea bass in a freshwater habitat within the Minho River, and to estimate the duration of their stay in these habitats. To characterize the freshwater contingent (e.g. age structure, sex ratio), sea bass collected with fyke nets between 2008 and 2023 from a fixed station in a freshwater habitat were analysed. A random subsample of 30 fish was selected for otolith chemical analysis to reconstruct habitat use throughout their lives and estimate time spent in freshwater habitats. An increase in the number of catches was observed from 2016 onwards, with sea bass numbers peaking during autumn and spring. The youngest and smallest individuals were collected during autumn and early winter, whereas the oldest and largest individuals were found more frequently during late spring and summer. The most common age was 4 years old, and 63% of the individuals collected were undifferentiated. Otolith chemical analysis revealed that nearly 50% of the individuals began their lives in brackish environments, suggesting that coastal habitats may also serve as spawning grounds. This finding raises the hypothesis that this population may also exhibit partial migratory behaviour. The continuous use of freshwater habitats for more than a year was documented, which should be considered in fisheries management.