Alternative habitat use strategies in the catadromous Chelon ramada inferred by otolith microchemistry

Otolith chemical analyses of Sr, Ba, and Ca composition were conducted on thinlip grey mullet (Chelon ramada) collected from a freshwater (n = 45) and an estuarine (n = 55) site in the Mondego River (central Portugal) using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) to investigate the species' habitat use patterns. The reconstruction of habitats used provides evidence for a behavioral continuum along the marine-freshwater environments in the Atlantic region. Overall, the estuarine environment is the habitat with the highest residency, yet four distinct behavioral profiles were described, taking into account the time spent in each habitat, the frequency of marine contacts, and the age of first contact with this environment. About 37 % of the individuals showed a high prevalence of movements between the estuarine and marine habitats, with residual or no more prolonged freshwater contact and a high frequency of marine contacts (Marine-Frequent Spawner). In contrast, 15 % of the individuals had reduced and irregular exposure to the marine habitats and spend more time on freshwater (35 % of their lifetime, i.e. Freshwater-Irregular Spawner). The remaining fraction (48 %) displayed a higher estuarine residence and similar use of freshwater and marine habitats. However, one-third of the specimens exhibited later and irregular contact with the marine environment (Estuarine-Irregular spawner). Our findings also support the facultative nature of the annual reproduction, at least for a population fraction.