Understanding habitat use and connectivity in estuarine fishes is essential for effective conservation and management. In this study, otolith microchemistry was applied to investigate habitat use and connectivity of the estuarine catfish Genidens genidens across three estuaries in southeastern Brazil. A total of 58 individuals were analyzed using laser ablation inductively coupled plasma mass spectrometry, focusing on strontium-to-calcium (Sr:Ca) and barium-to-calcium (Ba:Ca) ratios. Variations in elemental ratios along otolith transects were used to infer individual ontogenetic patterns along the estuarine–marine gradient. Most individuals exhibited combined use of estuarine and marine environments, while trajectories restricted to freshwater were rare. The apparent complexity of chemical profiles tended to increase with age; however, this pattern disappeared after correction for size-related bias, suggesting that age itself did not significantly influence habitat-use transitions. These patterns are consistent with ecological plasticity and partial migration within populations of G. genidens, although they may also reflect exposure to variable environmental conditions. Sr:Ca ratios were useful indicators of salinity-related transitions, whereas Ba:Ca ratios provided complementary information associated with continental influence. Overall, this study highlights the applicability of otolith microchemistry for investigating habitat-use patterns in estuarine fishes and reinforces the ecological importance of estuaries for feeding, growth, and recruitment in G. genidens, while acknowledging inherent limitations related to environmental variability and proxy interpretation.
Gil et al. (2026) studied this question.