This work aims at associating fish ecology and analytical chemistry in order to generate a tool able to study the migratory behavior of Micropogonias furnieri, an eurihalin fish species highly abundant in the South-West Atlantic Ocean. We analyzed strontium and barium concentrations in otoliths of M. furnieri collected from fresh (Mirim Lagoon) and marine water (Rio Grande coast) habitats from southern Brazil, and tested the hypothesis that the elemental signatures of strontium and barium result from water salinity. M. furnieri has been locked in the Mirim Lagoon since 1977, when a floodgate was built to prevent saltwater intrusion, and therefore it may be considered a natural freshwater control for elemental composition of fish otoliths. A sample of 10 M. furnieri was collected from each habitat. The otoliths were removed, sliced in 0.4 mm thick sections and cleaned before analysis. The sections were then scanned from the core to the edge using laser ablation inductively coupled plasma mass spectrometer for the measurement of strontium and barium concentrations. Otolith annual growth increments were measured through microscope images in order to assign the analyzed profile to the specific fish ages.
Results/Conclusions

Our results suggest a positive effect of the salinity on strontium and negative on barium deposition on otoliths of M. furnieri. The concentrations of strontium and barium in the analyzed otoliths were, respectively, 710 (±76) and 111.6 (±30) µg g^-1 for freshwater, 2069 (±238) and 16.7 (±20.4) µg g^-1 for estuarine, and 2990 (±635) and 2.7 (±3.3) µg g^-1 for marine waters. The results also show that ontogenetic processes increase the barium deposition in the otoliths producing a well defined peak around the initial life for both habitats. Barium concentrations for otolith from freshwater fish have presented higher concentrations than those observed for otoliths from marine habitats. For marine fishes, mean strontium concentrations increased from about 2000 to 2900 µg g^-1 along their life, suggesting a migration from estuarine to marine habitats. Meanwhile, strontium from freshwater fishes decreased to a low and regular level (about 710 µg g^-1) before one year of life and kept these levels for the rest of the life of the studied fish, characterizing a unidirectional sea-freshwater migration. We conclude there is interesting chemical information stored on M. furnieri otoliths and its life cycle can be studied using the approach presented here.