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Strontium : calcium (Sr : Ca) ratios were measured in sagittal otoliths of silverside (Odontesthes bonariensis) from a freshwater–marine environment (Paraná River Delta and Río de la Plata River, South America) in order to understand its seasonal movements. Conductivity was recorded and the water Sr : Ca ratio was determined by inductively coupled plasma–atomic emission spectrometry (ICP-OES). A calibration curve of water Sr : Ca ratio versus otolith Sr : Ca ratio from four isolated lentic water bodies was made to predict silverside displacements in the lower section of the Plata Basin during summer (lotic water bodies). Otolith Sr : Ca ratio of silversides from the Paraná River Delta was associated with the conductivity of water at the locations where the sample was collected. According to this association and the obtained results in the calibration curve, the silversides were displaced from zones where conductivity was 7.45 (mS cm–1), corresponding to the water Sr : Ca ratio in the outer section of the Río de la Plata River (1.92 mmol mol–1). The high values of otolith Sr : Ca ratio found for some fish may indicate that during summer they moved from estuarine waters where salinity increases gradually to the Argentinian Sea.

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