Use of otolith strontium:calcium ratio as indicator of seasonal displacements of the silverside (Odontesthes bonariensis) in a freshwater-marine environment

Abstract

Strontium:calcium ratios were measured in sagittal otoliths of silverside (Odontesthes bonariensis) from a freshwater-marine environment (Paraná River Delta and River 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 ICP-OES (sites: Paraná River Delta, Concepción del Uruguay, Punta Indio, Samborombón Bay and Punta Rasa, Argentina, and Fray Bentos, Uruguay). A calibration curve of water Sr:Ca ratio vs. otolith Sr:Ca ratio from 4 isolated lentic water bodies was made to predict the water Sr:Ca ratio of the places of 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 (lotic water bodies). The water Sr:Ca ratio increased with increasing conductivity along a latitudinal (north-south) gradient in the lower section of the Plata Basin, with a rapid increment in the outer section of the Río de la Plata River. According to this association and the obtained results in the calibration curve, the silverfish were displacements were in 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.