Variability in Sr:Ca and Ba:Ca ratios in water and fish otoliths across an estuarine salinity gradient

Retracing movements of euryhaline fishes using otolith chemistry analysis is a complex task, requiring detailed knowledge of the relationships between otolith and water chemistry for the target species and how these relationships change across dynamic environmental gradients in estuaries. In a controlled laboratory experiment, we quantified relationships between salinity and Sr:Ca and Ba:Ca ratios in ambient water and otoliths in euryhaline Australian bass Macquaria novemaculeata across a salinity gradient encompassing freshwater to marine values (i.e. 0.5, 5, 21 and 38‰). Results from the laboratory experiments in addition to analyses of water samples from 7 coastal rivers in Victoria, Australia, showed consistent positive (Sr:Ca) and negative (Ba:Ca) non-linear relationships between salinity and otolith/water chemistry, with the greatest change in elemental concentrations occurring at <5‰. Otolith Sr:Ca increased linearly with ambient values, while otolith Ba:Ca increased exponentially. Uptake of Sr and Ba into the otoliths varied with salinity, with partition coefficients (DSr and DBa) maximised at the lowest and highest salinities, respectively. Otolith transects revealed high individual variability in the timing and magnitude of Sr:Ca and Ba:Ca responses following transfer of bass from higher salinities to the lowest salinity (0.5‰). Substantial time-lags in elemental uptake were also evident, with Sr:Ca taking ≤40 d and Ba:Ca ≤30 d for concentrations to reach equilibrium at 0.5‰. Whilst otolith Sr:Ca and Ba:Ca are clearly useful as indicators of movement across coarse salinity gradients, our data suggest that the addition of alternative chemical markers to augment measurement of trace element concentrations is needed to improve resolution at finer scales.