Temporal and spatial variation in strontium in a tropical river: Implications for otolith chemistry analyses of fish migration

Analysis of otolith strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) is an increasingly utilized approach for studying fish migration. We analysed surface and ground water from the Daly River catchment in the wet–dry tropics of northern Australia over 2 years. Analyses of otolith⁸⁷Sr/⁸⁶Sr ratios were also conducted for freshwater sooty grunter (Hephaestus fuliginosus) and the putatively diadromous diamond mullet (Liza ordensis). Spatial variation in freshwater⁸⁷Sr/⁸⁶Sr was high (range: 0.71612–0.78059), and there was strong seasonality in water⁸⁷Sr/⁸⁶Sr, with highest values in the wet season. Temporal variation in water⁸⁷Sr/⁸⁶Sr ratios is attributed to seasonal patterns in surface runoff from geological formations with radiogenic compositions versus input from groundwater aquifers interacting with less radiogenic formations. Temporal variation in water⁸⁷Sr/⁸⁶Sr ratios precluded robust inference on movement within fresh water for both species, although movement across salinity gradients by diamond mullet was clearly identified. We conclude that temporally and spatially replicated water Sr data should be a general requisite for studies that analyse otolith Sr (⁸⁷Sr/⁸⁶Sr, Sr/Ca, Sr/Ba) to make inferences about fish movement and migration.