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

Abstract

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 otolith 87Sr/86Sr ratios were also conducted for freshwater sooty grunter (Hephaestus fuliginosus) and the putatively diadromous diamond mullet (Liza ordensis). Spatial variation in freshwater 87Sr/86Sr was high (range: 0.71612–0.78059), and there was strong seasonality in water 87Sr/86Sr, with highest values in the wet season. Temporal variation in water 87Sr/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 water 87Sr/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.

Quantifying 87Sr/86Sr temporal stability and spatial heterogeneity for use in tracking fish movement

“…We observed higher site specific ⁸⁷Sr/⁸⁶Sr seasonal variation than site specific ⁸⁷Sr/⁸⁶Sr year to year variation, as was also seen by Kennedy et al (2000) and Crook et al (2017). But the degree of temporal stability varied by orders of magnitude from stable to unstable across watersheds.…”

“…The largest seasonal difference we observed was 0.00318, in French Creek. The largest seasonal difference Crook et al (2017) reported was 0.05155 on the Edith River in the wet-dry tropics of northern Australia. Crook et al (2017) suggested changes in the source water (ground vs. runoff) between the two extreme wet-dry seasons likely drove observed extreme seasonal variability.…”

Do upstream migrating, juvenile amphidromous shrimps, provide a marine subsidy to river ecosystems?

“…Water 87 Sr/ 86 Sr ratio collected during the dry and wet season along an upstream gradient (water 87 Sr/ 86 Sr ratio data redrawn from Crook et al. ())…”

“…Water 87 Sr/ 86 Sr data were obtained from samples collected as a part of a separate project (Crook et al., ) along an upstream gradient from the river mouth to 405 km upstream. Samples were collected once during the wet season (eight sites, between the 5 and 13 March 2013) and once during the dry season (12 sites, between the 26 September and 7 November 2012).…”

“…Samples were collected once during the wet season (eight sites, between the 5 and 13 March 2013) and once during the dry season (12 sites, between the 26 September and 7 November 2012). Sr isotope analyses were carried out on a Nu Plasma multi‐collector ICPMS (Nu Instruments, Wrexham, UK) interfaced with an ARIDUS desolvating system operated by the University of Melbourne (detailed methods outlined in (Crook et al., )).…”