Relative contribution of water and diet to otolith chemistry in freshwater fish

Otolith chemistry is widely used to address key questions relating to fish ecology and fisheries management; however, there is limited research on the relative contributions of water and diet to elements within otoliths. This study explicitly tested the proportional contributions of water and diet in 3 Australian freshwater fish-silver perch, golden perch and Murray cod-in a controlled laboratory experiment. We independently 'spiked' both tank water and diet with enriched stable isotopes of strontium (Sr-86) and barium (Ba-137), key elements used in otolith chemistry. Hatchery-sourced fingerlings were used in the experiment and were independently exposed to a control, water-spiked or diet-spiked treatment for a period of 31 to 39 d. Otolith material laid down during the experiment was subsequently analysed for relevant isotopes using laser ablation inductively coupled plasma-mass spectrometry (LA ICP-MS), and water and diet samples were analysed using solution-based ICP-MS. An isotope mixing model was used to determine the relative contributions of water and diet to Sr and Ba otolith chemistry. For all species, water was the dominant source of elements in the otoliths, contributing between 64 and 71% for Sr and 88 and 92% for Ba. Diet contributed to a far lesser degree, with contributions ranging from 4 to 6% for Sr and 10 to 26% for Ba. The results from this study improve interpretations of otolith chemistry data in freshwater fish, thereby allowing more accurate reconstructions of the environmental and movement histories of wild populations.