Iodine content of fish otoliths in species found in diverse habitats

Expansion of aquatic deoxygenation has altered the quality and availability of habitats and worsened body condition for many fish species through past decades. Identifying complementary chemical redox proxies in fish otoliths, in addition to Mn/Ca, would strengthen the ability to identify hypoxia exposure in a diversity of aquatic habitats. I/Ca ratios have been used in marine sediments and bio-mineralized materials for reconstruction of past redox conditions. In order to explore influences from various endogenous and exogenous processes, a large data set of I/Ca ratios from cores of otoliths, including fishes from fresh, estuarine, and coastal waters across 30 species within 10 taxonomic orders, were reported with other chemical proxies (Mn/Ca and Sr/Ca) using LA-ICP-MS. Our results suggest no single abiotic factor, including redox condition and salinity derived from Mn/Ca and Sr/Ca, predicts I/Ca values, while they may be correlated for specific fish species. Iodine may be related to organic matter in the cores of otoliths for some species. Maternal transfer, habitat change, dietary source, phylogeny, and ecology may also influence I/Ca ratios. Based on these exploratory results, we suggest a range of future research directions to further evaluate the factors controlling biomineralization of otolith iodine and its utility as a redox proxy.