Using otoliths to understand historic and contemporary selenium exposure in Kokanee Salmon (Oncorhynchus nerka) in Kluane National Park and Reserve, Yukon Territory.

The Kokanee salmon population in Kluane National Park and Reserve (Yukon Territory) declined significantly between 2002 and 2012. Elevated levels of selenium (Se), which can affect fish reproduction, were recently measured in waters used by spawning Kokanee. To investigate whether Se may be contributing to long-term population declines, and in the absence of long-term data on aqueous Se concentrations, Se concentrations in sagittal otoliths - fish ear-bones that chronicle lifetime exposure - were measured and compared in Kokanee sampled in 1981 (a year with many spawners) and 2019-2021. Post-spawn Kokanee in 2019-2021 were also sampled for dorsal muscle and eggs. Selenium concentrations in otoliths indicate that Kokanee experience greater exposure to Se in the final year of life (represented by the distal otolith region) than in the first year of life (p < 0.001). Furthermore, 2021 Kokanee had greater Se concentrations in the distal otolith region, implying greater end-of-life exposure, compared to 1981 Kokanee (p < 0.01). While Se concentrations in the distal otolith region were positively related to Se concentrations in muscle tissue (p < 0.0001) and eggs (p < 0.01), the former was also affected by inter-annual differences in muscle lipid content, and thus historic Se exposure in the 1981 population could not be accurately estimated from otolith Se concentrations alone. Mean muscle Se concentrations increased from 2019 to 2021 (p < 0.001), and ∼30% of Kokanee across all years (n = 43) had Se levels exceeding muscle and/or egg tissue guidelines for the protection of aquatic biota. This is one of the first studies to relate Se concentrations in otoliths and soft tissues, the latter of which are an important endpoint for assessing Se toxicity. Our study demonstrates that otoliths can be a powerful tool for reconstructing Se in fish populations where historical data are limited, if inter-annual differences in the otolith-soft tissue relationship can be quantified.