Marine migration influences mercury accumulation in anadromous Arctic char (Salvelinus alpinus)

Mercury (Hg) is a globally distributed toxic metal present in northern natural environments that biomagnifies in food webs and bioaccumulates in apex predator fish, such as Arctic char (Salvelinus alpinus)-an iteroparous species with a circumpolar distribution. Arctic char exhibit a variety of life history types, with the migrating, anadromous life-history type supporting many subsistence and small-scale commercial fisheries in the Arctic and sub-Arctic. Despite anadromous Arctic char having generally lower Hg concentrations than non-anadromous conspecifics, the mechanisms that contribute to significant inter-individual differences within this life-history group have not been well investigated. A total of 119 anadromous Arctic char were captured from inner Frobisher Bay on Baffin Island near Iqaluit, Nunavut, in 2018 and 2019, to investigate causes of inter-individual differences in Hg concentrations in the anadromous life-history type of Arctic char. As hypothesized, Hg concentrations significantly decreased with increasing marine prey reliance; marine prey reliance-quantified using δ³⁴S ratios-explained 93% of the variability in Hg. Arctic char that relied more on marine (vs freshwater) prey had lower Hg concentrations. Delta³⁴S ratios and average somatic growth rate (size-at-age) were the best predictors of Hg concentrations among the set of biological variables assessed, and both were negatively related to Hg concentrations in common age groups (i.e., ages 9-13). For samples that had migration data available, Hg concentration increased with age-at-first migration (R² = 0.32) and decreased with number of marine migrations (R² = 0.57), suggesting that plasticity in life history tactics affects Hg concentrations in anadromous Arctic char. Overall, results indicate that differences in habitat use and feeding environment (i.e., freshwater vs. marine) play a significant role in determining Hg bioaccumulation in anadromous Arctic char.