Quantifying mercury isotope dynamics in captive Pacific bluefin tuna (Thunnus orientalis)

Analyses of mercury (Hg) isotope ratios in fish tissues are used increasingly to infer sources and biogeochemical processes of Hg in natural aquatic ecosystems. Controlled experiments that can couple internal Hg isotope behavior with traditional isotope tracers (delta C-13, delta N-15) can improve the applicability of Hg isotopes as natural ecological tracers. In this study, we investigated changes in Hg isotope ratios (delta Hg-202, Delta Hg-199) during bioaccumulation of natural diets in the pelagic Pacific bluefin tuna (Thunnus orientalis; PBFT). Juvenile PBFT were fed a mixture of natural prey and a dietary supplement (60% Loligo opalescens, 31% Sardinops sagax, 9% gel supplement) in captivity for 2914 days, and white muscle tissues were analyzed for Hg isotope ratios and compared to time in captivity and internal turnover of delta C-13 and delta N-15. PBFT muscle tissues equilibrated to Hg isotope ratios of the dietary mixture within similar to 700 days, after which we observed a cessation in further shifts in Delta Hg-199, and small but significant negative delta Hg-202 shifts from the dietary mixture. The internal behavior of Delta Hg-199 is consistent with previous fish studies, which showed an absence of Delta Hg-199 fractionation during Hg bioaccumulation. The negative delta Hg-202 shifts can be attributed to either preferential excretion of Hg with higher delta Hg-202 values or individual variability in captive PBFT feeding preferences and/or consumption rates. The overall internal behavior of Hg isotopes is similar to that described for delta C-13 and delta N-15, though observed Hg turnover was slower compared to carbon and nitrogen. This improved understanding of internal dynamics of Hg isotopes in relation to delta C-13 and delta N-15 enhances the applicability of Hg isotope ratios in fish tissues for tracing Hg sources in natural ecosystems.