Elevated Mercury Concentrations and Isotope Signatures (N, C, Hg) in Yellowfin Tuna (Thunnus albacares) from the Galápagos Marine Reserve and Waters off Ecuador

We examined how dietary factors recorded by C and N influence Hg uptake in 347 individuals of yellowfin tuna (Thunnus albacares), an important subsistence resource from the Galápagos Marine Reserve (Ecuador) and the Ecuadorian mainland coast in 2015–2016. We found no differences in total Hg (THg) measured in red muscle between the two regions and no seasonal differences, likely due to the age of the fish and slow elimination rates of Hg. Our THg concentrations are comparable to those of other studies in the Pacific (0.20-9.60 mg/kg wet wt), but a subset of individuals exhibited the highest Hg concentrations yet reported in yellowfin tuna. Mercury isotope values differed between Δ¹⁹⁹Hg and δ202Hg in both regions (Δ¹⁹⁹Hg = 2.86 ± 0.04‰ vs. Δ¹⁹⁹Hg = 2.33 ± 0.07‰), likely related to shifting food webs and differing photochemical processing of Hg prior to entry into the food web. There were significantly lower values of both δ¹⁵N and δ¹³C in tuna from Galápagos Marine Reserve (δ¹⁵N: 8.5–14.2‰, δ¹³C: −18.5 to −16.1‰) compared with those from the Ecuadorian mainland coast (δ¹⁵N: 8.3–14.4‰, δ¹³C: −19.4 to −11.9‰), of which δ¹³C values suggest spatially constrained movements of tuna. Results from the pooled analysis, without considering region, indicated that variations in δ¹³C and δ¹⁵N values tracked changes of Hg stable isotopes. Our data indicate that the individual tuna we used were resident fish of each region and were heavily influenced by upwellings related to the eastern Pacific oxygen minimum zone and the Humboldt Current System. The isotopes C, N, and Hg reflect foraging behavior mainly on epipelagic prey in shallow waters and that food web shifts drive Hg variations between these populations of tuna. Environ Toxicol Chem 2022;41:2732–2744.