Mercury stable isotopes reveal sources of mercury and prey in large (≥5 year old) Pacific bluefin tuna from the western North Pacific Ocean

Understanding the sources of mercury (Hg) to Pacific bluefin tuna (Thunnus orientalis, PBFT) is critical to predicting Hg accumulation in this high market value species and the risk of human exposure associated with tuna consumption. However, the contribution of various prey resources to Hg accumulation in edible PBFT tissues is unknown and is complicated by the broad migration patterns of this large predator across the North Pacific Ocean. To explore the prey species that are the dominant sources of Hg in PBFT, and the depths and locations in which they are consumed, we measured total mercury (THg, ~100% monomethylmercury) concentrations and Hg stable isotopes in 92 PBFT from the western Philippine Sea of the North Pacific Ocean ranging in age from 5 to more than 27 years (straight fork lengths of 182 to 266 cm). THg concentrations in PBFT muscle tissue varied from 0.80 to 4.96 μg g⁻¹ ww, (mean = 2.22±0.96 1SD) and increased significantly across three age classes (<10 years, 10 to 25 years, >25 years). δ²⁰²Hg and Δ¹⁹⁹Hg values also increased with age. We used a Hg isotope mixing model to estimate the relative contributions of epipelagic and mesopelagic prey from the western and eastern North Pacific Ocean to Hg accumulation in these fish. Model results indicate that epipelagic prey from the western North Pacific accounted for most (>60%) of the Hg accumulation in large PBFT, which did not vary significantly among age classes. Since PBFT accumulate all of their Hg from food, epipelagic prey from the western North Pacific also likely constitute the primary food source in these fish, consistent with a strategy to minimize migratory energy expenditure. These results suggest that the epipelagic food web of the western North Pacific is an important source of Hg and nutrition to large (≥5 year old) PBFT.