Mercury contamination of oceans is prevalent worldwide and methylmercury concentrations in the mesopelagic zone (200–1000 m) are increasing more rapidly than in surface waters. Yet mercury bioaccumulation in mesopelagic predators has been understudied. Northern elephant seals (Mirounga angustirostris) biannually travel thousands of kilometres to forage within coastal and open-ocean regions of the northeast Pacific Ocean. We coupled satellite telemetry, diving behaviour and stable isotopes (carbon and nitrogen) from 77 adult females, and showed that variability among individuals in foraging location, diving depth and δ13C values were correlated with mercury concentrations in blood and muscle. We identified three clusters of foraging strategies, and these resulted in substantially different mercury concentrations: (i) deeper-diving and offshore-foraging seals had the greatest mercury concentrations, (ii) shallower-diving and offshore-foraging seals had intermediate levels, and (iii) coastal and more northerly foraging seals had the lowest mercury concentrations. Additionally, mercury concentrations were lower at the end of the seven-month-long foraging trip (n = 31) than after the two-month- long post-breeding trip (n = 46). Our results indicate that foraging behaviour influences mercury exposure and mesopelagic predators foraging in the northeast Pacific Ocean may be at high risk for mercury bioaccumulation.

Foraging behavior of northern elephant seals and mercury concentrations in blood and muscleWe quantified variables for the full foraging trip (short and long foraging trip) to describe foraging behavior of northern elephant seals, using geography, diving behavior, and stable isotopes (carbon and nitrogen). Most seals were weighed upon recovery. If not, then RecoveryMassEstimated was recorded as 1, meaning that the recovery mass was estimated using body composition and morphometric measurements. The ToppID is the unique ID for the individual and the foraging trip that would link with the full diving and tracking file, housed at the University of California, Santa Cruz. The Seal_ID is an ID that corresponds to each seal in this study and links with their individual histories, also housed at the University of California, Santa Cruz.Peterson_et_al_Dataset.csv