Trophic pathways and spatial variation of mercury in Pacific tunas (Thunnus)

Large pelagic predators in the marine environment accumulate relative high levels of mercury (Hg). It is difficult to identify the ecological and oceanographic factors that control the fate and flow of contaminants in these systems, in part due to the difficulty in experimenting with large, dynamic ecosystems. This dissertation addresses critical gaps in understanding how mercury varies within and between tuna species at individual, food web, and oceanic regional scales. Chapter one explored whether the Hg concentrations of bigeye (Thunnus obesus) and yellowfin (T. albacares ) tunas vary regionally in the eastern and central Pacific Ocean and whether this variation corresponds to environmental characteristics that promote the bioavailability of Hg. Of the five regions sampled, I found significantly higher Hg concentrations in the eastern equatorial region (5°S - 5°N; 110°W - 120°W) with concentrations elevated by 0.22mug·g -1 and 0.17mug·g-1 for yellowfin and bigeye respectively, compared to the other regions. Chapter two explores the use of contaminants to estimate consumption rates of tunas by applying statistical estimation methods to coupled bioenergetics -- Hg mass balance models. The statistical estimation approach generated biologically plausible daily consumption rates (yellowfin: 6%-9.5%, skipjack (Katsuwonus pelamis ) : 4.5%-6.7%, bigeye: 8.7%-12% body weight for a 10kg tuna), swimming speeds, and slopes of the relationship between concentrations of prey Hg and tuna size when the concentration of prey for a mean sized tuna was assumed to be known. Chapter three developed coupled contaminant-trophic mass balance models to describe the fate and flow of Hg through the eastern tropical Pacific (ETP) and central north Pacific pelagic food webs. I estimated Hg concentrations or Hg-elimination rates for species in each region finding generally higher concentrations in the ETP model. Regional differences of Hg concentrations in tuna can be explained by differences in diets of tuna and their prey, although changes in Hg at the base of the food web can strongly influence Hg concentrations in top predators as well. Hg concentrations in tunas vary due to differences in diet and bioavailability of Hg at the base of the food web, and can be used as chemical tools to better understand physiological traits.