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

Abstract

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. © 2022 SETAC

Geographic Drivers of Mercury Entry into Aquatic Food Webs Revealed by Mercury Stable Isotopes in Dragonfly Larvae

“…Across all ecoregions and waterbody types within this study, dragonfly δ 202 Hg values spanned from −2.62 to 0.66‰ (Figure S5 and Table S5). These values encompass most of the known range of Hg isotope values in fish tissues for unimpacted forested, marine, − and large freshwater ,, ecosystems across the United States. To assess if this isotope range in dragonfly larvae was associated with proximity to Hg emission sources, we compared δ 202 Hg estimated marginal means between the eastern and western U.S (Figure and Table S6).…”

Analysis of Toxic Metals Found in Shark Fins Collected from a Global Trade Hub

“…Methylmercury (MeHg), the organic form of mercury, has been shown to account for between 90 and 100% of the total mercury found in the muscles of fish, ,, and can make up 40–80% of the MeHg found in dried shark fin. , Previous work has shown that phytoplankton at the base of food chains is an important MeHg entry point where bioaccumulation can rapidly occur through the food chain. ,, Pelagic and coastal food webs are distinct, pelagic food webs are dependent upon phytoplankton production, whereas coastal food webs tend to rely upon benthic producers . The higher concentrations of Hg we observed in pelagic shark species could be a consequence of phytoplankton at the base of pelagic food chains and the subsequent biomagnification of toxic metals at higher tropic levels, especially as phytoplankton are known MeHg concentrators .…”

“…Methylmercury (MeHg), the organic form of mercury, 51 has been shown to account for between 90 and 100% of the total mercury found in the muscles of fish, 5,52,53 and can make up 40−80% of the MeHg found in dried shark fin. 2,54 Previous work has shown that phytoplankton at the base of food chains is an important MeHg entry point where bioaccumulation can rapidly occur through the food chain.…”

“…Apex predators, such as sharks and tuna are particularly susceptible to the biomagnification and bioaccumulation of toxic metals within their tissues. − These naturally occurring, toxic metal elements are becoming increasingly common in marine ecosystems as human activities (e.g., the burning of fossil fuels, the refining of metal ores, or through their use in electrical components) release ever more emissions directly into the atmosphere, terrestrial, aquatic, or marine ecosystems . Shark products, especially the dried fins used in shark fin soup throughout Asia, are frequently consumed as luxury goods and are reputed to have numerous health benefits and life extending qualities .…”

Multiple anthropogenic stressors in the Galápagos Islands' complex social–ecological system: Interactions of marine pollution, fishing pressure, and climate change with management recommendations

“…Similarly, THg and methylmercury (MeHg), the main biomagnifying mercury form found in marine fish, was detected (ranging from 0.20 to 9.60 mg/kg wet weight) in yellowfin tuna (Thunnus albacres) taken in Galápagos waters (Figure 3; see Muñoz-Abril et al, 2022). Methylmercury is formed in high quantities in oxygen minimum zones (OMZ), in which upwellings are the main cause of oxygen decrease (Houssard et al, 2019), similar to those occurring in Galápagos and the nearby northern Humboldt Current Ecosystem in the southeastern tropical Pacific Ocean (Chavez et al, 2008;Fuenzalida et al, 2009).…”