In summary, we estimated the global amount of Hg exported from oceans via marine fisheries and the human exposure to Hg from seafood consumption worldwide. We used a database of Hg in marine seafood, which was then paired to biomass: (1) marine catches in the case of Hg export and (2) marine fish available for food consumption per capita in 175 countries in the case of human exposure.

Hg export via fisheries

The annual catch of marine fisheries from 1950 to 2014 was determined using the software FishStatJ from the FAO33. Catches were calculated by MFA and categorized into 41 groups based on the International Standard Statistical Classification of Aquatic Animals and Plants (ISSCAAP17,33; Supplementary Table 3). Inland waters catches as well as marine plant, coral, sponge, and pearl catches were excluded from the analysis. Catch values are reported in metric tonnes (1,000 kg) in the FAO database, except for 3 ISSCAAP groups: “Blue-whales, fin-whales” (baleen whales or mysticetes), “Sperm-whales, pilot-whales” (toothed whales or odontocetes) and “Eared seals, hair seals, walruses” (pinnipeds), which are reported in annual capture of individuals rather than by mass. A mean mass equivalence of 43, 1.29 and 0.231 tonnes were respectively used to transform values from number of individuals into tonnes for these groups (see Supplementary Table 5 for a literature review of mass values and references).

Mercury concentrations were assigned to each of the 41 ISSCAAP groups. Concentrations (mainly from muscle tissues) were derived primarily from a modified version of the online Seafood Hg Database from Karimi and colleagues34 available online: http://www.stonybrook.edu/commcms/gelfond/fish/database.html. Hg concentrations from market fish were excluded from the Seafood Hg Database because their origin was unknown. Hg concentrations of farmed fish from aquaculture were also excluded from the Seafood Hg Database. Moreover, farmed fish were not taken into consideration in terms of biomass in the calculation of Hg exported. The reason is that they represent a net null Hg budget when comparing Hg exported from vs. imported into the system (i.e., Hg accumulated during their growth period and exported from the system via capture is equal to Hg introduced to the system by feeding those fish during that period). Including farmed fish from the aquaculture sector could also lead to a double counting of Hg transport since fish feed often contain seafood products taken from the ocean (e.g., fish meal or oil)35,36. In addition to the Seafood Hg Database34, we searched the scientific literature to find Hg values for missing ISSCAAP groups, including some marine mammals, turtles, and invertebrates. An additional 2,952 data points from 68 taxa in 40 studies was added to the Seafood Hg Database34, which resulted in a total sample size of 52,224 from 416 taxa taken from 260 studies. This data was used to pair Hg concentrations to annual catches for each ISSCAAP group (see Supplementary Table 3 for a literature review of Hg concentrations and references). Mean THg concentrations in fish muscle for each ISSCAAP group were transformed into whole body concentrations according to the equation in Peterson and colleagues37:

$${{\rm{l}}{\rm{o}}{\rm{g}}}_{10}[whole\,fish\,Hg]=({{\rm{l}}{\rm{o}}{\rm{g}}}_{10}[muscle\,Hg]-\,0.2545)/1.0623$$ (1)

For whales, tissue Hg concentrations were converted to whole body concentrations using several tissues of odontocetes38 and mysticetes39 multiplied by the proportion of each tissue mass to the whole body mass40:

$$Whole\,whale\,Hg={\rm{\Sigma }}({C}_{i}\times {M}_{i})$$ (2)

where C i is the mean Hg concentration (μg g−1 w.w.) and M i is the mean mass (in tonne) of the ith tissue. For odontocetes, muscle, bone, and viscera concentrations were used to calculate whole body concentrations. For mysticetes, muscle, kidney, liver, spleen, and lung concentrations were used. For pinnipeds, we used the estimate of Yamamoto and colleagues41 who measured Hg in 15 tissues and calculated a whole body burden. For invertebrates, reported concentrations were assumed to be equal to whole body concentrations.

Weighted means (Hg w ), standard deviations (SD w ; as calculated in Karimi and colleagues)34, and 95% confidence intervals weighted for sample size (CI w ) were calculated for each ISSCAAP group to estimate error for the total Hg exported from the ocean (Supplementary Fig. 1). Mean THg concentrations in whole organisms from each ISSCAAP group were multiplied by the annual catch of each group to estimate the THg exported by marine fishing. Coastal THg removal by marine fisheries was estimated by removing oceanic taxa (58 epipelagic species and 62 deep-water species)42 as their captures were made outside the continental shelf area (considered to be between 0 and 200 m in depth)42,43.

To estimate the MeHg export by fisheries, we used percent THg as MeHg values for whole organisms rather than for edible portions. Consequently, %MeHg values are lower than the generally accepted 95% for edible portions. We used a %MeHg of 58% for whole fish (n = 39)44, 46% for whole invertebrates (n = 462)44,45,46, and 39% for various tissues of turtles (n = 9)47. For marine mammals, we estimated the whole body burden of MeHg based on equation (2) using published %MeHg48,49 resulting in an average of 19% of THg as MeHg (n = 206). Trophic levels for each ISSCAAP group were obtained from FishBase, an online aggregate database20. The annual Hg export by marine fishing was calculated for each MFA and for the entire ocean from 1950 to 2014. Averages for each MFA for periods 1950–1969, 1970–1989, 1990–2009, and 2010–2014 are shown in Fig. 2 and standard deviations are shown in Supplementary Fig. 4. A recent meta-analysis by Bonito and colleagues50 on marine fish found no spatial differences in Hg concentrations among oceans (East Pacific Ocean, West Pacific Ocean, Atlantic Ocean, Indian Ocean and Mediterranean Sea) for low predators (n = 65), mid predators (n = 417) and top predators (n = 276) or for all fish taken together (n = 795). A difference was only found for herbivore fish (n = 37). We therefore assigned Hg concentrations to all ISSCAAP without including a spatial component. The meta-analysis from Bonito and colleagues50 showed a decline in Hg concentrations in fish between 1969 and 2012 of 0.001 μg g−1 a−1 (converted from a slope of −0.01 ng g−1 a−1 on a log scale; R2 = 0.03, n = 2,662). Over a period of 64 years in our study, this indicates that mean fish Hg concentrations were approximately 0.062 μg g−1 higher in 1950 compared to 2014. However, we warrant caution on the interpretation of this trend given the low effect size of the published equation (R2 = 0.03) and consequently the uncertain predictive value. The Hg export estimates presented in our study were therefore not corrected for this potential time trend and are accurate for recent years but could be an underestimate for earlier years.

We tested for a significant change in slope in the time series of exported Hg and for the occurrence of a breakpoint (i.e., change in trend) using segmented regressions with the package segmented51,52 using the software R53. The Davies test was used to test for a non-zero difference-in-slope parameter of the segmented regression relationship54. Regression tests were two-tailed.

Seafood consumption by country

Risk of Hg intake is to MeHg, which is the bioaccumulative and toxic form of Hg. We evaluated potential mean MeHg per capita weekly intake (WI) of marine fish and seafood for each country in this study (μg kg of body mass (BM)−1 week−1) between 1961 and 2011 using the equation:

$$W{I}_{j}={{\rm{\Sigma }}}_{ij}(I{R}_{ij}\times {C}_{i})/B{M}_{k}/52$$ (3)

where WI j is the potential per capita weekly intake of country j, IR ij is the intake rate (kg capita−1 year−1) of a specific taxonomic group i per capita for country j, C i is the mean MeHg concentration of taxonomic group i, and BM k is the average human body mass for continent k. IR ij was estimated using data for marine fish and seafood supply which was available for consumption (kg capita−1 year−1) from the FAO22 for each country from 1961 to 2011 (years when commodity balance sheets were complete). There are currently 193 United Nations member states, but fish and seafood supply data were not available for all countries during the studied period. In 1961, data were only available for 139 countries, however the number of reporting nations gradually increased to 174 in 2011. The available food supply is calculated by the FAO by adding importations to production (i.e. catches and aquaculture) and by removing exportations, non-food uses, and waste22. Per capita food supply was used here as a proxy for per capita food consumption55 and the WI calculated here are therefore estimates of the potential intake (food available per capita for each country) rather than the actual intake (food consumed per capita for each country); for this reason, we use the term “potential” WI. C i was estimated using the weighted mean of MeHg (MeHg w ; Supplementary Table 6), for each available marine taxonomic group. Over 95% of THg in edible fish parts (e.g., muscle) was assumed to be in the MeHg form45,56. MeHg w of each available marine taxonomic group (“Cephalopods”, “Crustaceans”, “Demersal Fish”, “Other Marine Fish”, “Other Molluscs”, and “Pelagic Fish”; Supplementary Table 6) was multiplied by IR ij of that group to estimate the WI for each country. A different average BM of human population was used for each continent: Asia = 57.7 kg, Africa = 60.7 kg, Latin America and the Caribbean = 67.9 kg, Europe = 70.8 kg, Oceania = 74.1 kg, and North America = 80.7 kg57. We compared the calculated WI to the provisional tolerable weekly intake (PTWI) of 1.6 μg kg−1 week−1 for methylmercury24. We reported average WI (±SD) for the periods 1961–1970, 1971–1980, 1981–1990, 1991–2000, and 2001–2011.

Data availability

The authors declare that the data supporting the findings of this study are available within the article and the Supplementary Information. Time series fishery data and available food supply are available at the FAO website (http://www.fao.org). The Hg database used to pair fisheries data to taxa-specific Hg concentrations can be found in Supplementary Table 3 and at the author’s website (http://www.stonybrook.edu/commcms/gelfond/fish/database.html)34. Additional data supporting the findings of this study are available from the corresponding author upon reasonable request.