Assessing Mercury Risks for the Optimization of Nutrient Benefits from Wild-harvested Fish Consumption in the Northwest Territories, Canada

Abstract

Background: Fish are often rich in essential nutrients, such as omega-3 fatty acids (n-3 FAs) and selenium (Se), and thus can promote health. However, methylmercury (MeHg), a contaminant found in many species in the Canadian subarctic, can pose potential health risks. Fish consumption is important to traditional diets because these foods have nutritional, social, and cultural benefits. Objectives: The objectives for this study are to: 1) determine the concentrations of mercury (Hg), n-3 FAs, and Se in various freshwater fish species harvested from three lakes in the Dehcho Region, Northwest Territories (NWT); 2) evaluate the correlations between nutrient and Hg concentrations; 3) identify which fish species have the highest nutrient levels relative to their Hg content; 4) utilize a probabilistic optimization software (Crystal Ball’s OptQuest) to inform dietary recommendations that mitigate risks of Hg exposure and promote human health from traditional food consumption. Methods: Samples from seven freshwater fish species [Burbot (Lota lota), Cisco (Coregonus artedi), Lake Trout (Salvelinus namaycush), Lake Whitefish (Coregonus clupeaformis), Longnose Sucker (Catostomus catostomus), Northern Pike (Esox Lucius), and Walleye samples (Sander vitreus)] were harvested in August 2013 from Ekali, Sanguez, and Trout Lakes in the NWT. The laboratory analysis for Hg involved freeze drying the tissue muscle samples prior to analysis and quantifying Hg in the samples by a direct mercury analyzer; n-3 FAs levels were determined by a lipid extraction on pulverized fish tissue and measured by a gas chromatograph with a flame ionization detector; and Se levels were determined by a tissue digestion and measured by an inductively coupled plasma mass spectrophotometer. Thereafter, a probabilistic optimization method was assessed using OptQuest, a feature in Crystal Ball (Oracle). Results: The average total Hg (HgT) concentrations varied among fish species according to their trophic guild, from 0.057 mg kg-1 for benthivorous/planktivorous species to 0.551 mg kg-1 for predatory species. There were also substantial differences in n-3 FA composition among fish species, with averages ranging from 101 mg/100 g for Burbot to 1,689 mg/100 g for Lake Trout. However, in contrast to the HgT and n-3 FA results, average Se concentrations were relatively similar among species, ranging from 0.140 mg kg-1 for Northern Pike to 0.195 mg kg-1 for Walleye. Strong, negative correlations were observed in HgT concentration and nutrient content in the muscle tissue in particular fish species, including Lake Trout, Northern Pike, Walleye, Lake Whitefish, and Cisco. These analyses indicated that Lake Whitefish, Cisco, and Longnose Sucker had the highest nutrient levels relative to their HgT content. For the OptQuest probabilistic model, the term best solutions refers to the optimum food choices that maximize nutrient intake, while also limiting the upper percentile of Hg exposure from the consumption of freshwater fish. The total amount of fish within the best solutions from the OptQuest model for the women of child-bearing age were 546 and 1,359 g/week to achieve nutritional adequacy for EPA+DHA and Se, respectively, while also limiting the upper percentile for the toxicological reference values (TRVs) for Hg exposure. Whereas, the total amount of fish within the best solutions for the general population age were 851 and 1,848 g/week, to achieve nutritional adequacy for EPA+DHA and Se, respectively, while also minimizing Hg exposure. The models indicated that the consumption of Burbot, Cisco, Lake Whitefish, and Longnose Suckers would most help people achieve nutritional adequacy without exceeding the TRVs for Hg. Conclusion: Some fish species are particularly rich sources of nutrients; however, Hg concentrations occasionally exceed Health Canada’s retail Hg guidelines. My research shows that people can frequently consume fish that are lower in the food chain without exceeding the Hg TRV. Larger, long-lived predatory fish species also offered nutritional benefits in terms of n-3 FAs and Se; however, some limits on intake are necessary to avoid the adverse effects of Hg. The nutrient:Hg ratios and OptQuest model approaches that were utilized to determine the optimal food choices that achieve nutritional sufficiency, while not exceeding the Hg TRV yielded different answers. Future research will be necessary to determine which of these approaches, if either, yield the most useful information for promoting healthy eating in ways that balance nutrient benefits and contaminant risks.