Percentage of methylmercury in the muscle tissue of freshwater fish varies with body size and age and among species

Abstract

It is commonly assumed that most (>95%) of the mercury (Hg) found in fish muscle is the toxic form, methylmercury (MeHg), due to its efficient assimilation and retention in biotic tissue. However, this assumption is largely based on studies examining the percentage of MeHg (%MeHg [the fraction of total Hg as MeHg]) in muscle from mostly large-bodied predatory fish; less is known about the %MeHg in smaller bodied individuals or those of different trophic guilds. The present study analyzed MeHg and total Hg concentrations in the muscle of 2 large-bodied piscivores (walleye and northern pike), one large-bodied benthivore (white sucker), and 2 small-bodied forage fish (sculpins and shiners) across a broad size range. We found substantially lower %MeHg than the commonly assumed 95% in several fish (e.g., 17 individuals had <70% MeHg). Muscle %MeHg significantly increased with size and age in all species except walleye, which had significantly higher %MeHg than pike or suckers, particularly in smaller and younger fish (e.g., 18-21% higher at 10 g; 5-11% higher at 500 g). Results of predictive modeling suggest that muscle %MeHg is higher in pelagic-feeding fish and those with lower lipid content, although model results varied significantly among species. According to our findings, total Hg measurement in muscle is not an appropriate proxy for MeHg in smaller fish from all species, an important consideration for future piscine Hg studies and monitoring. Environ Toxicol Chem 2018;37:2682-2691. © 2018 SETAC.

Mislabelling and high mercury content hampers the efforts of market-based seafood initiatives in Peru

“…Duplicate samples were analysed every 20 samples, and the mean relative difference percentage was 9.23% ± 8.12% (range 0.04-28.22%; n = 20) ( Supplementary Table S4). Total mercury (THg) was used as a proxy for methylmercury (MeHg) since previous studies have found that most of the THg in fish tissue is MeHg (i.e., > 95% 54 ; 83.5 ± 19.7% 35 ; 68.1-88.8% 34,36 ). The assumption that > 90% of THg is MeHg applies to predatory species but does not necessarily apply to young fish and to fish from lower trophic levels where the percentage of MeHg is lower 35 .…”

“…Total mercury (THg) was used as a proxy for methylmercury (MeHg) since previous studies have found that most of the THg in fish tissue is MeHg (i.e., > 95% 54 ; 83.5 ± 19.7% 35 ; 68.1-88.8% 34,36 ). The assumption that > 90% of THg is MeHg applies to predatory species but does not necessarily apply to young fish and to fish from lower trophic levels where the percentage of MeHg is lower 35 . Thus, assuming that > 90% of THg is MeHg for lower trophic level fish (i.e., Flathead Grey Mullet) would overestimate the risk to consumers.…”

“…Most of these "best choice" species are fish at lower trophic levels which are expected to have lower levels of THg. The percentage of MeHg in THg has also been found to be lower in fish at lower trophic levels such as forage fish [34][35][36] . Nevertheless, the "best choice" list includes eight species of predatory fish that are high in mercury ( Supplementary Table S3).…”

“…Often, fish are in display cases, already cut into pieces, or cooked. These size recommendations also do not consider that THg and the percentage of MeHg in muscle often increase with the size and age of the fish 35 . A better strategy would be to simply remove predatory species that are expected to be high in mercury from these lists.…”

Food frequency questionnaire assessing traditional food consumption in Dene/Métis communities, Northwest Territories, Canada

“…For the findings from the focus group data in Table 2, food consumption was grouped for the fish, land animal and bird categories by preparation type (cooked; smoked), and by type of fish in the food chain (predatory, non-predatory), potentially having an impact on the dose reconstruction of mercury exposure as predatory fish generally have mercury levels that are higher than those in non-predatory fish [22]. Cooked fish tended to be more frequently consumed than smoked fish and the meats from wild game were more frequently consumed than fish and game birds.…”