>Mercury in Fish Tissue

Mercury in Fish Tissue

Physical / Chemical
Maintain Water Quality

QT: Indicators

What is it?

Mercury is a trace metal of particular concern because it can exert negative toxic effects on humans and wildlife at low concentrations (Selin 2009). Methylmercury has been shown to cause massive neurological damage because of its ability to cross the blood-brain barrier, while inorganic forms can cause nephrotoxicity (Boelsterli 2007). Mercury primarily enters aquatic ecosystems in inorganic dissolved ionic or particulate form through wet and dry deposition from the atmosphere to water body surfaces, or via runoff from watersheds (Selin 2009). Although mercury occurs naturally in the environment, human activities such as gold mining and recovery, burning fossil fuels and waste, mining and smelting metals, and using mercury in products and industrial processes have added to the amount of mercury in the global environment. Ecosystems near point sources may be characterized by higher mercury concentrations, but due to global transport ecosystems that are far from any point sources may also have high mercury concentrations (Scheulhammer et al. 2007). In the Feather River Watershed, mercury was used in the late 1800's to recover gold mixed in sediments sluiced from mountain-sides during hydraulic mining. Mercury was literally poured into sluice boxes in order to trap small gold particles. The mercury was then burned away to recover the gold. Most of this mercury was lost in the sluicing and in the burning.

Why is it important?

Pollution is a major concern for all watersheds, especially those that have been heavily modified and urbanized. Biological and chemical contaminants have a wide range of sources and can exert negative effects on species, ecosystems, and humans (primarily via drinking water and fish consumption). Trace metals such as mercury are natural components of rocks and soil and can enter the aquatic environments as a consequence of weathering and erosion (Garrett 2000). However, following gold-mining in California and global industrialization, unnatural quantities of many elements, such as mercury, were and continue to be released into the riverine and coastal ecosystems, altering natural biological equilibrium (Haynes and Johnson 2000). Anthropogenic activities have altered both the distribution of metals in the environment as well as metal speciation, or biochemical form (Goyer and Clarkson 2001).

Biological magnification is the process by which toxins become more concentrated in organisms at each successive trophic level up the food chain. Due to the tendency for mercury to bioaccumulate and magnify up the food chain, organisms at higher trophic levels on the food chain and with long life spans tend to be at greatest risk (Scheulhammer et al. 2007). Mercury concentrations in the tissues of top predators can be over one million times greater than those in the water column (Selin 2009). Furthermore, the proportion of methylmercury to total mercury increases at each trophic level in the food chain; in predatory fish, almost 100% of the mercury is methylmercury (USEPA 1997, Morel et al. 1998).

The major pathway for human exposure to methylmercury is consumption of contaminated fish. Dietary methylmercury is almost completely absorbed into the blood and is distributed to all tissues including the brain; it also readily passes through the placenta to the fetus and fetal brain (USEPA 2001). Based on available data, human exposures to methylmercury from all media sources except freshwater/estuarine and marine fish are negligible. Therefore, the USEPA methylmercury water quality criterion is a concentration in fish tissue (USEPA 2001).

Data Sources and Analysis

All data for mercury concentrations in fish were obtained from the CVRWQB, who maintains a database of concentrations for all water bodies sampled in the Central Valley Region.

Included in this analysis are mercury concentrations in edible-sized fish caught at sites within the designated subwatershed boundaries. Though this data spans several decades, preliminary analyses determined that there were no trends in mercury concentrations in fish tissues over time. To assess current risk to human consumers, we limited our analyses to data for the latest year at each site, except in the event of extremely small samples sizes where we pooled data from the latest five years. We also only included fish above certain minimum sizes (Shilling et al. 2010). We calculated the levels and variation at both the subwatershed and site levels.

Mercury concentrations in fish were transformed into a corresponding score based on 1) low concentrations of mercury found in certain species (e.g., trout in montane systems) and smaller individuals of edible species and 2) USEPA Reference Dose ( Rf D ), Relative Source Contribution (RSC) and Tissue Residue Criterion (TRC) (USEPA 2001). The quantitative health risk assessment for a non-carcinogen relies on an Rf D , an estimate of a daily exposure to the human population (including sensitive subgroups) that is likely to be without an appreciable risk of deleterious health effects during a lifetime. The RSC is used to adjust the Rf D to ensure that the water quality criterion is protective, given other anticipated sources of exposure. Finally, the TRC is the concentration in fish tissue that should not be exceeded based on a total fish and shellfish consumption weighted rate. According to the USEPA consumption guidelines (2001), the Rf D for mercury is 0.1 g/kg bw/day. After subtracting contributing mercury exposure via marine fish (RSC), the TRC for freshwater/estuarine fish= 0.3 mg methylmercury/kg fish based on an assumed weighted rate of 0.0175 kg fish/day, and resulting in the score of 0. Other consumption guidelines have also been published which suggest levels for safe exposure may be higher, such as the Food and Drud Administration (FDA) at 1.0 mg/kg (FDA 2001). However we chose to scale our scoring system by the stricter USEPA guidelines in order to be conservative and to reflect higher fish consumption rates found along the Sacramento River among communities that my be similar to or may fish in the Feather River Watershed. The highest score cutoff is for the lowest concentrations found in the Sacramento River Basin, 0.05 mg/kg, which may be similar to background.

Scoring based on:

After transformation, this corresponds to scores according to the following scale:

  • 0 for concentrations > 0.3mg/kg (corresponds to significant human risk via fish consumption)
  • 100 for concentrations < 0.05 mg/kg (corresponds to little to no human risk via fish consumption)
  • Concentrations between 0.05 mg/kg and 0.3 mg/kg were given a corresponding proportional score using a 1:1 linear transformation.

This transformation allowed us to report the general level of concern for the human consumption of fish in each area.

What did we find out/How are we doing?

Based on pooled data from fish samples for each subwatershed, 7 of 11 subwatersheds within the Feather River Watershed on average exceed the methylmercury USEPA tissue residue criterion (TRC) safe level for human consumption (Table 1; Figure 1). We can conclude from these basic statistics that human and wildlife consumers of fish in the Feather River may be at risk of methylmercury bioaccumulation and possible toxic effects in these subwatersheds.

The East Branch North Fork of the Feather had the lowest concentrations and highest score and the Deer Creek had the highest concentrations and shared the lowest score, 0, with other subwatersheds.

Analysis Confidence

Though some subwatersheds were limited by sample sizes and/or were not normally distributed, most had enough data to consider these statistics robust. Therefore, we have high confidence that mercury levels in fish in many of the Feather River subwatersheds are higher than safe thresholds set by the USEPA. However, there are several considerations to consider when interpreting these data. First, we chose to pool species of trophic levels 3 and 4 in order to roughly estimate the risk of exposure to the average human consumer of fish. However, certain subwatersheds, communities, or individuals may only have access to certain types of fishes or preferentially choose to consume certain species. Therefore, the risk may be very different than the values calculated here and may be either higher or lower. Many previous studies have shown that higher trophic level fish typically have higher concentrations of mercury as well as other persistent pollutants such as polychlorinated biphenyls (PCB). Thus, consumers preferentially eating carnivorous or top predatory fish should be especially cautious if they consume fish in these subwatersheds.