SEATOR partners strive to ensure that Tribal citizens and Southeast Alaska community members are able to safely harvest and consume traditional foods. Harbor seal (Phoca vitulina) is a prized traditional food for coastal communities. Seal meat and organs are widely consumed, while oil made from rendering the blubber is prized. Because harbor seals are long-lived and apex predators, they have potential to have higher mercury concentrations due to bioaccumulation. Mercury is a potent neurotoxin that can pose a risk to traditional harvesters who rely heavily on marine-based foods, especially on halibut or seal. Despite harvester concerns about potential mercury contamination, there is not regular testing for mercury in harbor seal and other marine mammals. Sitka Tribe of Alaska (STA) are collaborating with the University of Arizona and University of Alaska Fairbanks (UAF) on a two-year project to screen harbor seals in Southeast Alaska for mercury contamination and to better understand the risks to traditional harvesters.
Mercury Risk Levels
Marine subsistence foods such as fish and marine mammals are valuable sources of protein, vitamins, minerals and other important nutrients. Though they are healthy foods, most contain a measurable amount of mercury. Mercury is passed up the food chain, “bioaccumulating” and concentrating in the longest-lived predators. Because of this, women of childbearing age, nursing mothers and children should follow guidelines to limit their dietary intake. The Alaska Department of Health and Social Services (ADHSS)’s Eating Fish Safely Guidelines for Alaska Women and Children helps consumers understand their mercury risk level. While the guidelines specifically reference fish, they serve as a useful framework to understand mercury risk levels from marine mammals. It assigns points for the average expected amount of mercury per meal (6 ounces of uncooked fish) for a given species or size of fish. ADHSS recommends that women of childbearing age and children plan their meals to that a maximum of 12 points are consumed per week.
Total Mercury vs. Methylmercury
For this study, both total mercury and monomethylmercury (referred to here as methylmercury) were analyzed. Mercury is an environmental contaminant found in two general forms, inorganic and organic. Total mercury is the summation of inorganic and organic mercury. Most inorganic forms of mercury are readily excreted through feces and urine and do not represent a pressing health risk (Clarkson 1997). Methylmercury is a type of organic mercury that biomagnifies up the food chain. When ingested, methylmercury can cross the blood-brain barrier. Methylmercury is of particular concern because of its negative effects on the neurological development of fetuses and children (Bose-O’Reilly et al. 2010, Debes et al. 2006, Oken et al. 2005). Total mercury is easier to analyze, but methylmercury levels more accurately describe risk.
To address the marine mammal data gap, STA, UAF, and the University of Arizona are collaborating to collect baseline data on total mercury and methylmercury in harbor seal tissues throughout Southeast Alaska. Tissue samples were collected from three seals harvested by SEATOR partners in Sitka, Haines and Kake. Samples were analyzed for total mercury and methylmercury by the UAF Wildlife Toxicology Lab. UAF conducted total mercury analysis on a DMA 80 Total Mercury Analyzer™ using EPA Method 7473. Methylmercury was analyzed with a Brooks Rand MERX automated cold vapor fluorescence detection system using EPA method 1630.
Analyzing Muscle vs. Liver Tissue
Traditionally, both the muscle and liver of harbor seals are consumed. These tissues store mercury in different ways, so both must be tested individually. For this study, muscle and liver samples were collected from seals harvested near Haines and Kake, and a muscle sample was collected from a seal harvested near Sitka. Seal oil was not tested, as previous studies have found only negligible amounts of mercury in marine mammal blubber (Moses et al. 2008, Wagemann et al. 1998). In fact, Gmelch et al. (2017) found that consuming seal blubber was highly beneficial to children’s neurological development due to its high level of omega-3 fatty acids.
All of the tested harbor seal tissues are recommended for consumption based on their methylmercury levels. Results from the pilot study shown in the table below. Total mercury and methylmercury concentrations are in parts per million for each sample. In the “Points per Meal” column, a 6-ounce serving of either muscle or liver is assigned points based on the ADHSS guidelines for methylmercury in fish.
The fish species with equivalent points per serving based on their mercury concentrations are shown in the colored chart below. Limiting methylmercury consumption to 12 points a week is recommended for all women of childbearing age and young children.
Results By Area
- Sitka: A serving of muscle is 0 points per meal per week.
- Haines: A serving of muscle is 3 points per meal per week, comparable to a meal of a 40-80 pound halibut or a 35-40 inch lingcod. A serving of liver is 6 points per meal per week, comparable to a yelloweye rockfish or a 140-220 pound halibut.
- Kake: A serving of muscle is 6 points per week, similar to a serving of seal liver from Haines. A serving of liver is 12 points per meal per week, comparable to top predators like large halibut (over 220 pounds), lingcod longer than 45 inches, salmon sharks, and spiny dogfish.
STA has secured funding for another round of baseline data collection in 2019. We will be collection harbor seals samples from thoughout Southeast Alaska again.
A big thank you to the staff of Chilkoot Indian Association, Organized Village of Kake, and Sitka Tribe of Alaska for submitting seal samples for this study. Your contributions are greatly appreciated!
- Clarkson, T.W. 1997. The toxicology of mercury. Critical Review of Clinical Laboratory Science 34: 369-403.
- Debes, F., Budtz-Jørgenson, E. Weithe P., White, P., White, R.F., and Grandjean, P. 2006. Impact of prenatal methylmercury exposure on neurobehavioral function at age 14 years. Neurotoxicology and Teratology 28(3): 363–375.
- Gmelch L, Hintelmann H, Hickie B, Kienberger H, Stern G, Rychlik M (2017) Risk–Benefit Assessment of Monomethylmercury and Omega-3 Fatty Acid Intake for Ringed Seal Consumption with Particular Emphasis on Vulnerable Populations in the Western Canadian Arctic. Front Nutr 4:1–8.
- Moses SK, Whiting A V, Bratton GR, Taylor RJ, Hara TMO (2008) Inorganic Nutrients and Contaminants in Subsistence Species of Alaska : Linking. Int J Circumpolar Health 68:53–74.
- Hamade, A.K. 2014. Fish Consumption Advice for Alaskans: 48-50.
- Oken, E. Wright, R. O., Kleinman, KP, Bellinger, D., Amarasiriwardena, C.J., Hu, Howard, Rich-Edwards, J.W., and M.W. Gillman. 2005. Maternal Fish Consumption, Hair Mercury, and Infant Cognition in a U.S. Cohort. Environmental Health Perspectives 113: 1376-1380.
- National Academy of Sciences (NAS) Toxicological Effects of Methylmercury. Washington, D.C., 2000. Online. Available online: https://www.nap.edu/resource/methylmercury/summary.pdf (accessed 6 February 2019).