Lisa Ayers Lawrence, Virginia Sea Grant, Virginia Institute of Marine Science
In water, microorganisms convert mercury to methylmercury, a more lethal compound. Through a process known as bioaccumulation methylmercury builds up in the body tissues of predators when they feed upon contaminated prey. Large predatory fish in lakes and rivers are susceptible to bioaccumulation of methylmercury as well as oceanic top predators such as shark, swordfish, king mackerel and tilefish. Current scientific research seems to indicate that smaller oceanic fish are not showing high levels of methylmercury.
Mercury is a neurotoxin, affecting the brain and nervous system, especially developing brains. For this reason, it is very important that pregnant and breastfeeding women and young children avoid any foods that may contain high levels of mercury. The U.S. Environmental Protection Agency (EPA) determined a reference dose (RfD) for methylmercury of 0.0001 mg per kg of body weight per day. Therefore, a person weighing 60 kg (or 132 lbs) should not consume more than 0.006 mg of methylmercury per day or 0.18 mg of methylmercury per month (0.006 x 30 days).
Since 1993, the EPA has issued guidelines on which fish are safe to eat based on the species of fish, its size (smaller fish will have less methylmercury accumulation), and the water body in which it lived. For reducing contaminants other than mercury, there are even guidelines for preparing fish such as trimming skin and fat and grilling to allow fat to drip away. Unfortunately, these methods do not work for mercury because it is stored throughout the fish's body; not just in its fat. From 2002 to 2003, the number of advisories related to mercury in fish increased from 2,100 in 44 states to 3,089 in 48 states. To find out if fish in your local water body have mercury contamination problems, check out the National Listing of Fish Advisories. Click on "Map Advisories", then select "Mercury" as your pollutant and your state.
Seafood is a very important low-fat, high protein food source and should not be eliminated from our diets. Instead, we need to be aware of which species of fish may contain high levels of methylmercury and then limit consumption of those fish to the RfD level.
Based on the EPA's RfD for methylmercury, we will calculate the amount of fish a person can eat per month in order to stay within acceptable limits. Print copies of the blank worksheet.
Scenario: A family, consisting of a husband, wife and 8 year old daughter, have spent the day fishing on a nearby freshwater lake. After a successful day, they bring home several walleye and intend to eat them for dinner that night. As they are preparing the fish, the mother remembers an article in the paper warning of mercury in fish. According to the article, walleye from their lake have been averaging 1 part per million (ppm) of methylmercury. The article also provides them with the EPA reference dose of 0.0001 mg per kg of body weight per day. Not wanting to ingest more than the acceptable level of methylmercury, the family sits down to see how much, if any, of the walleye each person can eat.
Based on the EPA advisory and known contamination level for walleye in the lake, determine how much of their day's catch the family can eat to avoid exceeding the EPA methylmercury guidelines. Assume the family has not eaten any fish in the last month.
(Note: 1 ppm methylmercury = 0.001 mg methylmercury/ g of fish)
Step 1. Determine how much methylmercury (MM) is safe for each of the family members to consume in a given month.
Step 2. Determine the safe amount of walleye each family member can eat in a month.
Compare your answers to the Teacher's Answer Sheet.
Using mean methylmercury levels of other seafood species (USFDA Table 3), ask students to calculate the allowable portion sizes for each family member.