Mercury (Hg) is a naturally occurring element that can be toxic to people, wildlife and the environment. While naturally found in our air, water, and soil, it is also released into the environment through numerous human activities.
Mercury released into the atmosphere from industries--for example those burning coal, can travel long distances and eventually return to earth in rainfall. Mercury can then enter lakes, rivers, and the ocean. Once in these water bodies, it can be converted by bacteria methylmercury, a especially toxic form of mercury to humans. Fish and other aquatic animals can accumulate mercury in high enough concentrations to potentially endanger a person's health. This is why many nations issue fish consumption advisories.
According to a 2010 survey*, about 18.5% of respondents who reported eating fish caught recreationally in South Carolina, obtained the majority of the fish they ate from large public lakes or reservoirs. Given these water bodies are the main source of recreational fish that are consumed in South Carolina, DHEC administers a robust advisory program for the consumption of fish harvested or caught from large public water bodies .
Almost as many respondents (14.1%) said they mainly ate fish obtained from small ponds. Since these smaller water bodies are still an important source of recreational fish that are caught and eaten in South Carolina, DHEC's MyFish Mercury Calculator can help consumers make informed decisions about eating fish from small ponds and reservoirs (2-50 acres in size). The calculator predicts mercury levels in fish by looking at variables that include:
- Type of fish - Typically, large predators such as largemouth bass have more mercury than bream.
- Length of fish - For a given fish type, larger fish tend to have more mercury than smaller ones.
- Location of pond - Where a pond is located (the mountains versus the coast, for instance) is important in estimating the amount of mercury that may be in the fish.
- Type of pond - Two common construction methods for ponds are building a dam or removing soil to produce a depression. Most ponds in the South Carolina Mountains and Piedmont are built by dam construction. Nearer the coast, ponds are sometime built by excavation, with the resulting depression filling with water. The type of pond can affect mercury concentrations.
- Whether a pond is actively managed - Management practices include the addition of lime and fertilizer to increase the pH which may result in more and larger fish. Most private ponds are not managed, though some small public ponds are.
How does mercury get into fish?
Microscopic organisms absorb the methylmercury that is in the water column. When larger aquatic animals eat these small organisms, the methylmercury becomes slightly more concentrated. As the process continues, methylmercury becomes even more concentrated, particularly in the larger predatory fish such as largemouth bass. This results in high mercury concentrations in these fish and similar species. Sometimes, mercury concentrations can be high enough to potentially cause health problems for those who eat the fish on a regular basis.
How are humans exposed to mercury and what are the health effects?
One of the most common ways people can be exposed to mercury is from eating fish that may have been contaminated by the mercury in the water. Mercury can have significant human health impacts. Specifically, high levels of mercury in the bloodstream of unborn babies and young children may harm their developing nervous system. Most people have trace amounts of mercury in their tissues because of its widespread presence in the environment. Whether mercury exposure will harm a person's health depends on a number of factors, including the:
• Chemical form of mercury
• Dose (how much was taken in or absorbed)
• Duration of exposure
• Route of exposure (eating, breathing, injecting, touching)
• Specific characteristics of the person (age, health, etc.).
The MyFish Mercury Calculator is a predictive model/tool that calculates the amount of mercury in fish caught in small ponds/reservoirs (2-50 acres in size) across South Carolina. It provides fish consumption advice based on the answers given by the user.
Because some fish contain higher levels of pollutants like mercury, there has been widespread confusion around the topic of eating fish. However, many healthy diets include fish, a protein food with fewer calories and lower fat than other meat sources. Evidence shows that fish that are high in omega-3fatty acids ("good fats") and low in contaminants like mercury can have many health benefits. Eating fish may lower blood pressure and reduce the risk of a heart attack or stroke, for instance. To get the benefits from eating fish, the American Heart Association and DHEC recommend eating fish for at least two meals a week, but remember to choose the right types of fish to eat.
To help you make informed fish consumption decisions that take pollutants into consideration, follow the guidance offered by our:
- Advisory program for the consumption of fish caught in public waterbodies
- MyFish Mercury Calculator for fish caught from small ponds/reservoirs .