Candlewood Lake in Connecticut, with several boats on the water

A recent analysis of tissue samples by the University of Connecticut (UConn) detected largemouth bass with high mercury concentrations in several waterbodies. Human consumption of contaminated fish tissue can result in cardiovascular and nervous system damage, and in the case of prenatal health, may lead to developmental and cognitive issues for fetuses. Mercury can also have negative effects on aquatic organisms. 

“Fish contaminated with mercury experience impaired growth and development, reduced survival and reproductive success, and altered behavior,” said Maryann Dugan, a NEIWPCC environmental analyst. “High levels of mercury can disrupt ecosystems, affecting the overall health and biodiversity of aquatic habitats.” 

Mercury primarily enters the water when contaminants from the atmosphere travel to the Earth’s surface and waterways, in a process called atmospheric deposition. A variety of other human activities such as mining, industrial manufacturing and coal combustion can also release mercury into the water. 

A largemouth bass being held from a canoe on a lake

Researchers chose to measure mercury concentration levels in the tissue of largemouth bass (Micropeterus salmoides), a popular gamefish found in nearly every waterbody in the state. As one of the prevailing predators in Connecticut waters, the largemouth bass’s position on the food chain makes it highly susceptible to mercury contamination. The fish accrues considerable amounts of mercury due to bioaccumulation, which occurs when a chemical gradually builds up in an organism’s body through the consumption of contaminated prey. Bioaccumulation results in older and larger specimens having higher concentrations of toxins. 

The Connecticut Department of Energy and Environmental Protection (CT DEEP) first conducted research on largemouth bass in 1995. The study was repeated in 2006 and, most recently, in 2019. This latest version, which was funded and managed by NEIWPCC and CT DEEP, recently published a report on their findings after being delayed by the COVID-19 pandemic. 

During the project, UConn researchers collected tissue samples from 701 largemouth bass at 51 lakes and ponds throughout the state. The fish were collected using boat electrofishing surveys, a process that uses an electrical current to temporarily stun a fish. Other specimens were taken from the catch at local angler tournaments.

The majority of samples from this survey were completed via a non-lethal biopsy punch, a change from the two previous studies. The punches allow for the collection of tissue without sacrificing the fish, which also reduces processing time. To ensure that the biopsy results were accurate and comparable to previous surveys, one largemouth bass from each waterbody was taken for a whole-fillet analysis. The results showed the two samples to be similar and the report recommended the use of non-lethal biopsy punches for future fish tissue surveys.  

The most recent assessment found that individual fish with high mercury concentrations were still prevalent in certain waterbodies. In general, mercury levels in fish tissue have been declining since the late 2000s, but the rate of change from the first survey in 1995 to the present has not kept pace with the decrease in regional mercury emissions; in fact, the rate of decline has slowed even more in this most recent survey. The report notes this could be due to a variety of external factors including atmospheric mercury emissions, climate change and/or the slow response time of largemouth bass to changing mercury levels. 

Currently, CT DEEP advises that pregnant women and young children under six eat no more than one meal of wild-caught freshwater fish per month due to health concerns surrounding mercury; for everyone else, they recommend no more than one meal per week. CT DEEP also produce annual fish advisories to regulate which species are safe to consume at each waterbody.  

“By accessing data generated from scientific studies and monitoring programs, regulatory authorities set fish consumption advisories that take into account health risks associated with specific contaminants present in fish tissue,” said Dugan.  

Fish consumption advisories are especially important for certain communities, as mercury exposure risks are not the same for everyone throughout the state. Research has shown that low-income and minority groups are more frequently exposed to environmental hazards such as contaminated fish due to their proximity to sources of contamination like industrial manufacturing plants.  

“Fish consumption advisories play an important role in protecting public health by notifying individuals about contaminants in fish and assisting them in making informed choices to minimize their exposure to harmful substances,” said Dugan. 

To help eliminate some of the language barriers these communities face, the Connecticut Department of Public Health has produced several factsheets on fish consumption safety which are available in multiple languages. These factsheets provide information on what catch is safe to eat, recommendations for people who are pregnant and guides to selecting fish from the supermarket. The results of this study on largemouth bass will be used to update future public health advisories and outreach efforts.