While it's clear that thousands of young snapper are killed and wasted after being inadvertently "by-caught" in shrimp nets, new research from Texas Christian and Louisiana State universities finds shrimp trawling also may be raising the level of toxic mercury in juvenile snapper.
"Our study demonstrates that mercury concentrations are elevated in juvenile red snapper in coastal areas where commercial shrimp trawling occurs," says Matthew Chumchal, Ph.D., an assistant professor of biology at Texas Christian University in Fort Worth.
Titled the "Effect of Trawling and Habitat on Mercury Concentration in Juvenile Red Snapper from the Northern Gulf of Mexico," the paper is in the November 2008 issue of the journal Transactions of the American Fisheries Society, vol. 137, number six.
"To our knowledge, this is the first study to document a relationship between the presence of trawling and fish Hg (mercury) concentration"
Matthew Chumchal, Ph.D., an assistant professor of biology at Texas Christian University in Fort Worth
Co-authors include R. J. David Wells and James H. Cowan Jr., both of the Department of Oceanography and Coastal Sciences, Louisiana State University (LSU), Baton Rouge.
Chumchal, an aquatic ecologist who specializes in mercury contamination of fish and other aquatic organisms, notes that Red Snapper typically have lower mercury levels than many other marine fish on the menu, such as larger tuna and swordfish. He also emphasizes that nothing in his study suggests that trawling in itself raises mercury to levels that pose a public health concern.
The key point, he suggests, is that this study establishes a clear relationship between mercury levels in fish and the disturbances caused by dragging a huge net through delicate coastal marine environments, a process that stirs up sedimentary deposits of mercury and may alter the composition of the predatory food chain.
"We're not seeing a dramatic increase in mercury levels of fish in areas that are trawled, but that's not surprising given the very small size of the fish we're sampling," says Chumchal. "Still, when you consider the ability of mercury to accumulate in ever-increasing levels as you move up the food chain, and the intense fishing effort throughout the northern shelf of the Gulf of Mexico, it's definitely an issue worth watching."
Mercury pollution in the ocean can be tied to human activities, such as coal burning and other industrial emissions. Mercury discharged into the air eventually returns to the earth's surface with rain or snow, either falling directly into the ocean or making its way there from land via groundwater, streams and rivers. Once deposited in ocean sediments, mercury is converted by sulfate-reducing bacteria into methyl mercury, a known nerve toxin that can accumulate in the tissues of marine animals as they consume smaller fish and microorganisms.
Methyl mercury is dangerous to everyone, but it can be especially harmful to the developing nervous systems of fetuses, young children and animals. For that reason, both the Environmental Protection Agency and the Food and Drug Administration have cautioned pregnant women to limit the consumption of certain types of fish due to concerns about mercury contamination.
"Most people don't eat enough fish with elevated mercury concentrations to put themselves at risk," Chumchal says. "It typically isn't a problem for adults."
As part of a larger LSU-led study investigating potential impacts of shrimp trawling on the continental shelf, Chumchal and colleagues studied mercury levels in juvenile red snapper less than 250 millimeters in total length, looking for connections between mercury contamination and factors such as fish size, habitat type and the prevalence of shrimp trawling in the area.
Specifically, they compared fish taken from heavily trawled sand, shell, and natural reef habitats with fish taken from similar habitats within artificial reef permit areas, which shrimp trawlers tend to avoid for fear of losing their nets on debris dumped there to create artificial reef structures. Findings indicated a clear elevation in mercury levels among juvenile red snappers in areas of the northern Gulf Coast where shrimp trawling occurred.
Chumchal and colleagues hypothesize that increases in mercury contamination are related to the bottom-scouring effects of trawling operations, which churn sediments into the oxygen-rich upper levels of the ocean, making embedded mercury more accessible for easy uptake in the food chain.
Young snappers in heavily trawled areas may also have higher concentrations of mercury contamination due to differences in feeding strategies. The researchers suggest that small fish in these trawled areas may have the unusual opportunity to feast on fish and other organisms that have been killed or injured in the shrimping operations, thus providing a diet of organisms from higher up in the food chain where higher levels of mercury contamination may have accumulated.
"To our knowledge, this is the first study to document a relationship between the presence of trawling and fish Hg (mercury) concentration," the researchers conclude, noting that the findings are part of a growing body of evidence that man-made sediment disturbances often lead to increased contaminant concentrations in marine organisms.