In the mid 1980s, people fishing on Presque Isle Bay began catching catfish that had strange bumps on their lips—like flattened raspberries beneath the skin.
After the anglers reported finding the bumps, the U. S. Fish and Wildlife Service asked Eric Obert, who then worked for the Pennsylvania Department of Environmental Resources, to investigate the stories. "When I looked at catfish in the bay in 1990, 86 percent had developed skin tumors," he says. The catfish, a kind called Brown Bullheads, are bottom-feeders—they ingest along with their food any chemical that may have sifted into the sediment. Obert, now working with Pennsylvania Sea Grant at Penn State Erie, wonders if pollution on the bay floor could be causing their tumors.
While environmental groups near Presque Isle worked to clean up the area, Obert tried to find out what chemical, if any, was affecting the fish. "Based on studies from other parts of the Great Lakes, it could be polycyclic aromatic hydrocarbons, or PAHs," Obert explains. PAHs are emitted by burning coal or oil and are formed naturally by forest fires. Studies in 1992 and 1995, furthermore, did show signs of PAHs in the bullheads' gall bladder bile, but the researchers could not correlate the chemical with the tumors. Obert adds, "PAHs are not the only possible chemical. If there is sewage in the sediment, there is a lot of nitrogen, with no oxygen, that causes nitrosamines to form, which could also cause tumors."
In 1997, Daniel Weinstock, Arthur Hattel, and Thomas Drake, veterinary pathologists in Penn State's Animal Diagnostic Lab, began a study to see if the incidence of tumors in the catfish could indicate pollution levels in the bay. They examined the health of the fish, compared the tumor rate to previous studies, and checked the liver for signs that the fish had ingested and metabolized PAHs. After looking at thousands of preserved fish tissues for abnormalities, the researchers found that the number of fish with tumors had actually decreased from 1992 to 1997. Why the rate was decreasing—as well as the cause of the tumors—was still unknown.
"We did have the advantage of looking at previous studies to compare our results, but although we had a lower tumor rate, our fish were also younger," explains Drake. "The tumors could have been related to many factors, such as age, chemicals, or genetics." Word of their studies reached Jack Vanden Heuvel, a molecular toxicologist at Penn State, who used his background in the study of poisons to attempt to find the reason the tumors had developed. "The big difference between this study and previous studies is that we took the molecular approach. Most people try to guess what single chemical to analyze. I look at large classes of chemicals. I chose to test for dioxins, and for certain PAHs that act like dioxins," he explains.
Vanden Heuvel first looked for traces of dioxin in the soil near the bay and in the sediment from the bottom using a Chemical Activated Luciferase Test (CALUX). He made a cell line that had a gene for luciferase—the enzyme that makes a firefly's body glow—spliced with a gene that responds to dioxin. When he added soil samples from the bay, the cell glowed—revealing that the sediment was full of dioxins and of PAHs that act like dioxins.
But were the fish eating the dioxin? Vanden Heuvel knew that when dioxin enters an animal cell, the cell reacts by producing an enzyme to get rid of the chemical. "The fish in the bay have a higher amount of that enzyme than fish in clean areas," he explains. "It shows that the fish are ingesting the toxicant and the cells are responding to it"—though that response may or may not lead to a tumor.
Vanden Heuvel also studied samples from other areas near Presque Isle, including a tributary, Cascade Creek. While he expected his results to reveal that dioxin was present in the bay, the creek samples were surprising—dioxin activity levels were ten times higher than those in the bay. "Cascade Creek has been identified as a source of pollution," explains Obert. "Hopefully the results of what Vanden Heuvel has shown will encourage the Pennsylvania Department of Environmental Protection to speed the clean-up process in that area."
Based on his research, Vanden Heuvel has developed an interesting hypothesis. While the tumor rate has continued to decrease, the dioxin and PAH levels are still high, which could indicate that the bullheads are genetically adapting to the chemical. "People have been looking at the health of the bullhead as an indicator of the health of the environment—with the decline of the tumor rate, people think conditions are getting better. If the fish are adapting, we would have a false sense that the environment is improving," he says. On the other hand, the declining tumor rate despite the high dioxin levels could also indicate that the chemical has nothing to do with the bullheads' tumors. Though dioxin is a possibility, researchers need more studies before they can determine what exactly is affecting the fish.
Eric Obert is extension director for Pennsylvania Sea Grant, Penn State Erie, 1000 Glenhill Farmhouse, Behrend, Erie, PA 16563; 814-898-6453; firstname.lastname@example.org. Jack Vanden Heuvel, Ph.D., is associate professor of veterinary science in the College of Agricultural Sciences, 226 Fenske Lab, University Park, PA 16802; 814-863-8532; email@example.com. Daniel Weinstock, D.V.M, Ph.D., is senior research associate for the College of Agricultural Sciences; firstname.lastname@example.org. Arthur Hattel, D.V.M., is senior research associate and associate professor of veterinary science; email@example.com. Tom Drake, D.V.M., is senior research associate for veterinary science; firstname.lastname@example.org. Funding came from the Pennsylvania Department of Environmental Protection.