The cause may be a virus or other contaminant, according to the lead researcher, Fred Pinkney of the U.S. Fish and Wildlife Service's Chesapeake Bay Field Office in Annapolis.

"If you read detective stories, I imagine we're trying to sift through the evidence," Pinkney said. "Sometimes they give it up easily, sometimes you have to work on them."

The tumor-ridden brown bullhead catfish were first documented in the South River in 2004 after they were found during a separate survey involving other fish.

More than half of the fish collected then had unsightly tumors growing around their mouths and 20 percent had liver tumors.

Catfish riddled with such tumors in the Chesapeake Bay region had been prevalent in only the heavily contaminated Anacostia River.

A team lead by Pinkney collected more catfish between 2004 and 2008 to assess the problem. One sampling of South River catfish was not enough to declare a serious problem, Pinkney said.

In the additional samples from the South River, between 19 percent and 58 percent of fish had tumors, mostly around the mouth.

That validated the findings from 2004, when 53 percent of fish had skin tumors.

"We definitely have a problem with the fish. It's consistently high for skin tumors in the South River," Pinkney said.

The rate of liver tumors in South River catfish decreased, from 20 percent in the original sample to 0 to 6 percent in the additional samples, indicating that liver tumors are less of a concern.

Fish also were collected in the Severn, Rhode and Choptank rivers for analysis.

The Severn had a wide range of fish with skin tumors (2 percent to 52 percent). A few Rhode River catfish had skin tumors (6 percent) and almost no skin tumors were found in Choptank catfish.

Scientists performed a variety of tests on the fish, including looking at their DNA, to determine the cause of the tumors.

They did not find a correlation between the tumors and the presence of polynuclear aromatic hydrocarbons, chemicals found in gas, oil, coal, car exhaust and runoff from highways.

They also didn't find a link between the tumors and another group of chemicals called alkylating agents.

Because brown bullhead catfish live and eat at the bottom of rivers, they can be affected by toxins such as PAHs and alkylating agents that latch onto sediment particles.

Pinkney's team's findings will be published in the journal Science of the Total Environment.

Diana Muller, the riverkeeper for the South River Federation, said anglers often find the tumor-laden catfish, especially in the northern reaches of the river near a waterskiing area. She wonders if the tumors could be caused by old landfills that leach contaminants into groundwater.

"We definitely have to do more research as to why. If it's causing cancer in the catfish, what else is it doing?" Muller said.

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