When the U.S. Fish and Wildlife Service caught some yellow perch from Maryland's South River in spring 2005, they found a surprising feature on brown bullhead catfish that came in with the nets.

Some bullheads had ugly, puffy red growths on their lips, prompting the Chesapeake Bay Field Office to alert the South River Federation, a volunteer group formed in 1999 to protect this tributary south of Annapolis.

The Federation gave $3,200 for testing, and the U.S. Fish and Wildlife Service confirmed Tuesday that South River bullheads have the highest percentage of skin tumors of any Chesapeake Bay tributary.

In fact, the skin tumor rate of the South River brown bullheads matched that of brown bullheads taken from the Great Lakes, which had the nation's highest rate.

One fifth of the South River bullheads also had liver cancer, second only to the rate found in the Anacostia River in 2001, where nearly 70 percent had liver tumors, according to Tuesday's study.

"The fish are clearly exposed to cancer-causing agents, and at this point, we really don't know what chemicals are responsible," Fred Pinkney, the Fish and Wildlife Service biologist who conducted the study, told The Washington Post. "We suspect it's from [polluted] runoff."

The report raises questions about development in Anne Arundel County and its effect on public heath, said David Koslow, the South Riverkeeper for the Federation. While bullhead cancers do not pose a human health risk through contact, anglers should not eat fish with skin abnormalities, he said.

"A lot of people understand that the state's fish is contaminated," he said, noting bullheads are more of an indicator species than a popular food. "It's kind of a canary-in-the-coal-mine thing."

The Federation, the Fish and Wildlife Service and the Maryland Department of the Environment all suspect that local runoff and exhaust are to blame for the bullhead cancers.

Tests are still needed to confirm this, but similar problems in the Patapsco and Back rivers point to polynuclear aromatic hydrocarbons from fuel combustion, said Richard McIntire, spokesman for the MDE.

"This has been a known problem for quite some time," McIntire told the Post. "If you catch any fish that looks strange, throw it back."

PAHs embed into the sediments on the river's floor, where bottom feeders like bullheads live, he said.

"The most obvious solution is usually the answer," Koslow said, referring to PAHs as the likely polluter. "Yesterday my computer conked out, so I went to the most obvious solution and a plug had come out of the back."

Pollution in the South River is most likely the result of increasing development in Anne Arundel County, Koslow said. As a boy, Koslow said he could run for miles in Fairfax County, Va., and never see a house or road.

The development he saw in Fairfax has reached Anne Arundel County and "is really reaching critical mass."

"We're at an important time when we look at who comes first, the economy or the quality of life of people who live here now," he said. "Health is a big part of that, and it seems like it's taken a back seat."

While PAHs are the likely source of bullhead cancers, the evidence is not strong enough to jump to conclusions before proper testing. PAH has killed fish populations in other rivers, but bullhead deaths have not been observed in the South River, McIntire said. Also, Tuesday's fish and wildlife report said available data for the South River does not suggest high concentrations of PAHs in the sediment.

Koslow noted the river is crossed by Route 50 and other roads, so "maybe it's coming from the runoff off the highway — or ski boats."

"We don't have a lot of industry on the river," he said to the Post, "so we're pushing hard to figure out what's going on and deal with it."

Proposed testing will look at the DNA of the fish's bile and metabolites to see what caused changes in cells, Pinkney said.

The service will conduct a competitive bid process to find a lab that will test the South River bullheads, added Pinkney. An original estimate placed testing costs at $5,500, but that figure may rise, he said.

A proposal is due at the end of April.

The South River Federation is leading the effort to find test funding. It originally wanted to seek grant money, but might instead use funds from donations, said Lee Ann Candon, Federation communications coordinator.

"This is something people want to see done," she said. "They don't want to wait any longer."

If tests confirm that PAHs are responsible for cancer in the South River bullheads, there is no "quick, easy answer," McIntire said. Scientists and environmentalists disagree whether it is best to dig out polluted sediments, risking a release into the general waters, or to let new sediment bury the polluted sections.

Cancerous lesions on bullheads are not considered an acute problem at present, McIntire said, but the likely polluter will be hard to stop.

"Unless we're going to abandon fossil fuels anytime soon," he said, "unfortunately there will be PAHs in the environment."

The Associated Press contributed to this report.