Good news for the District’s fishermen, and better news for its fish: Tumor rates among fish in the city’s waterways have dropped substantially.

According to a survey just released by the U.S. Fish and Wildlife Service, 42 percent of female brown bullhead catfish in the Anacostia River and 14 percent of males had liver tumors during the 2009-2011 study period. That may not sound like cause for celebration, but those figures are down sharply from an earlier survey conducted between 1996 and 2001, when 78 percent of females and 43 percent of males were estimated to have tumors—-the highest reported rates in the country (though only a handful of waterways had data).

Skin tumors are also down by about 40 percent, but according to the Fish and Wildlife Service, this is not a statistically significant drop, while the decline in liver tumors is.

Fred Pinkney, a biologist at the U.S. Fish and Wildlife Service, says it’s difficult to point to a precise cause for the decline. The tumors have been linked with chemicals called polynuclear aromatic hydrocarbons (PAHs), which come from burning fossil fuels and enter rivers through runoff. “One of the key control actions over the past 10-15 years has been capturing the old coal tar remnants from the Washington Gas site,” Pinkney says. The plant, near the 11th Street Bridge, supplied gas lamps in the District and stopped operating around 1940, Pinkney says, but the PAHs it leaked into the groundwater continued polluting the river for decades. Other factors include the cleanup of auto repair facilities in the Hickey Run watershed (near the Arboretum) and broader efforts to reduce the amount of stormwater entering the river.

“But there’s nothing I can point to and say, this is clear,” Pinkney says. “The best stories are when there’s a single action, when you have one facility that’s contaminated the sediments and then it closes down. We don’t have that.”

Though these figures refer to the Anacostia River, Pinkney says the rates in the Potomac are similar.

The estimated 17,000 residents of the Anacostia watershed who consume fish from the river each year shouldn’t get too excited by the declining tumor rates, though. The chemicals that pose the greatest threat to humans aren’t PAHs but polychlorinated biphenyls, or PCBs, which accumulate in the muscle tissue of fish. And Pinkney says there’s no reason to believe that PCB concentrations have declined.

So a quick public service announcement: Even if they don’t have tumors, don’t eat fish from the Anacostia!

Photo from the U.S. Fish and Wildlife Service study