They Paved Paradise and Killed the Fish

But the scientists use the Severn as their example because they have studied its decline so closely. Until the 1950s, the Severn had so many yellow perch that state officials took fish from there to start their hatcheries. But then came waves of development -- weekend cottages, then a web of waterside neighborhoods -- that have left about 17 percent of the watershed covered by hard surfaces, they say.

The impact of that development has shown up in a crash of the yellow perch population, officials said. Eggs do not hatch, possibly because of toxic contaminants or a low level of oxygen. Even adult fish that migrate in from other areas are under stress in the Severn, trying to find a place where they can breathe.

One statistic hints at the scope of the problem. Before 1955, state officials estimate, somewhere between 60 and 98 percent of the yellow perch eggs in the Severn were viable, or capable of hatching fish larvae. Now, about 10 percent are.

"You end up with murky water. You end up with reduced wildlife habitat. You end up with lousy yellow perch habitat, for example," said John Page Williams, a senior naturalist with the Chesapeake Bay Foundation who has studied the Severn and the creeks that feed it. When you develop an area like the Severn's watershed, Williams said, "the tradeoff is in the creek and the river."

After studying the Severn, Uphoff and the other biologists embraced a new trend in the world of wildlife management, in which biologists try to prevent fish population declines before they occur. They have set up a presentation that explains the consequences of increasing impervious surfaces beyond the 10 percent threshold.

So far, they have presented it to a group concerned with the Choptank River, whose watershed on the Eastern Shore is still largely rural, and to Charles officials considering the fate of Mattawoman Creek. The Mattawoman, a Potomac tributary, is frequently cited as one of the best-preserved waters in the D.C. area, with large populations of bass and other fish. But more growth is projected there.

"We're kind of taking it on the road and trying to at least make [people] aware that there's a limit" to what watersheds can take, Uphoff said.

Jim Long of the Mattawoman Watershed Society said that environmentalists appreciated the help. Officials in the county are considering how to curb growth and by how much.

"These folks have the degrees that are needed; they live it," Long said. "So people are going to believe them more than they're just going to believe an activist."

The long-term prospects for the Severn are also murky. Environmentalists say that more recent developments have done more to control storm water and that they have seen rebounds in oysters and underwater grasses. But scientists say it is unclear whether the treasured yellow perch will ever return to their previous populations.

"They're drop-dead gorgeous. They're fun to catch. They're good to eat," said Williams, the bay foundation naturalist. "You really lose something when you start losing these fish."