Over the last few years, biologists monitoring small streams in Shenandoah National Park have observed something that is happening almost nowhere else - the number of eels is steadily growing.

Most places along the East Coast - including the Chesapeake Bay - have had declining numbers of American eels over the last several decades.

The decline is dramatic enough that the U.S. Fish and Wildlife Service recently announced it would review whether the species should be protected under the Endangered Species Act.

But in small headwater streams of the Rappahannock River on the East slope of the Blue Ridge Mountains, biologists are seeing the opposite trend as they survey 100-meter stream segments.

"Historically, in a lot of these places we were seeing one or two individuals," said Jeb Wofford, a biologist with the National Park Service. "In more recent years, we were picking up 15, 17 or 20 individual small eels in some of the same places."

To explain the surge, Wofford and Than Hitt, a biologist with the U.S. Geological Survey, point almost 100 miles downstream to the former site of Embry Dam, just outside Fredericksburg, VA, which was blown up and removed in 2004. The biologists began to see a steady increase in the number of small eels about two years later.

Ironically, the primary beneficiaries of the Embry Dam removal were expected to be other migratory fish such as shad and river herring. The 22-foot-high Embry had not fully blocked the river to eel migration. Unlike most fish, eels are able to work their way up, or crawl around, some obstacles.

But Wofford and Hitt's work suggests that eels have been a big beneficiary of the project, and that dam removals may have a role to play in helping the species recover from record low levels. While surveys upstream of the Embry site have seen relatively small numbers of shad and herring, the overall number of eels in the headwaters has more than doubled.

"Essentially, the dam was a filter," Hitt said. "Some individuals were getting through, but not all." The dam appeared to be keeping small eels from making it upstream. Before it was removed, most eels in the Shenandoah survey were a foot long or larger. Now, most eels that turn up are only a few inches long.

What's more surprising is that the numbers of eels in the upstream tributaries have continued to increase even as eel numbers in the tidal portion of the Rappahannock and other Virginia rivers have continued to decline in recent years, following a coastwide trend.

It's possible, biologists say, that the removal of the dam is allowing eels that make it to the river to spread into new habitats, and potentially improve their survival.

"Eels have a pretty strong drive to get as far upstream as they can until they find a barrier to migration," said Leonard Machut, a fisheries biologist with the Virginia Institute of Marine Science who previously studied the impact of dams on eels in the Hudson River system. "So if there isn't a barrier there, they may continue swimming upstream just to get away from the competition."

Machut said that eels below dams tend to grow more slowly and be in poorer health than those that make it farther upstream. It's not certain why, but he speculated that it could be because high concentrations of eels below the dams increase competition for food and habitat.

Also, he said, dams may lead to higher predation on eels, particularly small ones that may get trapped in large waterways with big predators. "If it looks like a worm, something like a bass is going to be more likely to take it," Machut said.

Allowing those eels to get upstream may ultimately help increase eel reproduction. Eels are spawned in the Sargasso Sea of the Atlantic and migrate to freshwater where they live most of their lives before returning to the Sargasso, often a decade or more later.

The sex of eels is not determined until they are several years old, and eels that disperse into the headwaters tend to become almost exclusively females. Those that go the farthest also become the largest females - sometimes reaching lengths of 3 feet - and produce the greatest number of eggs.

That means getting more eels farther upstream may be an important element of bolstering the sagging eel population, Hitt said.

"If fewer eels are making it into the estuaries, that suggests that it is all the more important that the ones that do make it be allowed to grow to be females so they can reproduce," he said. "Anything you can do to benefit reproduction given a declining overall population is really important, and we've shown that dam removal can do that."