Bay states removed 11 dams in the Chesapeake watershed during the past year, opening 148 miles of river habitat to migratory fish, according to figures compiled by the Bay Program.

Dam demolition represents the preferred method of opening new habitat in the hope of spurring the recovery of migratory fish such as shad and river herring. Only one traditional fish ladder was built last year.

In contrast, when Bay Program fish passage efforts moved into high gear in the 1980s and 1990s, constructed passages such as ladders got most of the attention.

"That is the trend in the Chesapeake right now," said Mary Andrews, an environmental engineer with the National Oceanic and Atmospheric Administration's Restoration Center, who chairs the Bay Program's Fish Passage Work Group. "We are by far favoring dam removals over passage projects, without a doubt."

Dam removal is a goal shared by the rest of the nation. According to the environmental group American Rivers, the United States removed its 1,000th dam in the last 100 years in 2011. In September, work began to remove the 210-foot-high Glines Canyon Dam on the Elwha River in the state of Washington, the largest dam removal in the world.

Nothing in the Bay watershed approaches that scale, but the pace of dam removals in the region is among the fastest in the nation, largely because Pennsylvania has been emphasizing removals for nearly two decades.

But some sizable dams came out, including the 8-foot-high and 175-foot-wide Riverton Dam on the North Fork of the Shenandoah River in Virginia, and the 10-foot-high, 200-foot wide Simpkins Dam on the Patapsco River in Maryland.

(Both of those came out near the end of 2010, but the Bay Program tracks data based on "water years" which run from Oct. 1 through Sept. 31.)

According to Bay Program figures, 95 dams have been removed in the watershed since 1989, mostly in the last decade, and another seven have collapsed.

Still, thousands of dams remain in the Bay watershed - the exact number is unknown. They were built over hundreds of years to power mills, provide electricity, store water, create recreation opportunities and for other purposes. Bay tributaries had so many dams that there were no free-flowing rivers that connected the Bay with the mountains until the Embry Dam on the Rappahannock was removed in 2004.

A policy adopted by the Bay Program that year explicitly prioritized projects that "favor dam removals as opposed to structural fish passages, where practical."

The change in emphasis stems from the realization that traditional fish passages, even when they work well, are not ideal ways to get fish upstream, and the numbers of fish that use them are often small.

At Boshers Dam on the James River in Richmond, about 700 American shad used the fish ladder that was built in 1999 during the 2011 spawning run. That was a huge increase from the roughly 100 that had used it other recent years, but only a fraction of the shad in the river, said Alan Weaver, fish passage coordinator with the Virginia Department of Game and Inland Fisheries. "Even with good fishways, it's still a barrier," he said.

While the Bay Program has - on paper - opened 2,025 miles of river to migratory fish, much of that area has few fish because it relies on old, often ineffective, fish passages.

Dam removal, Andrews said, "is a huge step forward. The sad part of it is that we are realizing a lot of the ladders that were put in in the early 1980s and 1990s are not going to give you 100 percent efficiency."

Also, fish passages typically have to be designed for specific species.

For instance, a multi-million dollar fish lift built on the Conowingo Dam on the Susquehanna was designed to help shad migration. But shad tend to migrate along the strongest currents, so the fish lift is in the middle of the river. As a result, it is of no benefit to many other species, such as eels, which migrate upstream along the slower flows on the side of the river. In recent years, biologists working with eels on the river have been collecting them and driving them upstream in trucks to get around the dam.

In contrast, the removal of Embry Dam on the Rappahannock River cleared the way for all migratory species. The result has been a surge in American eel numbers in the headwaters.

With dam removals, Andrews said, "you are basically making every fish species your target because you are no longer designing for one individual species."

Andrews said it is not certain how many dams will be removed in the coming year. Some that will be counted, including two on the South River in Waynesboro, VA, have already been completed.

A federal Bay restoration strategy adopted in May 2010 called for opening another 1,000 miles of river habitat to migrating fish by 2025, which equates to an average of 66 miles a year. "We fully expect to meet that goal," Andrews said.