BREWER, Maine — For the last century, many types of fish — including endangered Atlantic salmon, alewives, sea lamprey and eels — have been blocked from Sedgeunkedunk Stream by two hydroelectric dams.

Two of those dams, the Mill Dam in Brewer and the Meadow Dam in Orrington, have been removed, and a study started four years ago shows the fish have returned, Stephen Coghlan, assistant professor of freshwater fisheries ecology for the University of Maine, said Thursday.

Within days of the Mill Dam’s removal in August 2009, “we began to see Atlantic salmon throughout most of the stream,” he said.

Other fish have followed and are spawning up and down the waterway, Coghlan said.

The century-old Mill Dam, the Meadow Dam and the Brewer Lake Dam used to provide water to the Eastern Fine Paper Co. mill, which closed in 2004.

Orrington acquired the abandoned and failing Meadow Dam and the Brewer Lake Dam through tax liens when the mill closed. Town leaders decided to replace the leaking Meadow Dam in 2008 with a rock-ramp fishway dam specifically because it allows fish to pass into Fields Pond.

The fishway dam looks like natural rapids created by a pile of brook rocks but maintains water levels with a hidden wall underneath the rocks.

A year after the first dam was bypassed, Brewer officials removed the Mill Dam.

“The goal is to open this stream up to create a fish passage,” Frank Higgins, Brewer city engineer, said at the time.

“Now, we basically see Atlantic salmon all the way up to the Meadow Dam fishway,” Coghlan said.

Coghlan is leading a group of UMaine students who are testing the water to see how aquatic species are responding to the removal of the dams. In addition to studying the type of fish now using the stream, the study also focuses on sea lamprey.

“We wanted to look at a fish that was abundant” in the stream and decided to “monitor sea lamprey before and after the dam removal,” Coghlan said.

In 2008, the group found 50 of the fish, which “basically look like a snake and have a sucker disk mouth,” all located below the dam in Brewer, he said.

“This year, we found 150 lamprey,” Coghlan said. “They colonized the stream all the way up” to the fishway dam.

Orrington used a U.S. Fish and Wildlife Service grant to survey the habitat and water levels before the Meadow Dam was removed and also tapped the National Oceanic and Atmospheric Administration, an agency under the U.S. Department of Commerce, to pay for an engineering study.

NOAA paid for the first two years of Coghlan’s study, and he is using a Maine Sea Grant, issued through UMaine, to fund the rest.

“We want to make the Sedgeunkedunk Stream study a pilot,” he said. “We want to have a long-term data set, a rigorous story to quantify exactly what does happen” when dams are removed.

Rory Saunders, fishery biologist for NOAA’s National Marine Fisheries Service, said the goal is to collect data about how removing dams affects watersheds.

“There has been a lot of river restoration that has gone on in the past, but it’s rare that we have an opportunity to” collect data on impacts on fish, flora and fauna, from before and after a dam’s removal, he said. “This is one of the few cases where we have the whole story to tell, water quality, habitat and fish community.”

Saunders had “crews out [Thursday] doing habitat surveys on how the physical habitat has changed as a result of the removal of the dam.”

Preliminary results show that removing the dams has created a good environment for a variety of fish, which were blocked before, to colonize and spawn, he said.

“The lesson is: You take the dam out and the fish come back,” Coghlan said.