BANGOR, Me. — Joseph Zydlewski, a research biologist with the Maine Cooperative Fish and Wildlife Research Unit of the United States Geological Survey, drifted in a boat on the Penobscot River, listening to a crackling radio receiver. The staccato clicks told him that one of the shad that his team had outfitted with a transmitter was swimming somewhere below.

Shad, alewives, blueback herring and other migratory fish once were plentiful on the Penobscot. “Seven thousand shad and one hundred barrels of alewives were taken at one haul of the seine,” in May 1827, according to one historian.

Three enormous dams erected in the Penobscot, starting in the 1830s, changed all that, preventing migratory fish from reaching their breeding grounds. The populations all but collapsed.

But two of the dams were razed in 2012 and 2013, and since then, fish have been rushing back into the Penobscot, Maine’s largest river.