An endangered species of sturgeon has rediscovered long-inaccessible habitat that could be a key to improving the fish’s reproduction, University of Maine scientists said.

The shortnose sturgeon, listed endangered for nearly 50 years, has returned to the portion of the Penobscot River that is beyond the former Veazie Dam, a central Maine structure that was removed in 2013, the scientists said. The sturgeon had not been seen in the area, which is part of one of the largest river systems in northern New England, for more than 100 years, the university said.

The shortnose sturgeon is among the most primitive of the bony fishes, and its population was devastated by overfishing, pollution and other threats.

The fish still have not been observed spawning in the Penobscot River, and scientists believe they need to go farther upstream to spawn, said UMaine marine science professor Gayle Zydlewski. Living in a river during winter can be a precursor to spawning the following spring.

“If there is suitable habitat, will they use it? This is kind of big for that potential,” Zydlewski said.

Researchers said they had implanted sturgeon with small sound-emitting devices to see if they would use the newly accessible parts of the river. The researchers confirmed evidence of three female shortnose sturgeon in mid-October. About 1,000 shortnose sturgeon live in the Penobscot during winter, and 20 of them are tagged, Zydlewski said.

The fish, endangered throughout their range, live along the East Coast and spawn in coastal rivers from the St. John River in New Brunswick, Canada, to the St. Johns River in Florida, according to the federal National Oceanic and Atmospheric Administration.

The removal of the Veazie Dam was part of the Penobscot River Restoration Project, which restored all of the sturgeon’s historic habitat in the Penobscot. Molly Payne Wynne, monitoring coordinator for the Penobscot River Restoration Trust, called the project a “monumental restoration effort.”