FPL Energy has tentatively agreed to build four fish passages on dams along the Saco River, a plan that over 20 years would open 25 miles of water from Bar Mills to Hiram to migrating fish.

Florida-based FPL, which owns nine dams on the Saco, would build the fishways on the only dams on the river that lack such passages. The company also plans to build separate passages for eels on all nine dams.

The plans are spelled out in the final draft of a fisheries agreement negotiated over two years by FPL and state and federal agencies. Officials involved in the talks expect to sign a deal within a month.

The negotiations are linked to the federal relicensing process for the energy company’s Bar Mills dam, the first that fish migrating upriver cannot pass. Fish passages were installed at five dams downriver under a 1994 agreement.

Mark Woodruff of the Saco River Salmon Club, who is taking part in the negotiations, said that because the licensing period for a dam can last up to 50 years, the agreement with FPL is likely to establish for decades how far fish can travel as they migrate up the Saco River.

“It sets the groundwork for many years to come,” said Woodruff, whose main interest has been to ensure safe passage for salmon. Salmon populations have remained stable on the river, but shad and alewives are on the rise, he said.

Officials representing the state Department of Marine Resources in the talks sought to have the power company open up more spawning grounds for those species, Woodruff said.

FPL spent $16 million a decade ago to build fish passages at four dams near Saco Island and at the Skelton Dam on the Buxton-Dayton line, said Al Wiley, a company spokesman. Wiley said he could not provide a cost estimate on the proposed projects.

The draft agreement calls for completing the first fish passage at Bar Mills by 2016. Progressing upstream, the company would build three other passages at three-year intervals, with the one at Hiram completed by 2025.

The timetable for creating upstream and downstream passages for American eels at all FPL dams on the Saco would begin in 2008 with dams closest to the mouth of the river; the work would end in Hiram in 2032.

An eel passage is less difficult to build than a fish passage, Wiley said, because eels can wriggle over solid surfaces. The typical eel passage, he said, consists of a trough lined with artificial turf.