SEATTLE -- Puget Sound steelhead were listed Monday as a "threatened" species under the Endangered Species Act, the National Marine Fisheries Service said.

The agency proposed the listing about a year ago to cover naturally spawned steelhead from river basins in the Puget Sound, Hood Canal and the eastern half of the Strait of Juan de Fuca. Also covered by the listing are two winter-run hatchery stocks: the Green River natural south of Seattle and the Hamma Hamma River on the Olympic Peninsula.

Agency biologists said the decline in the steelhead population has been widespread, likely because of degraded habitat, man-made barriers, unfavorable ocean conditions and harmful hatchery practices.

The steelhead listing will help focus more attention on the condition of Puget Sound, said Brian Gorman, spokesman for the National Oceanic and Atmospheric Administration.

"I don't think there's going to be an enormous change over and above protections already afforded to Puget Sound chinook," said Gorman. "My guess is the steelhead listing is going to refocus concentration and effort improving Puget Sound water quality and keeping it clean."

The Northwest chapter of environmental group American Rivers praised Monday's action.

"Anyone who has had the privilege of fishing for or watching wild steelhead on Puget Sound rivers knows why we must restore them. Steelhead are a wild symbol of this place and a reason why so many of us love living here," said Rob Masonis, Northwest regional director for American Rivers.

Puget Sound has three other fish species protected by the federal Endangered Species Act: Puget Sound chinook salmon, Hood Canal summer-run chum salmon and bull trout.

Steelhead are a popular game fish and unlike most other Pacific salmon, they do not necessarily die after spawning. Some can remain in fresh water as rainbow trout, which are not covered by Monday's listing.

The steelhead in Monday's listing include more than 50 stocks of summer- and winter-run fish. The Skagit and Snohomish rivers support the largest populations.

An "endangered" species is in danger of extinction. A "threatened" species is likely to become endangered within the foreseeable future.