BOZEMAN, Mont. (AP) - The U.S. Fish and Wildlife Service on Tuesday decided against protecting the fluvial arctic grayling under the Endangered Species Act.

Conservation groups called the decision "politically motivated" and said it could spell doom for the unusual, river-dwelling fish. Its numbers have been declining for years.

"It's a species on the brink of extinction," said Noah Greenwald, a conservation biologist for the Center for Biological Diversity, one of the groups that pushed for federal protections. "They basically said that the existence of the grayling in the lower 48 states is not significant."

An appeal by conservation groups is likely, he said.

The ruling also raises questions about the future of a major conservation project in the Big Hole valley aimed at improving grayling habitat.

Twenty-seven landowners controlling 130,000 acres have already enrolled, agreeing to improve their irrigation projects to help keep water in the Big Hole River, regional fisheries manager Bruce Rich said.

In return, participating ranchers are assured they won't be saddled with other demands if the fish is listed.

It's unknown if any additional federal money will be available to complete the project, Rich said.

U.S. Fish and Wildlife based its decision on findings that genetics of the river-dwelling grayling are similar enough to more abundant lake-dwelling grayling that ESA protection is not necessary.

Agency officials also decided the fish should no longer be a candidate for listing, a status it gained in 1994.

The agency has said in the past the grayling is in trouble. In a conservation proposal last year, FWP officials said its overall population appeared to be at risk still because of low abundance and irregular recruitment.

But officials now say fluvial grayling are abundant in northern reaches of the continent.

The status of the fish has been in and out of court since 1991.

In 1994, Fish and Wildlife determined the grayling merited listing, but said other species were higher priorities. Ever since, however, its status has been reviewed yearly by the agency.

The latest litigation occurred in 2004, when Greenwald's group sued seeking emergency protections for the fish. FWS settled the case by vowing to decide this month whether to protect the grayling under the Endangered Species Act.