JACKSON - In an apparent attempt to sate both sides of the snowmobile debate, the National Park Service is proposing to reduce the number of sleds allowed per day in Yellowstone National Park from 720 to 540.

That change comes as agency studies have shown even existing numbers of snowmobiles - about 250 a day - have harmed soundscapes and natural resources in some areas of the park.

In the final environmental impact statement, released Monday and now the fourth in a string of expansive studies, park officials reveal their preferred plan of scaling down daily entries, while still requiring visitors to travel with guides in groups of no more than 11. Snowmobiles would have to be "best available technology" machines to reduce noise and pollution.

The preferred plan also calls for the closure of Sylvan Pass, citing safety concerns of using a howitzer and helicopters for avalanche control.

The new rules, which must be approved by regional Park Service director Mike Snyder, are scheduled to take effect beginning December 2008. This winter, the park will operate on current temporary-use plans, which call for a maximum of 720 sleds per day and avalanche control on Sylvan Pass.

News of the preferred alternative did not come as a surprise to most Monday, as many were briefed in the past week of what to expect from the forthcoming plan. The park is expected to finalize its plan in 30 days, but can make changes based on new information.

Jack Welch with the Blue Ribbon Coalition said his organization was thankful the Park Service decided to continue to allow "some level of continued snowmobiling in the park." But he said his group was not satisfied with the 540 number or the guiding requirement.

He said he was pleased to see the park acknowledge it will use "adaptive management" to determine if more or fewer sleds should be allowed, or if guiding should be eliminated, and a new in-depth review won't be required for those changes.

Tim Stevens with the National Parks Conservation Association said his organization appreciates that the Park Service recognized the draft plan calling for 720 sleds per day was flawed. But the lower number will still result in harm to the park, he said.

"Ten million dollars and four in-depth environmental studies point to the fact that the best way to protect the park and maintain visitor access is through snowcoach access," he said. He said what "science is telling us and the public is demanding" is snowmobiling be phased out in favor snowcoach access only in winter.

A park spokesman said the recommendations released Monday balanced the competing pressures to preserve the park and allow visitors access to its resources.

"Any human activity has some impact," said Yellowstone spokesman Al Nash. "Reducing the maximum number of snowmobiles per day, from 720 to 540, is one action that helps us better provide resource protection, especially with the noise issue, and also be responsive to access concerns."

More than 100,000 people from all over the country and the world submitted comments on the latest park plan, with more than 70 percent calling for a phaseout of snowmobiles.

In the Park Service's proposal, the number of snowcoaches - vans mounted on motorized tracks that can carry up to 20 people - would be increased by five to 83 daily. By 2011, those machines would have to meet noise and pollution requirements.

The agency analyzed seven options ranging from elimination of snowmobiles to allowing as many as 1,025 per day to enter Yellowstone.

In Grand Teton National Park, 65 snowmobiles per day will be allowed under the preferred plan.

The document said wildlife could experience "moderate" impacts from displacement, and impacts to natural soundscapes in the park could also be "moderate." The socioeconomic impacts to Cody and Jackson would be "minor beneficial," and to West Yellowstone "major beneficial."

The plan to close of Sylvan Pass has rankled many in Wyoming, including elected officials from the governor to the congressional delegation. Park County residents have said keeping Sylvan Pass open is helpful to the winter economy and represents a cultural history the town wants to maintain.

Public comments to close Sylvan vs. conduct avalanche control numbered 13,627 and 562, respectively. Sylvan Pass is the only area in any national park subject to avalanche control with explosives.

Wyoming's elected officials said the Park Service's preferred plan was not acceptable for Wyoming.

In Grand Teton and the John D. Rockefeller Memorial Parkway, 140 snowmobiles a day would be allowed this winter. Most of the snowmobiles would have to meet "best available technology" requirements.

Access to the Continental Divide Snowmobile Trail, Grassy Lake Road, Jackson Lake and adjacent public and private lands would continue under the same regulations used the past three winters.

Grooming and motorized oversnow travel on the Continental Divide Snowmobile Trail between Moran Junction and Flagg Ranch would be discontinued under the revised preferred alternative. People interested in through travel on the Divide trail could transport their snowmobiles on trailers between these locations.

Twenty-five snowmobiles a day would be allowed to travel on the Grassy Lake Road under the revised preferred alternative. There would be no technology or guiding requirement on the Grassy Lake Road, and traffic would be allowed both ways.

Forty unguided, "best available technology" snowmobiles a day would be allowed on Jackson Lake to facilitate ice fishing. Snowplanes would be eliminated.

Environmental reporter Whitney Royster can be reached at (307) 734-0260 or at royster@tribcsp.com.