For the 400-some Jackson Hole pronghorn migrating south over Trappers Point overpasses, traversing and wintering in developed gas fields is a fact of life.

The Pinedale Anticline Project Area and Jonah gas fields have been the most well-researched developments from a wildlife-impact standpoint. Now, a proposed 220-square-mile Encana project, called the Normally Pressured Lance field has biologists and conservationists turning their heads.

The NPL field in Sublette County, currently being studied for approval by the Bureau of Land Management, overlaps both prime winter range and migration corridors for pronghorn, Wildlife Conservation Society biologist Renee Siedler said.

“Almost all of them are wintering, in an average winter, on the Pinedale Anticline Project Area,” Seidler said of the small Jackson Hole herd. “In a bad winter, it’s likely they then have to move through the Jonah or the proposed NPL. That’s what our data clearly shows.”

Because the precise location of winter range shifts year to year depending on weather, it’s difficult to pinpoint the implications of gas developments on pronghorn, Seidler said. The NPL project’s impacts to migration corridors are clearer, as shown by GPS data collected by Seidler and the Wildlife Conservation Society.

At least three migration routes, one a primary route, intersect the proposed 3,500-well, 141,000-acre gas field.

Officials with Encana, which also operates the Jonah field, tout NPL as being an environmentally benign relative to the Jonah and Anticline fields — two of the largest gas fields in the world.

“The NPL project will rely heavily on directional drilling and consolidated production facilities,” Paul Ulrich, NPL project lead, said. “It’s our belief that it will have a very, very minimal surface impact.”

“To some degree we believe we’re breaking new ground with coupling significant use of directional drilling with consolidated production facilities,” Ulrich said.

Production facilities include roads, transmission lines, compressor stations   and other infrastructure necessary to operate a gas well.

“Right now in the Jonah field we have 120 production facilities servicing 120 wells,” Ulrich said. “There will be a maximum of 11 production facilities serving 3,500 wells at the NPL.”

The new gas field would have four pads per square mile but the size of each was not available. Critics of the project say Encana does not plan to drill in a way that preserves pronghorn migration corridors.

“The pronghorn that come to Jackson Hole each year are just a small segment of the overall herd, but they’re probably the most vulnerable because they have to migrate through the Anticline, Jonah and now the NPL,” Linda Baker, executive director of the Upper Green River Alliance, said.

“This is a death by a 1,000 cuts,” she said. “Your leaders need to know that your pronghorn may be sacrificed.”

Ulrich said the development would be pronghorn-friendly, but he wouldn’t commit to siting well pads with respect to winter range or to directionally drilling underneath the ungulate’s GPS-tracked seasonal corridors.

“We believe that our plan of development — the way we have built it — is a significant advantage for pronghorn,” he said. “We believe that the very fact that we’ll be using directional drilling throughout the entire project area will help in that regard.”

Working with Wildlife Conservation Society-provided data, the Wyoming Game and Fish Department recently redrew the lines on critical winter range for the Sublette pronghorn herd, which is managed for a population objective of 48,000.

When asked, Game and Fish officials did not provide the new map in time for publication.

Seidler said where the pronghorn migration corridors that end in the NPL in the adjoining map are “likely” wintering areas.

Wyoming’s mostly stable population of about 400,000 pronghorn comprise nearly 60 percent of all Antilocapra Americana. The diminutive Jackson Hole herd, typically numbering around 300 to 400, makes the second-longest terrestrial migration in the Western Hemisphere, venturing 100-plus miles.

The effects of dense natural gas development on pronghorn’s use of winter range are well understood.

Seidler, along with Jon Beckmann, Kim Murray and Joel Berger, published research about the effects of the Jonah and Anticline fields on pronghorn winter range in an August 2011 journal article in “Biological Conservation.”

“In fact, patches of habitat which were predicted to be of very high use during winter months inside the [Pinedale Anticline Project Area] and Jonah gas fields have declined in abundance over the 5-year period from 2005 to 2009 by 82 percent,” the biologists wrote.

“The denser the gas fields got, the less likely pronghorn were to use the habitat,” Seidler said in an interview.

The percentage of habitat disturbed at NPL will be less than at the Anticline or Jonah fields. But Baker said she’s still worried about wildlife.

Baker was a member of a 16-person working group called the “operator committed practices committee” that met from March 2011 to November 2011 to discuss the NPL project with Encana.

“The wildlife discussions in the operator’s meetings were a little confusing,” she said. “Encana, on one hand, said they knew where the migration corridors were and that they would protect them. But when Game and Fish came to the meeting, they said they didn’t know where the migration corridors were.”

“Biologically, it’s a relatively unknown area,” Baker said.

A draft environmental impact environmental statement on the NPL project is due out this spring, Ulrich said.

A wildlife survey and impact report that will go into the document was conducted by a consulting group the Calgary-based natural gas company hired, Baker said.