ELLENSBURG — The monitoring and research to ensure unimpeded passage for fish and wildlife impacted by the massive Interstate 90 construction project at Snoqualmie Pass is the largest-scale study of its kind ever conducted in the United States.

Because it isn’t about only the largest animals.

Yes, the study is monitoring the elk, deer and bears the state Department of Transportation wants to see crossing below — or, eventually, well above — I-90 instead of actually on the roadway.

But the study goes far beyond them, and it doesn’t even stop at the little mice, shrews and pikas skittering between the rocks — all of which are being counted, logged and, in some cases, tagged for monitoring.

Field researchers, most of them from the biology programs at Central Washington University, have also been monitoring the amphibians. Those members of the animal world are the ones most likely to become roadkill — or to die in other ways, with 30 percent of all amphibian species already threatened with extinction.

That’s why CWU researchers have been capturing, tagging and monitoring movement of such critters as salamanders, toads and the most common amphibean in the I-90 corridor, the Cascades frog. Any time a Pacific giant salamander moves out of a creekbed in the construction zone, it’s noted and logged.

“To me, one of the key things is that we’re not just focusing on the big animals. We’re going down to the small stuff,” said Craig Broadhead, the Transportation Department’s program manager for biology and mitigation on the $551 million construction project funded by gas-tax revenues.

“We’re trying to connect populations for everything, not just the big things that can move pretty well.”

Most of the larger species don’t even try to cross the interstate, as evidenced by the fact that only about 10 larger mammals annually die while trying to cross the interstate in the project area. That’s a tiny number, considering that in 2013, motion-detector cameras installed by the department and Conservation Northwest have logged 246 elk, 238 deer, 94 coyotes, 28 bobcat, 16 black bears, dozens of smaller mammals and a cougar.

“What that’s indicative of is that I-90 has such high traffic that animals don’t even try to cross it,” Broadhead said. “So what you’re talking about is totally isolating the animal population there.”

Another important wildlife population that the I-90 construction project has removed from isolation, as it turns out, is bull trout. Since the 1950s, Gold Creek was the only stream in the Snoqualmie project area known to have spawning bull trout, an imperiled species considered a critical indicator of clean water systems.

Two other tributaries to the Keechelus reservoir, resort and Rocky Run creeks, had previously been impassable for upstream-bound fish because of culverts and concrete slabs associated with the old roadway.

Since the removal of those obstacles during the current construction, though, CWU researchers have found bull trout moving into both creeks.

“If we can get (distinct bull trout populations) established in Resort and Rocky Run, then we have more genetic diversity,” CWU biology professor Paul James said at a Thursday meeting updating project partners.

“You guys” — James said, directing his comment to the Transporation representatives — “are going to be heroes when we find bull trout rearing in these creeks where there haven’t been bull trout in more than 50 years.”

In addition, James said, the construction work didn’t seem to be endangering or even bothering the resident cutthroat fish in the several streams that feed into Keechelus, based on the high recapture rate of fish tagged by field researchers in those streams.

“That’s been kind of amazing,” James said, “which tells me we probably have a pretty good idea of the population size. And we know our methods aren’t harming these fish because we’re capturing some of these fish four and five times.”

The only “wildlife connectivity” structures completed so far are four lengthy bridges — two underpasses crossing both east and westbound lanes — over Gold Creek. The new crossings, 1,100- and 900-feet long, replace what had been 160-foot-long bridges that didn’t provide any safe passage for wildlife.

Construction of the signature structure of the project — the massive, 800-foot-long animal overcrossing at milepost 61 that will be some 50 feet above the roadway — is scheduled to begin in 2015 and be completed by 2021. Fencing along the interstate will serve to guide animals to the structure.