Preble’s mouse, fish populations rebounding along St. Vrain Creek

Boulder County biologists studying Preble’s meadow jumping mouse and fish populations along the St. Vrain Creek have been encouraged this summer by signs of species rejuvenating since their habitats were altered by the 2013 flood.

Preble’s mouse researchers hesitate to say the flood caused the mammal’s decline in prevalence along the South Boulder Creek and its seeming disappearance from the Rocky Flats National Wildlife Refuge over the last five years. But flood recovery restoration work by local governments along the St. Vrain — especially west of Longmont, downstream from Lyons — has created conditions conducive to a comeback for the Preble’s mouse.

Restoration efforts also have been conducive to bolstering populations of certain “transition zone” fish that need shady habitat and water temperatures between the colder sections of stream at higher elevations and the warmer sections through Longmont and east of the city.

Fish trends promising, concerns remain

As Boulder County wildlife biologist Mac Kobza has tallied fish caught by his team of six research assistants this summer along the St. Vrain, he’s chalked up seven species, an improvement in the river’s fish diversity over the past five years.

White suckers, brown trout and minnows like johnny darters and longnose daces were among those caught and documented Tuesday, adding to the counts of fish in the area tracked this summer and for the past five years by Kobza.

Only this summer, though, has the data provided him particular satisfaction for the health of the river.

“After the flood … the river was rough and tumble with lots of pools. We saw quite a bit of trout. Some other species dropped off,” Kobza said. “I think what we’re seeing now is a normalization of the river. We’re seeing pretty good numbers, pretty even numbers between all the species. … It’s really hard to say right now where this is going, but it looks good.”

But for fish — especially stonecats and common shiner, whose populations declined significantly after the flood — living downstream closer to and within Longmont, where the water is warmer and less ideal for some species calling the area home, man-made obstacles prevent access to slightly cooler water upstream.

“We need to work together to take those diversion dams and other features that are getting perhaps in the way of the fish and (make) them more passable,” Kobza said.

Diversion dams and other irrigation ditch structures that pull water from the St. Vrain to deliver it to other water rights holders since the flood have been redesigned in some places to allow easier fish passage, but some ditch companies have hesitated to modify their apparatus.

“I think they’re resistant because it can be expensive. Colorado Parks and Wildlife is trying to work with those ditch companies and provide some of the funding to allow (more fish passage) to happen,” Longmont Land Program Administrator Dan Wolford said.

Preble’s mouse an indicator

For both the Preble’s mouse and fish, future growth of trees and vegetation recently replanted along the St. Vrain will enhance the area’s suitability, Boulder County biologists predict.

Native willows and shrubs yet to mature along the St. Vrain should eventually provide more refuge for the Preble’s mouse, as well as additional shade — and therefore cooler waters — for fish.

“Preble’s is a sufficient indicator species of riparian ecosystem health. Because they’re naturally rare on the landscape and are particularly dependent on a habitat type or arrangement, they should be sensitive to changes in that environment,” said Tim Schaefer, another Boulder County wildlife biologist.

But the Preble’s mouse — a federally protected species — seems to be thriving along the St. Vrain more so than near South Boulder Creek, where populations were sizeable before the flood but appear to have declined.

“For South Boulder Creek, post-flood, they’re still there. They’re not absent. We don’t have a reason why they’re lower numbers there,” U.S. Fish and wildlife biologist Leslie Ellwood said.

Further south in the Rocky Flats National Wildlife Refuge, the Preble’s mouse has been undetectable for the past five years since the flood, according to Ellwood.

Schaefer, who has led Preble’s mouse tracking along waterways for five years, is still unsure of what is causing the discrepancy.

“When we talked to people after the floods, a lot didn’t know what happened to (the Preble’s mouse). They said they all drowned, or washed down to Nebraska or Kansas,” Schaefer said.

But he suspects the landscape of the St. Vrain helped them prevail.

“Where we found they were surviving are places where we had larger, mature vegetation like cottonwoods and larger willows, where flood waters were slowed down and the sediment was held in place,” Schaefer said.

It is still too early to conclude the flood specifically caused the downturn of the Preble’s mouse down south, though.

“These areas of the St. Vrain have become of regional importance recently. … If we can uncover with any level of confidence what it is that is causing those populations down south to decline, there are certainly some lessons to be learned that we could apply toward how we manage our lands, and our waterways more specifically,” Schaefer said.

Sam Lounsberry: twitter.com/samlounz.