Vole, it’s what’s for dinner. These three rodents were pulled from the stomach of a brown trout. Ralph Stewart photo

By Matt Miller, senior science writer

As an avid fly fisher, I had heard the stories.

By day, the trout of Silver Creek—a clear, spring-fed stream in southern Idaho—fed on tiny mayflies and caddis flies. The water dimpled as trout sipped the profuse insect life from the surface. People like me used equally tiny artificial flies to try to  mimic said insects, often an exercise in extreme frustration.

By night, though, river monsters ruled: giant brown trout cruised the depths, occasionally surfacing to gulp down any hapless rodents that fell into the stream.

There’s something appealing, at least to an angler, about a trout that attacks mammals. Maybe it’s the thought of our favorite water transforming into a scene from Jaws.

Maybe it’s an antidote to the frustrations of tying delicate flies that practically require a microscope:  If I came back at night, I could just chuck a giant hairball!

But these mice-gulping trout always carried a strong whiff of, well, the classic fishing story. High on drama. Short on fact.

Silver Creek, after all, is one of the most-studied trout streams in the world. And there were no confirmed reports of trout dining on rodents.

Silver Creek also has one of the highest densities of aquatic invertebrates anywhere. The trout surely had easier prey than the occasional mouse.

Then biologists examined some brown trout stomachs.

What they found wasn’t pretty.

But it sure did validate some heretofore questionable fishing stories.

Preserve interns assist biologists in electrofishing Silver Creek. Matt Miller/TNC

Shocking Trout Yields a Shocking Discovery

The Nature Conservancy, who owns Silver Creek Preserve, is constantly working with agencies and biologists to study aspects of Silver Creek ranging from water quality to bird populations to vegetation patterns.

One of the regular studies is a trout population survey. Biologists and interns stand in the stream and send electric voltage into the water, a technique known as electrofishing. The trout surface, stunned by the current, and are netted. They are weighed, measured and, for the most part, released.

These surveys help biologists determine the overall trout population, its age structure and the ratio of rainbow trout to non-native brown trout.

During each trout survey, the researchers sample the contents of select trout stomachs see what they have been eating.

And as they opened brown trout stomachs they found them: montane voles, small, cigar-shaped rodents that are common along Silver Creek. One trout, pictured in this blog, had three fresh voles in its stomach.

It’s not a very appetizing picture, unless, perhaps, you’re a night-time fly fisher.

The montane vole. Or, if you’re a coyote or long-eared owl or brown trout: dinner. Matt Miller/TNC

Vole Patrol

The year this survey occurred an unusual phenomenon was happening at Silver Creek. The vole population exploded.

These rodents have population cycle booms and busts, much like their better-known relatives, the lemmings.

There were voles everywhere—you’d see them in fields, in sagebrush, running across roads. One night, I took a dark walk through the preserve and constantly heard the click-click-click of tiny teeth, as thousands of voles fed on grass.

I had started to hear stories that fly fishers were catching truly monstrous brown trout by using vole patterns. But again, there was no actual verification of this – and it was so often second-hand information, a story passed on by someone’s friend or uncle.

It made sense, of course. The vole explosion was like a small-scale Serengeti. On land, there were many scenes of carnage as coyotes, weasels, hawks, owls and rattlesnakes devoured the rodent hordes. Why wouldn’t an aquatic predator do the same?

The trout stomach samples backed this up. Do brown trout feed on rodents when there isn’t a huge vole population?

It’s likely just as anglers say: when a small mouse falls into the stream, brown trout emerge from the depths and devour it.

Science is not about anecdotes, of course. But keen observers of the natural world—be they fishers or birders or urban walkers—still can contribute to our knowledge and assist conservation efforts.

Watch carefully, note your sightings and share. Strive for detail. Your observations may be an interesting side note, like the trout dining on voles. Or they could  lead to important conservation actions.

In the evenings when I’m visiting Silver Creek Preserve, I like to walk along the paths, listening to night sounds. Sometimes I hear a slight commotion on the edge of the water. I imagine the little rodent struggling for shore, for an overhanging limb, anything to get out of the terrifying water world, where monsters wait.

I also know now what invariably follows. A loud,  violent splash.

Then silence.

As I continue on my way, I wonder what other secrets are hidden here at the creek, secrets that can be revealed not only by scientists, but by anyone willing to watch and listen.

This is the first in an ongoing series on Conservancy fisheries research and how that work intersects with some of the biggest, wildest fish in freshwater. 

The Conservancy’s Central Idaho conservation manager Mark Davidson hoists a monstrous brown trout, caught at night during the “vole hatch.” Mark says he could feel the large lumps of unfortunate prey in the fish’s stomach.

Opinions expressed on Cool Green Science and in any corresponding comments are the personal opinions of the original authors and do not necessarily reflect the views of The Nature Conservancy.

Matt Miller is a senior science writer for the Conservancy. He writes features and blogs about the conservation research being conducted by the Conservancy’s 550 scientists. Matt previously worked for nearly 11 years as director of communications for the Conservancy’s Idaho program. He has served on the national board of directors of the Outdoor Writers Association of America, and has published widely on conservation, nature and outdoor sports. He has held two Coda fellowships, assisting conservation programs in Colombia and Micronesia. An avid naturalist and outdoorsman, Matt has traveled the world in search of wildlife and stories.