Fooling trout on big, vermin-shaped flies is a spectacular thrill, but is tougher than you might think.
[by Joshua Bergan]
A 16-INCH BROWN TROUT BATTED MY CHEAP MOUSE IMITATION like an orca on a seal, launching itself and the fly into the air. A few minutes later, another trout of about 15 inches repeated the commotion. It was a cloudy April afternoon near Twin Bridges, Montana, and the fauna seemed particularly active.
A family of otters squealed and wrestled under the bridge, the streamer bite had been strong all day, and I even swung up a trout with my switch rod before a 17-inch brown trout slammed my little brown mouse fly—my first mouse-caught trout ever. It was a smashingly successful half hour.
But that’s rare. Most anglers in the lower 48 don’t have the confidence to stick with a mouse longer than 20 minutes, and rarely see any action in that time. So it made me wonder, what happened that particular afternoon? What made it so special that the fish decided mice were on the menu? Was there a full moon? Was there an overly active mouse population in that stretch of river? Or did my flies just look like succulent protein moving too seductively to ignore? Je ne sais quoi?
So I set off to get some answers. If anglers better understood the what, when, and why of fishing mouse flies, maybe it would instill the confidence to change the game when nothing else seems to be working. The first step is becoming acquainted with the mammals themselves, similar to how we know stoneflies, scuds, and sculpins.
There are actually several different mammals that fall under anglers’ “mouse” classification. Mice, voles, shrews and lemmings are, for all intents and purposes, the same thing to those of us this high up in the food chain—small, furry, beady-eyed scampers. But to trout, for whatever reason, these animals are quite distinct. After I read a few studies and talked to both mammal experts and fishermen, one thing stood out; salmonids can prefer shrews over other small mammals, including mice.
HOOK: Gamakatsu B-10S, size 1/0.
THREAD: Black Big Fly thread.
WEED GUARD: 40-pound monofilament.
TAIL: Black rabbit strip with ½” tag of 20-pound monofilament tied to tail to keep it from fouling.
BODY: Black rabbit strip.
LEGS: Three yellow medium round rubber legs knotted and trimmed to create feet.
SHELLBACK: 1/8” black Evasote foam.
Shrews are small, cylindrical-snouted mammals, similar to mice (to humans anyway), except smaller. They are common throughout North America and usually range from about 8 to 15 cm long from nose to the tip of the tail, are gray or tan, and are not actually rodents; they’re part of the mole family.
A recent finding published in the journal Ecology of Freshwater Fish (Episodic Predation of Mammals by Stream Fishes in a Boreal River Basin by Peter J. Lisi, Kale T. Bentley, Jonathan B. Armstrong, and Daniel E. Schindler) revealed fish have a taste preference for shrews over mice after a 13-year study in Alaska’s Wood River basin. In the more than 2,400 rainbow trout and 1,500 grayling examined, researchers found 75 shrews and one vole in the fish’s collective stomachs—zero mice and zero lemmings, even though populations of all four mammal species were present in the geographic area.
As further evidence of the study’s findings, a photo showing a dissected rainbow trout from Alaska’s Togiak National Wildlife Refuge (NWR) with the remains of 20 creatures laid out beside it recently made the social-media rounds. At first, all 20 looked like mice, but it was later determined they were actually shrews.
Furthermore, on a recent three-day “mousing bender” on New York’s Upper Delaware River, Field & Stream magazine’s fishing editor Joe Cermele found small mouse flies to be more effective than standard large ones.
“When most people think of mouse flies, they think of big, bulky patterns made out spun deer hair,” Cermele says. “And although we did have some success with flies like that, most of our hits and fish came on a really simple foam mouse with a really slim profile.”
One theory as to why trout prefer shrews to mice relates to the shrews’ inability to swim, thus making it easier to capture, as opposed to its size. But only scattered empirical evidence supports that idea.
The Right Time
The study previously mentioned also noted fish showed more shrew predation than others in some years, but unfortunately, those high-predation years were not regular or predictable, unlike other non-annual fly-fishing events like a cicada hatch or years when New Zealand’s mouse populations explode.
“We found that both large-bodied arctic grayling and rainbow trout episodically consumed shrews, every two to three years, with particularly large peaks occurring in two of the 13 years,” the study said. Those two peak years were 2003 when researchers found eight total shrews consumed among all sampled fish, and 2011 when they found 56. The other two- to three-year spikes (2001, 2006, and 2008) were relatively minor in comparison.
McNeil’s Mad Mouse
HOOK: Gamakatsu B10S, size 2 or 4.
THREAD: Brown UTC 140 denier.
WEED GUARD: 40-pound monofilament.
TAIL: Black ultra chenille, trimmed Zonker strip, or rubber band.
BODY: Deer hair, spun and trimmed into a teardrop shape (leave tips long on top).
LEGS: Knotted black round rubber.
EARS: Leather from used elk- or deer-hair patch.
EYES: Red stick-on eyes.
WHISKERS: Thin moose hair.
“The population swings of most small mammals in the continental U.S. are not very predictable,” Dr. Tim McCay, head of the Biology Department at Colgate University in upstate New York said. This makes it difficult to anticipate full seasons of enhanced mouse fishing, but doesn’t mean those years don’t happen unannounced, or that smaller stretches or individual days won’t be hot, like the day I experienced on the river mentioned earlier.
It is also noteworthy that about a third of those fish from that study (at least 13 of 40) that consumed any shrews, consumed multiple shrews, with up to eight found in a single fish in 2011. This fact, combined with the trout’s documented gluttony at the Togiak NWR suggests there are times or situations when fish gorge on mammals.
Explaining the “why” is another issue altogether.
There are many possible explanations for the high-predation events and years, like high populations of fish large enough to eat mammals, high water that flushes mammals into the rivers, cooperative weather that allows for the mammals’ large-scale survival, lack of other food for the trout, and plenty of other food for the shrews’ other predators. Or could it be that fish simply get hot on shrews for the same, seemingly enigmatic reasons they prefer any other food source. There’s simply no data to support any conclusive claims. (continued on page 2)