A Long Pondered Question, Answered

Hebgen Lake ranks as one of the world’s greatest dry fly lakes. Actually, it’s probably the greatest, but that’s a discussion for another time. Suffice it to say that from ice-out around the end of April until late October, barring gale-force winds and possessing  just a modicum of local knowledge, you can find trout surface feeding every day. Plenty of them, without fail. Rainbows and browns both, averaging 17-18 inches. Pretty good stuff. But in the four decades that I’ve known the lake, serious Hebgen anglers have always had a slightly uneasy feeling about the rainbow portion of the fishery. As good as it is—and as much fun as it is—we’ve always been left to wonder: to what degree is this fabulous fishing a function of stocked trout?

For years the state of Montana stocked fingerling rainbows in Hebgen Lake, a fact not widely known among visiting anglers (all browns are wild). The numbers of fish planted varied dramatically over the years. I can’t locate the stocking records offhand, but I recall anywhere from around 50,000 fish per year to as many as 200,000 per year. Now, neither I nor anyone I know has ever caught a rainbow from Hebgen that looked or acted in any way “artificial”. Not a big surprise, considering that by the time a fingerling grows large enough to surface feed on aquatic insects, for all intents and purposes it is wild. Despite not having any sensation that we’ve been catching stockers, many of us have wanted to know for our own edification the contribution these planted fish make to the fishery. So, too, has Montana Fish, Wildlife and Parks (though I’m pretty sure the heart of their interest lies in the economics of raising and planting fish). So with help from Montana State University, they decided to study this matter.

Every body of water has a unique chemical makeup. That chemistry is mirrored in the inner ear bones (otoliths) of fish that were hatched there. By comparing the chemical signatures of sampled otoliths to the signatures of Hebgen’s tributaries and the hatchery waters where stocked fingerlings originated from, it’s possible to assign a place of origin to a fish with a pretty high degree of confidence. According to the results of the study, it appears that approximately 84% of Hebgen rainbows have wild origins.  Hatcheries accounted for 13%. The remainder couldn’t be assigned accurately. (If you’re given to science, I’ve glossed over a lot of fascinating details here. The study results can be found on the Montana FWP website.)

And from which tributaries, exactly, did the wild rainbows come? Here the study isn’t as definitive as serious anglers might wish for, but that’s okay. It’s purpose wasn’t to breakdown the contribution of each individual tributary to the fishery as a whole. But the study was able to segregate most of its sampled fish into two tributary groups. One group was comprised of Grayling Creek and Duck Creek. The other included the Madison River, South Fork of the Madison, Firehole River, Gibbon River, and Cougar Creek. (If the Cougar Creek placement seems odd—it’s actually a tributary of Duck Creek—the study recognized this aberration and deemed it a spatial anomaly.) The Madison River group produced the most rainbows, in a roughly 3:1 ratio to the other. Without knowing more specific details, that seems reasonable enough.

As of 2017, the state of Montana has ceased stocking rainbow trout in Hebgen Lake. Hebgen is viewed as an important fishery for the state, and reductions in catch due to a cessation of stocking is something they’re sure to hear about. So far, not much to report on this front. There are still plenty of rainbow trout rising. Most likely with no concern at all for where they began their lives.