Stream Sampling for Fly Fishing

In the game of water poker played with trout, we anglers are always looking for the fish to show us their hand. One of the quickest and most simple ways to get trout to fold in our favor is with a stream sample.

A quick example from the recent trip our guide team took to the San Juan River illustrates this point very well. We headed down to New Mexico, armed with flies chosen based upon recent stream reports from the local shops (the nutrient rich tail water is known to be a midge mecca as well as chock full of annelids…see SAN JUAN WORM). In any case, we hit the water immediately following our 7 hour car ride from Denver; rigged with bugs we THOUGHT would work. Sure enough, we hooked a good number of fish. Additionally, in the areas where we noted fish to be most active, we took several stream samples. We did do some quick on-stream fly adjustments based upon quick glances into our samplings. More importantly, we collected the samples into vials in order to complete a more thorough inspection later. Wrapping up day one on the water, back at the motel with some PBR’s, better light, elbow room, and time to kill, we inspected our aquatic collection a bit more closely.

The entomology process here was not super scientific (fish don’t speak latin, as they say), but it was entertaining and effective:

We poured out our vials onto a white paper plate
We removed the junk (vegetation, etc.)
We checked out the bugs (noting things such as size, color, insect order, stage, shape, etc.)
Finally, we did some simple math & measurements to determine percentages and sizes (figuring to first match our flies/rigs the next day with the prevailing percentages of those in the sample).
Here were our general results:
- About 40% midge pupa, olive/gray & black, sz 24-26
- About 30% very small olive brown annelids, super slender sz 18ish
- About 25% deep red midge larvae sz 20-22
- A smattering of small scuds and baetis

With the luxury of a vise and tying materials, we took turns spinning up our best imitations (admittedly, Marcelo and Jeremy both crushed me in every way). If you aren’t already into tying your own flies or you just don’t bring tools and materials along on your trips, don’t worry. Simply snap a few pics with your smart phone and head into the nearest fly shop. There’s a good bet they’ll have the bugs you need!

As for our trip, the proof was in the catch rates. We had a good first day when we thought we were fishing what they were eating. We had GREAT second and third days because we KNEW what they were eating.

Check out some pics of the process below: