I frequently spend time snorkeling trout streams so that I can observe the aquatic world in action. While snorkeling, I will grab ahold of rocks or boulders, so that I can hold myself in the current and gain a vantage point to observe fish holding in the current. I will watch fish moving around in small holding areas mouthing almost everything in the drift. Most times the object - be it a small twig, leaf stem, seed or any manner of detritus that gets blown into the current- is investigated and most likely ejected. Sometimes, this morsel is a real food form and is taken deeper into the mouth and crushed with pharyngeal teeth before being ingested. I can imagine that twigs and most artificial flies might be too tough to crush, so are immediately ejected out of the mouth with a quick reversed opercule pump motion. In the case of an artificial fly, often the rejection occurs before we can detect the take.
A recent discussion I had on facebook with an angling friend in the Pacific Northwest has me re-examining what I know about habitat conditions, namely water temperatures and metabolic demands placed on the fish throughout the season. He made an observation that fish are not as selective in temperature ranges in the 40º-50ºF range, while above 50ºF the fish become more selective.
The optimal temperature range for trout growth is between 42º-52ºF. Between these temperatures, fish are at their peak and can afford lower degrees of selectivity so they feed more actively and gain weight as rapidly as possible while the temperatures are optimal. Also, it’s important to note the stream order we are observing. Lower stream orders stay cooler while higher stream orders generally run warmer. Higher stream orders also have noticeable changes in food availability. In higher stream orders - lower in the watershed - the diversity of macroinvertebrates changes to include a higher ratio of aquatic larvae such as the water snipe and crane fly. I have sampled large rivers such as the Madison, Colorado, Roaring Fork and routinely pull out large numbers of Athericidae larvae, the water snipe fly. This aquatic larvae is easily imitated with a common olive green rock worm pattern and is often mistaken for a green rock worm.
When times are lean and the guides are casting #22 baetis or #24 trico imitations in the late summer, trout will often move further for a big juicy aquatic larvae as it drifts by (stream order 3 and greater). This is opportunistic feeding at its best, more caloric reward per unit of effort. I regularly defy the conventional wisdom of casting small flys in the late summer when I regularly catch very large trout with one inch long larvae pattern used as an anchor fly while the smaller beatis is often ignored.
One other point I would like to make is that as the season progresses, mayflys, stoneflys and caddis that are available tend to diminish in size yet the aquatic larvae and free living caddis larvae keep growing. By late summer I can be fishing an aquatic larvae or even a free living caddis almost an inch long. I also notice that my daily benthic macro samples start revealing more and more water snipe larvae and less of the big three bugs on which we often focus our attention.
Fly patterns such as Sawyers Killer Bug, the Utah Killer Bug and my Latex larvae are the perfect imitation.