Entomology (the branch of zoology that deals with the study of insects)
As a trout guy...mostly, I am always interested to know about the bugs. I guess one of the most often asked questions I hear while on the water is "What in the heck are they eating?" If you have ever ask this question, it is usually because they were not eating what you were offering...right? Well, you need a little entomology (the study of insects) lesson.
I guess the simple answer to the question I posed above is "probably a mayfly, a caddisfly or a stonefly". Unfortunately, that answer is rarely sufficient. It is correct, however...just based on percentages and probabilities...these three aquatic insects almost always comprise the primary diet of trout. The reason that answer is not sufficient is that each of these three insect groups (also called "orders" has multiple life stages and the trout may very well be "keyed" on one of these stages and their interest can change at almost any time. That is not to say that they will not eat other insects or other stages of the same insect...it just means that the fish are most interested in a particular life stage at that given time. The reasons for this "keyed" in behavior may be that a particular stage is more abundant or it may be that a particular stage is simply an "easier" meal. As an example, many times I have fished a good mayfly hatch successfully only to suddenly find that what worked 10 minutes ago is no longer working very well...a situation I am sure many of you have experienced. The fish are still "rising" as before but you are no longer being very successful. In many of these instances, the fish have started to take the emerger or perhaps the spinner...not the dun you had been using. When this happens, it is very good to notice this change sooner than later. I always find it useful to go sit on the bank and really watch the water not only to see if there are different insects drifting down but to observe the trout's rise form. A trout taking food from the surface (dun, spinner) will often leave bubbles behind...if they are taking nymphs or emargers just below the surface, they will leave either fewer small bubbles or no bubbles at all. This requires very close observation and some experience.
Without an understanding of the complete life cycle of a particular aquatic insect, it is more difficult to adjust to these changes in a trout's feeding behavior. In his book Spring Creeks, author Mike Lawson makes a very important distinction... "It is important to understand that the surface of a trout stream is not a single plane. If a trout breaks the surface during a rise, it does not mean the he has accepted something that is drifting on the surface. The surface should be considered in three parts: on the surface, in the surface and under the surface. Emerging nymphs and pupae struggle to push their way through the surface film.Many of these drift a considerable distance and are never able to escape, and feeding trout eagerly pluck them off." Mike continues this by saying "Emergers and cripples drift partially in and out of the surface film. When insects drift under the surface film or in the surface film in the flat surfaces common to spring creeks and limestone streams, trout can see them before they enter the trout's window of vision. It isn't hard to understand why trout concentrate on nymphs and emergers in the surface film during an intense hatch. From my personal observation and the stomach samples I have taken from feeding trout, I have found that ninety percent of the insects ingested during a mayfly hatch are nymphs, emergers, and cripples."
I should note here that these often subtle changes in a trout's feeding behavior are much more common to mayflies than when other insects (caddisflies and stoneflies) are the food of choice. The reason for this I think, it that the life cycle of the mayfly is more complex than that of the other main aquatic insects. Mayflies have more "stages" than do these other bugs so there are more situations that you need to be aware of.
For many years, I carried a small aquarium net in the back of my vest. If you are going to try to fool a trout, it is a very good idea to know what "fish food" is available to the fish in the particular section of the stream you plan to fish. By simply sitting by the edge and carefully watching the surface, you will start to see what is available to the fish. However, as most food a trout takes is underwater, you will need to know "what's down there?" That is where the net comes in. Simply hold it on the bottom and start turning over rocks and moving the gravel around. Shortly, you will know what the fish have available subsurface. Once you know a little entomology and can identify what you have found, you are well on the way to fooling a fish. Here is a video to demonstrate..."Match the Hatch" by Ken Tanaka of Wish4Fish.
I have been very fortunate to have friends that even know all of the latin names of these bugs. I will share some of that information here. I will do my best to keep the latin to a minimum.
For those of you that may have a special interest in aquatic entomology, here is an incredible resource. This site was developed by Roger Rohrbeck, a fly fisher from the state of Washington, USA. Following a lengthy career in information technology, retirement provided the opportunity to pursue a long-held interest in fly fishing entomology. This site contains more than (200) pages of highly structured information with more than (80) illustrations and more than (3,200) internal links to facilitate access. In addition, multiple databases provide query capability. To visit this site click here Fly Fishing Entomology...
For more detailed information on these insects...
If you are interested in aquatic insects or you like to tie nymphs, here is a site for you... lifeinfreshwater.net. There are incredible images of virtually all of the nymph or underwater forms of aquatic insects.
Should you be interested in capturing and saving aquatic insects from the river/stream, to use when tying nymphs, here is a short video that explains the process very well...see it here...