Learn What Trout Really Eat
Has This Ever Happened to You?
by Michael McAuliffe
photo by John Collins
You did your homework. All winter long you studied your hatch charts, researched the best patterns, tied flies, and figured out where and when to fish. You fastidiously gathered all of this information in the hopes of constructing an epic season of fly fishing. It is early afternoon on a crisp April day. You have confidently positioned yourself downstream of the fast water that the Epeorus Pleuralis inhabits.
You tie on a Quill Gordon just as the first bugs begin to struggle free of the surface tension. The trout should start eating any minute as the flotilla increases in numbers. After an hour of watching the big mayflies pour by unmolested, you concede defeat. Naturally you wonder, “What did I do wrong?”
Conventional wisdom would have you believe you’ve done nothing wrong. Many of our trusted advisors coin phrases to justify why the fish didn’t eat on a particular day, or better yet, why we can’t catch them all the time. Does this sound familiar? I thought so. Don’t feel bad, I’ve been there; and now I intend to chronicle how I stopped falling victim to angler’s theories, hatch charts, speculation, conjecture, and the pontifications of “experts.”
Long before I began guiding professionally, I simply wanted to be a better angler. I did all of the things mentioned above and continually came up short of my expectations. The simple truth of the matter is that hatch charts represent a small view of the complex relationship between trout, food sources, behavior, and environment. It is a common and accepted belief that trout do 90% of their feeding sub-surface. So why do we put so much stock in writings that primarily detail feeding behavior and food sources that are visible on or above the surface of the river? If you are from my region, the Mid Atlantic States (notably NJ and PA) and your interest is fishing dries flies in the evening, hatch charts are a fantastic tool. They should put you in the ball park and guide you to what trout are eating during a window of time and what flies you should have in your box. However, for the rest of us that fish beyond May and June Evenings, many of the hatch charts I’ve seen leave out much of a trout’s primary food sources. If you want to consistently catch more Trout, especially on tough days, you will need to educate yourself through observation made on the stream.
Looking Beyond the Text
How did I learn to see beyond the Hatch Charts and get the complete picture? Enter my good friend and fellow NJ Fly Fishing Guide, John Heaney. John is the one of the best fisherman I have ever had the good fortune to share a stream with, and he taught me that the greatest tool a fly angler can arm himself with is a regiment of focused observation. The tools we use for our research are a seining net, stomach pump, notebook, and digital camera with a macro function. Armed with these devices we started cataloguing what the trout were eating during every month of the year. I like to jokingly call this the Salmo Gastronomic Index for New Jersey.
The first step to understanding a trout’s diet in a particular river is by looking at what food sources are present. There are several simple ways to quickly identify what food sources are available. The first is to look at and under submerged rocks along the side of the stream. The second is to pull up a few submerged sticks or trapped leaves and examine the insect life you are finding. A good course of action is to spend a few hours sampling at least once a month. It is important to do this in diverse types of water. Varied insects and food sources inhabit slow/silty, moderate, and fast/broken stretches of water, so make sure to sample a diverse cross section of habitat. I like to pick 3 spots on a body of water with landmarks that will not be erased by floods, and sample the exact spot each time. I would also recommend you do this in the same stretches of river you fish the most. Remember to put stones, leaves and sticks back where you find them, as they are important parts of the ecosystem.
If it is legal in your area, a seine or kick net is a better option. Have one person hold the net or screen a few feet downstream while you disturb the streambed with your boots. The current will carry and trap macro-invertebrates in your net. Make sure to get good clear pictures of what you find. Now you can catalogue and research your findings in a notebook.
The second and most revealing observations you can make are through the use of a stomach pump. Over the course of a little more than one fishing season, I watched and learned from John Heaney how to safely and effectively pump a trout’s stomach. This is a subtle art that takes a lot of practice. I would strongly recommend you seek out someone with a few years experience if you intend to employ this technique. It is very easy to harm a trout if you do not do it properly.
To get stomach samples, you need to catch the “right” fish. We like to pump stocked fish in the 12-14 inch range. It is very easy to harm a smaller trout, due to the size of the stomach pump and the opening between the gills. A larger fish is much stronger, harder to control, and also likely to be injured in the process. After netting the right sized fish, make sure to wet your hands before handling a trout. Get a little bit of water in the tube of your stomach pump, and turn the trout upside down. This will disorient the fish and should stop it from struggling. This is the critical point at which we choose to pump or just release a trout. If it continues to wiggle or the slightest resistance is felt when inserting the tube, we release the fish without sampling. If the tube slides in easily, we gently squeeze the bulb and allow just the water to be pushed from the tube. In one steady motion, slowly back the tube out while letting go of the bulb. If you do this properly, it should take eight seconds for the entire operation from net to release. If you miss and do not get a sample, release the fish and try again on the next one. If you did it right, you will see bugs in the tube. Squirt the contents into a white dish or container and photograph your findings. These stomach contents will reveal something that most entomology texts are missing, concrete facts. Stomach contents are not debatable, and you will undoubtedly discover times when the trout are not focused on the mayfly that has landed on your shirt, but rather, something you did not see.
In collecting our data we were able to get more information than we imagined because several of us were doing this concurrently. We would schedule trips, send emails, and indulge in many long-winded phone calls to share the data with each other. Our good friend and fly tying guru John Collins was also a valued compatriot and conspirator. He was a huge asset in identifying insects, and developing new patterns that better represented our local bugs. This is when things really got interesting.
The Big Payoff
The most surprising revelation that we encountered with our studies was the abundance of midges in the stomach samples. There are lots of great tail water and limestone streams in our general region that are famous for their midge populations. To this point, Chironomids were barley even mentioned in relation to our local NJ freestone streams. Month after month, we would find countless tiny midge pupa and larva in our samples. As it turns out, there are month long periods in our local waters when the trout feed almost exclusively on midges.
Our stomach samples also revealed that caddis larvae appeared much more frequently than we would have imagined. We knew our local trout ate caddis larvae, but we had no inkling that it would present itself as the major food for the entire winter and a great deal of the spring. Once we began fishing imitations of the Hydropsyche and Rhyacophila (both commonly referred to as Green Rock Worms) in the winter, our catch rates went through the roof. Our winter sessions turned into some lights-out fishing, rather than a desperate move to get out of the house.
After a few years of us using these techniques and recording our findings, we discovered one last revelation. Our local trout do not eat that many mayflies! Sure, trout will opportunistically eat a mayfly nymph that is well presented. There are also some events in the year that get the trout feeding on mayflies like the Sulphur, Slate Drake, and Blue Wing Olives. However, the fact is that our NJ Trout are mostly eating scuds, caddis, stoneflies, and midges. Looking at our sample results, caddis and midges are the predominate species in almost all of our local trout water. Based on what many of our clients and people who attend our lectures tell us, representations of these bugs are not their “go to” flies. I believe this is due largely to the simple fact that midge larvae/pupa, and caddis larva are not easily seen when looking at the streambed. Chalk it up to out of sight, out of mind. Once anglers understand the importance of these “other” food sources, fly selections change and catch rates go up.
I would implore any fly angler that reads this to change your habits slightly. Spend a few minutes observing the water and insects before and during any lull in your fishing trips. Always take the time when you are finished to record your findings. If you establish small changes in your routine, just by adding a few minutes of curious observation, you will catch more trout and become a more capable angler.
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