Every angler has stood in a river and wondered what the fish were eating. On this episode of the Fly Fishing Insider Podcast, our guest Donny Benkendorf, an ecologist with a PhD from Utah State University, answers that question from the scientist's side of the bank. Here is what I took from the conversation and how you can use it on your next trip.
Bugs as a report card for the river
Donny explained that scientists use aquatic insects to judge water quality, a practice called bioassessment. Instead of testing the water once, you look at who lives there. A common starting point is counting the number of mayfly, stonefly and caddisfly taxa in a sample. A diverse mix of those three groups generally points to good water quality. A more advanced approach compares the insects found with the ones models predict should live in a healthy version of that stream. If many expected species are missing, the stream is probably impaired.
He also pointed out the limits of the shortcut. Even within mayflies, some species need pristine cold water while others tolerate warm, somewhat polluted water. Healthy rivers are judged by the whole community, not by a single bug.
Reading the stressors
Donny's research looked at going a step further: using each species' sensitivity to specific stressors, such as warm temperatures or nutrients, to work out what is wrong with a river rather than just whether something is. He described this as preliminary work that could one day help management agencies decide what a stream needs. For anglers, the takeaway is that the bugs you see reflect temperature and water quality, which is the same information that drives where trout feed and which flies work.
How to use this on the water
You do not need a lab. Donny said he often fishes with no gear for this at all: he picks up a few rocks and looks at the undersides, where many insects cling. Nymphs wiggling on your finger tell you size, shape and color. Sometimes he drops a few in a small container for a closer look. If you find a pile of small brown mayflies, a size 12 pheasant tail is a smart place to start.
He also described how life cycles shape your fly choices. Some species live several years, so different age classes share the water at once, while others complete a cycle in one year. He fishes larger nymph patterns as a species approaches its spring emergence, and he treats adults flying in the air as the cue to switch to dries.
Why trout key in on hatches and drift
Hatches are an easy buffet. Trout stock up on pupae rising to emerge and on spinners when the adults die. Nymphs also become available outside hatches through behavioral drift, when insects let the current carry them, and when heavy rain dislodges them. Knowing that helps you decide whether to fish near the bottom, in the film or on top.
A lesson in downsizing
Donny shared a story from the Frying Pan River in Colorado. Fish were rising all around during a Baetis hatch, but he and a friend were getting refusals. A local guide stopped to help and suggested a smaller fly, a floating pattern and lighter tippet. The same fish that had been refusing began to eat. The lesson is simple: when fish rise and refuse, match the size and color more closely, and thin your tippet before you change anything else.
Gear
If you want to build a simple nymph and dry setup around what you find under the rocks, the Tips and Tricks for Fly Fishing eBook is a handy companion. For a small bug net, vial or seine, your local fly shop is the best place to ask what suits the rivers you fish, and they will have the local hatch information too.
