Biology of Fly Selection

Tom Logan

Most fish are predators and a basic understanding of their “predator/prey relationship” to the natural foods they eat can be an advantage in our selection of the flies we fish for them.

We often hear fly patterns referred to as Trout Patterns, Bass Bugs and Bream Flies. But such terms are misleading when we consider the biology of what fishes eat and why they select these foods. We actually tie artificial flies to imitate the natural foods fish eat and most freshwater fishes’ survival is dependent upon capture and consumption of specific life forms of aquatic insects. This is true whether brookie, pike, bass or bream and it applies to fishes around the world. Sure, they eat crustaceans and larger fish prey on small fish, but it’s the many species of aquatic insects that are important prey for most freshwater fishes. Many species of fishes also feed on the same aquatic insects present in the waters they share.

Most fish are predators that prey on other animals for food. This is where knowledge of the predator/prey relationships of fish and the insects they eat can be helpful to fly selection and our enjoyment of fly fishing.

All animals, and especially predators, must be able to find and consume enough food to mature, maintain their health and ultimately reproduce successfully. The biological reality of this is that no species can survive unless a sufficient number of its members live and reproduce. Food is essential to these processes. Two variables are critical to a predator’s ability to eat enough food to survive…its prey must be abundant and it must be readily available for capture and consumption on a regular basis. No animal can survive if it expends more energy finding prey than the calories it consumes.

Aquatic insects generally are abundant in waters throughout the world. More than 80 species of mayflies and 200 species of caddisflies are described in my waters of Florida, for example. We often see the adults, sometimes in large numbers, on streams and lakes. But we seldom see the other life forms or stages that are very important in the diets of fish, nor do we see them eaten. All insects are most abundant at the egg stage, but mortality takes its toll at every life event until very few individuals survive as spinners that return with eggs to continue this cycle.

Insect numbers decline continuously from the time eggs are deposited but they are relatively abundant in the earlier life forms through the first adults that emerge to the water surface. Not all life states are necessarily available to feeding fish, though.

Nymphs develop and grow using the protective cover of bottom structures. They become available as food when they become dislodged and begin to tumble or swim in the water currents. Fly patterns that imitate nymphs certainly catch fish. Spinner patterns are effective when they return to deposit eggs, spinners represent the life form of least numbers. Fish, like other predators, are opportunistic. It is the emerger life forms of insects that rise through the open water column, hold in the film and molt on the surface becoming adults that most represent a prey that is both vulnerable and abundant. This is the interaction of factors, especially vulnerability, that makes this life form of aquatic insect ideal prey for feeding fish.

So, how does a biology lesson in predator/prey relationships of fish and insects apply to our selection of artificial flies? A fish rising to insects on a stream or lake certainly gives us a clue to fly selection when fish are feeding at the surface. But what about when the fish and insects are quiet? Even when you don’t see fish feeding down in the water column or on the bottom, the fish are there and the variables of prey form and availability always apply. Sure, other factors influence fish and insect behavior. But the fish must eat and an artificial fly that fits a fish’s “search image” is a candidate meal.

We’ve already talked about nymph patterns and now know why they catch fish. Natural nymphs are abundant and vulnerable when they leave cover and any predator will key on numbers that can be caught. Many species of fish take them, even a big bluegill will readily eat a Prince nymph.

A dry fly pattern on the surface can tease rise even when you may not see fish rising to insects. Key here is the availability of an adult imitation sitting on the surface. Dry patterns I fish for all species from warm waters to mountains are Western Coachmans, Adams parachutes or cripples, X-caddis, Klinkhammers and several of the Wulff patterns. Others will do, but stick with those you know will catch fish, you have confidence in them and you will catch fish.

My favorite patterns though, are the soft-hackles and other historic wet patters that have been catching fish for centuries. There’s a reason why a partridge and orange, that’s been around for 400 years, are still catching fish and will take a bluegill as quickly as a brook trout. These and other wet patterns imitate the insect life form that best meets the biological importance of numbers and vulnerability to feeding fish as they rise through the water column to the surface film. My favorites are the partridge and orange, winged hare’s ear, Irish fiery brown and my very favorite, the Irish Invicta.

These patterns attract predatory fish, not because of specific insect they represent; rather, because they imitate the natural life forms of insects that are most abundant and vulnerable, and fish must eat them to survive. Size, shape, color and location are key. This may suggest why some “attractor” patterns take fish so well. Now that we know something about the biological variables that drive fish to eat what they do, when and where, perhaps this information can give new purpose to the fly patterns we choose and expand our enjoyment of fly fishing.