Entomology: what trout like to eat
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Aquatic Insects: a crash course in entomology
Although trout in western North Carolina streams commonly feed on many species of aquatic and terrestrial insects, the three families of aquatic insects that trout feed upon most often are May Flies, Caddis Flies, and Stone Flies with Midges and Terrestrial insects comprising the the rest of their insect diet. However, be aware that there are several less common species of aquatic insects that trout also feed upon when presented with the opportunity such as Dobson Flies, Damsel Flies, and Crane Flies. In addition, each of these insects with the exception of Terrestrials has an aquatic nymph or aquatic larval stage as well as a terrestrial adult stage and thus, since fly fishing is all about imitation, people who tie flies have developed many effective patterns to imitate each stage of each family and even specific patterns to imitate specific species.
May Flies -
When most fly fishermen think of dry flies, the May Fly with its upright wings and two or three-pronged tail is the first image to come to mind since the May Fly is often the most abundant aquatic insect species on an given stream and thus, fly anglers are often presented with prolific hatches of these insects. In fact, on larger streams and some mountain lakes here in western North Carolina, hatches of these insects can appear as dense, swirling, clouds rising from the water. However, May Flies spend the very large majority of their lives as aquatic insects called nymphs which come in two varieties called "swimmers" and "clingers". Swimmers are highly mobile, free living, nymphs that inhabit lakes and calm pools in streams while clingers are found living among the rocks in the swift sections of most mountain streams. After spending one to four years in the nymphal stage depending on species, they reach a particular point of maturity and then prepare to hatch en masse because the more insects that are hatching at once, the greater chance of survival each individual has. Thus, when the time comes, the nymph undergoes a metamorphosis inside of its exoskeleton in which they transform into immature adult insects. Then, when they are ready, they inflate their exoskeleton with air and gradually rise through the water column to the surface where they suspend in the surface film. At that point, the nymph causes their exoskeleton to split open along the back which allows the adult insect (now called a "Dun") to emerge and crawl out onto the surface film where they ride the current for a few moments while their wings unfurl, dry, and harden before taking off. Thus, while they are certainly vulnerable to trout in the nymphal stage, they are most vulnerable during this later stage while rising, emerging, and drifting.
May Fly Nymph
May Fly Adult
Caddis Flies -
Caddis Flies are similar to May Flies in that they spend most of their lives in an aquatic larval stage before hatching into adults. In addition, Caddis Flies also exist in two varieties with behaviors similar to May Flies. The first variety is a un-cased, predatory, free-living genus that hunts and consumes other aquatic insects. The second variety is a cased genus that inhabits both fast and slow sections of the stream. The ones that live in slow water choose to build a tube-like case out leafs and twigs (known locally as Stick Bait) which they drag around with them in their search for food and can withdraw into when they feel threatened. The ones that live in the swifter parts of the stream however choose to build their cases out of small pebbles which they often attach to the underside large rocks. However, unlike May Flies, in order to hatch into an adult insect, Caddis Flies must first pass though a pupal stage in which they spin a cocoon much like a caterpillar in which they molt and transform completely before emerging as adult insects. Thus, the free-living species often drifts for extended distances while in this pupal stage and are thus very vulnerable to being consumed by trout. However, once they have fully matured, all species of Caddis flies emerge as from their pupal cocoons as adult insects with wings that are completely dry and fully functional even though they emerge under water. Therefore, they are able to rise through the water column very quickly and are able to fly the moment they reach the surface. So, to a fly fisherman, Caddis Flies appear to suddenly bust forth from the water in large swarms with no warning and often draw vicious strikes from trout who are following them to the surface.
Cased Caddis Fly Larve
Cased Caddis Fly Larve
Adult Caddis Fly
Stone Flies -
Stone Flies in Southern Appalachian streams are most often found in sections of the stream where the water is swift, frothy, and highly oxygenated. Therefore, they tend to prefer sections such as Riffles and Runs where the stream bed consists of lots of gravel and small rocks. However, they are poor swimmers and thus, they tend to stay hidden in the crevices between the rocks when they are in their larval stage. But, when they are ready to hatch, the migrate en masse across the streambed to shallow areas where the emerge onto exposed rocks and/or logs above the water line before exiting their exoskeletons as adult insects.
Stone Fly Nymph
Adult Stone Fly
Midges are small aquatic insects that have a larval stage, a pupal stage, and an adult stage just like caddis flies. However, unlike Caddis flies, Midges only inhabit lakes or slow, calm, pools in streams with mud or silt bottoms. Thus, like Caddis Flies, they are most vulnerable to predation by trout when they are drifting in their pupal stage. But, because Midges are so small, an individual midge provides very little energy for a trout and thus, in order for them to be a viable food source, trout must consume them in very large quantities. Therefore, when feeding on midges, trout will often hold in at the tail of a pool where the stream narrows and thus concentrates the drifting pupae.
As the name implies, Terrestrial insects are non-aquatic insects that inhabit streamside foliage and occasionally fall into the water by accident. Thus, they consist of such insects as inchworms, ants, beetles, crickets, grasshoppers, wasps, flies, ect. Also, they are most active during the warmer months of the year starting in the late Spring and continuing through early Fall. Therefore, because they are non-aquatic insects, they are only available to the Trout as a food source when they become dislodged from the streamside foliage and slip into the water such as on windy days. However, just because Trout do not see Terrestrial insects on a continuous basis, do not underestimate their importance to Trout as a source of energy because Terrestrials are normally significantly larger than the aquatic larvae and nymphs that they are used to seeing and, because they are unable to swim, Terrestrials present and easy meal for a hungry Trout. In fact, cricket or grasshopper is akin to a large steak dinner to a Human and thus, Trout will often attack Terrestrials with abandon!
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