A discussion of the food habits of trout is of special interest to the angler because it often aids him in the selection of artificial lures. The normal diet of a trout may be determined from a study of the stomach contents of a number of fish specimens. Such a study will disclose that the foods of trout come from the land as well as from the water. In Yosemite National Park, even during the summer, the trout depends upon aquatic foods for 82% of the diet as determined by an analysis of 53 stomachs collected in 1948. Studies of 100 stomachs examined by Helen Howe and Avis Meigs in 1926 and 102 stomachs analyzed by H. John Rayner in 1936 bear out this finding.⁹

Insects constitute the most important group from which those foods are drawn. The extent to which trout depend upon the aquatic or terrestrial food source is governed by the weather, the temperature of the air and the water, the season of the year and other factors of the environment. Generally speaking, the trout are dependent upon aquatic forms for nearly all of their food during the winter months.

The contents of stomachs from rainbow, eastern brook, brown, golden and cutthroat trout fail to reveal any great difference in the quantity or kinds of foods eaten by each species at any given size.

Foods of aquatic origin vary with the type of bottom, swiftness of the water, the water temperature and the season of the year. Some of the forms are large while others are minute. The smaller forms often make up for their lack of size by their great numbers. Upon turning over a stone, a person will often find several types of aquatic organisms.

The four main types of aquatic insects are the caddisflies, mayflies, stoneflies and true (two-winged) flies. The larvae of many caddisflies construct small tube shelters or cases of gravel, bark, grass or twigs. These cases are attached to the rocks in

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swift water but in still water they are usually unattached. The larvae are, also, known as caseworms or perrywinkles. Both the larvae and the adults furnish one of the important trout foods.

Many of the two-winged or true flies are of aquatic origin. The young are found as small, worm-like larvae or as transforming pupae, while the adults are winged forms. Some of these flies are very small and are commonly referred to as “gnats.”

Mosquitoes are true flies, also. In certain waters, mayflies and stoneflies are of value as trout foods. The nymphs of mayflies or stoneflies may be found crawling around under the rocks in the stream bed. Mayfly nymphs possess three tails or cerci while stone fly nymphs have only two.

Other foods of aquatic origin include: dragonflies, damselflies, fresh water clams, waterboatmen, water striders and aquatic beetles. Trout foods derived from the land include principally flying insects which have fallen into the water where they are consumed by the trout. Beetles found in the stomachs include click beetles, lady bird beetles, and various kinds of bark beetles. During the period when the black ants are in the winged form, these furnish an abundance of food. True bugs, wasps, bees, grasshoppers, craneflies, and spiders are other common foods.

Larger trout will feed upon other fish and other large items. Two trout caught in 1936 contained mice in their stomachs. In June 1934, Ranger Oscar Irwin, stationed at Buck Camp, caught an 18 inch eastern brook trout in Chilnualna Lake which had eaten a full grown water ouzel and a seven inch trout.

If the angler has determined that the trout are feeding upon black ants or black flies, a black gnat fly will usually be successful in catching trout. Use a brown hackle if trout food consists of brown flies or a gray hackle or mosquito if the fod is made up of small gray flies or mosquitoes. In the early spring before the hatch of flies has occurred wet flies or nymph type lures will usually produce the best results. (see page 25.)

9. Howe, Helen Y. and Meigs, Avis F. 1926. Food habits of trout. Yosemite Nature Notes. Vol. 5: No. 12 (Dec. 31) pp. 92-94.
Rayner, J. H. 1937. Notes on the food of trout of Yosemite National Park. California Fish and Game. Vol. 27: No. 2. pp. 149-156.

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