The mean quantity of food per stomach had a significant direct relationship with annual trout growth irrespective of species of trout, habitat (lake or stream), genetics (wild or hatchery), and age class. The regression accounted for 80. 6% of the total variation. Since trout show extreme variability in their diet in most habitats, quantity, not quality, is the most important factor determining growth. The great difference between mean volume of food in stomachs and the potential capacity of their stomach indicate feeding in most environments is at a low level.

Empirical diet, in contrast to the real diet, tends to underestimate annelids, fish, and amphibians, and overestimate crustaceans and most insects. In adjusting for caloric content, fish become more important while crustaceans and insects diminish in importance.

Quantity of food in the stomachs of trout from streams is more variable than in trout from lakes. In lake habitats, forage fish comprise the single largest category in the diet, followed by crustaceans, and combined insects. In streams the trout diet was comprised of 50% annelids, 20% insects, 15% forage fish, and the balance mollusks, crustacea, terrestrial organisms, and unidentifiable material. Worms and terrestrial organisms make a greater contribution to the diet of trout in small streams than in large streams. Trout in large streams are more dependent on food produced within the stream whereas in small streams a greater amount of food originates outside the stream.

Experimentally, the stomach evacuation rate for trout varies with the kind of food organism and with temperature. The ratio of instantaneous stomach volume to daily ration is about 1:3. The observed (empirical) diet of trout was adjusted for taxa, temperature, and caloric content. Gastric evacuation rates can significantly alter the observed diet. Temperature corrections are minor in our environments but could be significant in marginal waters.