Before the introduction of trout Diaptomus leptopus, Holopedium gibberum, Diaphanosoma brachyurum, a rotifer, and two species of Chaoborus were abundant in both lakes. Daphnia pulex, the only species of Daphnia present, was rarely found in either lake. After the introduction of trout there was an immediate decrease in the Chaoborus population in the lake with trout, accompanied by a nearly 37-fold increase in the number of large (1.35 mm) daphnids. There was no such change in the control lake. The copepod D. leptopus, the most abundant zooplankter present in both lakes, was not affected by trout. However, this species declined in both lakes during the last 2 years of the study. After cessation of stocking, the density of Chaoborus increased to about one-half of its former pre-trout level. Water clarity was highest while fish were present and daphnids were abundant.

The most important food items in the gut of rainbow trout during the first year of stocking were Daphnia, Anisoptera, Tendipedidae, Amphipoda, and Chaoborus. During the second year, the most important food items were Daphnia followed in significance by Trichoptera, Tendipedidae, Zygoptera, Hemiptera, and Hirudinea. The absence of Chaoborus, Amphipoda, and Anisoptera in the second year indicated that predation on these items was severe. The growth of the trout the first year was exceptional with an average increase in length of 16.6 cm from May through October. The trout planted in 1977 grew at about the same rate but their condition appeared to decline. The stocking rate of 101 trout per hectare matched the potential production of the lake the first year but it appeared to be too high a rate for the lake on a sustained basis.

Many studies have shown that the introduction of planktivores can have a deleterious impact on the Chacborus population, but few studies have documented tremendous increases in large daphnids following an introduction of planktivores. Apparently the near elimination of Chaoborus by trout in this study lake was responsible for the large increase in daphnids. After most trout disappeared from the lake the number of large daphnids declined but in contrast to the pre-trout years they still comprised a significant part of the zooplankton population.

Other studies have shown that Chaoborus prey heavily on copepods. If this had occurred in this study lake, the near elimination of Chaoborus should have permitted the copepods to increase. However the abundance of copepods remained at about the same level and did not change until after trout had disappeared. The expected replacement of large planktonic crustacean species by smaller forms following fish introduction did not occur.