Feeding of Hatchery-reared Brown Trout (Salmo trutta L.) In Relation To The Diet In A South Dakota Stream

One thousand five hundred catchable brown trout (17-26 cm) were stocked into the South Fork of the Yellowbank River May 17, 1973. Samples were taken one month apart from May 22, 1973, through October 29, 1973. Fifteen-minute drift samples were collected every hour for 24 hours and trout were captured every 4 hours. Trout captured ruing the study varied in condition ® from 1.34 to 2.55. Condition progressively decreased through August, rose in September, and decreased again by the October sample. The average weight of brown trout increased through the September sample and decreased in October. Two peaks in number of drifting invertebrates occurred at night in the samples for May through September and one peak occurred at night in the October sample. All nighttime peaks were a result of changes in the number of drifting benthic and emergent invertebrates; the presence of terrestrial invertebrates influenced the daytime drift patterns. Adult chironomids accounted for 60-90 percent of the emergent drift, and chironomid pupae and larvae constituted a large portion of the benthic drift in the June, July, and August samples. Ephemeroptera were the second most abundant order of invertebrates in the drift; Baetidae and Caenidae contributed 95 percent of all mayflies collected. Hydropsychidae and Hydroptilidae comprised 93 percent of all drifting Trichoptera. Frogs and fishes were the primary food consumed by the brown trout examined. Trout ate a variety of invertebrates. Ephemeropterans comprised 45 percent by numbers, 27 percent by weight, and 21 percent by volume of all invertebrates consumed. Trichopterans constituted 20 percent by numbers, 34 percent by weight, and 38 percent by volume of all invertebrates consumed. Dipterans comprised 17 percent by numbers, 28 percent by weight, and 28 percent by volume of al invertebrates consumed. Correlation between mean drift rates (for the 4 hours prior to the trout sample) and mean stomach contents for all six sampling dates were not significant. Brown trout examined did not exploit the diel drift patterns of stream invertebrates. Hatchery-reared brown trout may require a longer acclimation time before full exploitation of invertebrate drift develops.