Abstract I examined how constant reduced discharge influenced emigration, abundance, growth, production, and distribution of underyearling rainbow trout Salmo gairdneri in small, seminatural experimental streams in New Zealand. Three stream channels had constant discharges of 0.125 m3/s from mid-November (spring in southern hemisphere) to mid-February. Discharge was then reduced in two channels, but remained unchanged in the third. Reduction of discharge by 32% (to 0.085 m3/s) and by 60% (to 0.050 m3/s) caused no increase in downstream emigration, the emigration rate immediately before reduction being 0.1–0.7 fish/d and that for 5 months after reduction being 0.0–0.3 fish/d. However, the results may have been influenced by unequal population sizes in the channels before the discharges were altered. Presumably, these differences were caused by unequal hatching success of eggs planted in the channel substrates. Population density was greatest (41/100 m2) in the channel selected for lowest discharge and leas...