Abstract

In this thesis I studied lotic trout predation and its ecological effects, and investigated invertebrate predator-prey interactions under natural and anthropogenically modified flow conditions. Given the growing concern about the reliability of extrapolations from small-scale studies to larger spatio-temporal scales, results of mechanistic small-scale experiments were scaled up by linking them to large-scale field surveys.

An intensive survey assessed changes in diel feeding periodicity, prey selection and daily ration of juvenile brown trout (Salmo trutta L.) over the course of the open water period. This survey provides the first field estimates of juvenile brown trout daily rations and indicates crepuscular feeding peaks. Trout selectively preyed on medium- to large-sized prey, shifting towards epibenthic feeding with increasing availability of suitable prey. In a small-scale field experiment, trout displayed clear size-related predation concentrating on invertebrate predators and cased caddisflies, a pattern that scaled up successfully in large-scale surveys. Further, predation effects on large-sized prey were also repeated in a meta-analysis on lotic salmonid predation.

While dense blackfly populations in lake-outlet streams are common, mass outbreaks of blackflies in short-term regulated rivers are poorly studied. In our studies the principal invertebrate predator of vernal benthic communities, the caseless caddisfly Rhyacophila, displayed significant preference for blackflies and was almost unable to capture any other prey, thus resulting in passive selection for larval blackflies.

Rhyacophila larvae displayed highest capture success in intermediate current velocities, whereas further increases in current velocities decreased capture success. Short-term regulation releases increased both predator and prey drift but, unlike for Rhyacophila, magnitude of drift was unrelated to substrate for blackflies. Indeed, field observations indicated that blackflies rarely face detrimental effects of short-term regulation due to their fast growing rates and early emergence. Moss was the most preferred habitat of Rhyacophila and provided the best buffer against sudden increases in current velocities. These results suggest that several factors maintain spring-time outbreaks of blackfly populations in short-term regulated rivers: exaptation of the dominant blackfly species to prevailing conditions, degradation of the key habitat of the predator, and recurring annual drift losses and diminished capture success of Rhyacophila during short-term regulation releases.