Contrasting predation risks presented by introduced brown trout and native common river galaxias in New Zealand streams

Alterations in the predation risk affecting macroinvertebrates in streams of the Taieri River system of New Zealand were assessed following the replacement of native common river galaxias (Galaxias vulgaris) in many sites by introduced brown trout (Salmo trutta). In laboratory channels, galaxias foraged on mayfly nymphs from benthic positions; they searched for prey at night but stayed in cover during the day except when attacking prey. Trout foraged from positions in the water column during day and night. There were no diel differences in foraging by galaxias but trout had higher reaction distances and consumed more during the day. No diel pattern was found in trout stomach contents in the field, probably because of a nocturnal increase in prey availability. Trout and galaxias populations in the field consumed similar biomasses of aquatic prey. However, these experiments indicate there have been subtle changes in the predation regime. Trout relied on visual cues to capture prey so predation risk was higher during the day and larger prey items were more at risk. In comparison, galaxias relied more on mechanical cues to capture prey and predation risk was similar during day and night.