Impacts of Introduced Salmonids on Native Galaxiids in New Zealand Upland Streams: A New Look at an Old Problem

Abstract

Since brown trout Salmo trutta and rainbow trout Oncorhynchus mykiss were introduced into New Zealand in the late 1800s, there has been a decline in the abundance of native fish that is often attributed to predation by them. The evidence is largely circumstantial, however, and habitat deterioration may also have played a role. Nor is predation the only means by which introduced trout might be having a negative impact on native fish. Studies of trout population stream ecology show that trout may consume the entire annual benthic insect production of streams, leading to a trophic cascade that is evident in the proliferation of periphyton in the relative absence of invertebrate grazing. Changes in the behavior of both benthic invertebrates and native fish are also believed to have resulted from trout predation. The observed level of benthos predation suggests that an additional impact of trout on native fish may function through a feedback loop: native fish may be less able to obtain food owing to changes in their own behavior, insect behavior, and insect abundance, all of which are induced by trout predation.