COMMUNITY-WIDE CONSEQUENCES OF TROUT INTRODUCTION IN NEW-ZEALAND STREAMS

Trout provide one of the prime illustrations of the willful introduction of living organisms for recreational purposes. In New Zealand, brown trout were first liberated in the 1860s; today trout are so ubiquitous in the country that they have become enshrined in New Zealand culture. There is abundant evidence that in many streams, trout have largely replaced the indigenous galaxiid fishes. It remains unclear, however, whether their effects in stream communities have propagated through multiple trophic levels. Here we performed a pair of field experiments to compare the relative impacts of introduced and native fishes on the structure of a New Zealand stream community. We found large differences in the insect assemblages colonizing experimental channels, depending on the fish species present. In general, insect densities and biomass were lowest in channels containing trout compared to either galaxias or no fish treatments. Effects of trout on insect diversity were, however, relatively minor. The influence of fishes on insect abundance cascaded to the bottom trophic level, as algal standing crop was greatest in the presence of trout. These results suggest that the consequences of trout may extend beyond the replacement of native fish species. Although our approach allows us to ask whether the potential exists for community-wide impacts of invasive fishes, it remains unclear whether multiple trophic-level effects will be readily detectable in many ecological settings. The high degree of spatial and temporal heterogeneity common to running water systems, and the confounding effects of land-use transformations, may impede our ability to discern community-level changes following this and other species introductions.