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

Trout provide one of the prime illustrations of the willful introducti on of living organisms for recreational purposes. In New Zealand, brow n trout were first liberated in the 1860s; today trout are so ubiquito us in the country that they have become enshrined in New Zealand cultu re. There is abundant evidence that in many streams, trout have largel y 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 th e structure of a New Zealand stream community. We found large differen ces in the insect assemblages colonizing experimental channels, depend ing on the fish species present. In general, insect densities and biom ass were lowest in channels containing trout compared to either galaxi as or no fish treatments. Effects of trout on insect diversity were, h owever, relatively minor. The influence of fishes on insect abundance cascaded to the bottom trophic level, as algal standing crop was great est in the presence of trout. These results suggest that the consequen ces of trout may extend beyond the replacement of native fish species. Although our approach allows us to ask whether the potential exists f or community-wide impacts of invasive fishes, it remains unclear wheth er multiple trophic-level effects will be readily detectable in many e cological settings. The high degree of spatial and temporal heterogene ity common to running water systems, and the confounding effects of la nd-use transformations, may impede our ability to discern community-le vel changes following this and other species introductions.