Invasion biology and ecological impacts of brown trout Salmo trutta in New Zealand

Brown trout Salmo trutta were introduced to New Zealand in 1867. Successful establishment was broadly predictable in terms both of the characteristics of brown trout and of the receiving community. There is evidence of impacts of brown trout on the abundance of some native fish and invertebrates, and brown trout have been responsible Jor the local extinction and fragmentation of certain species. An intensive study of the Taieri River has revealed that several native galaxiid fishes are now restricted to headwaters above large waterfalls that prevent the upstream migration of brown trout. Brown trout may profoundly affect the functioning of stream communities, reducing the abundance of grazing invertebrates and altering their grazing behaviour so that algal biomass increases. A trophic cascade was predictable on the basis of the attributes of the invader and of the stream community. Brown trout seem to have been responsible for the evolution among invertebrates of novel antipredator behaviours with far-reaching community consequences. The ecological and evolutionary consequences of the introduction of brown trout to New Zealand are probably reversible. Keywords: brown trout, invasion biology, acclimatization societies, Galaxiidae, trophic cascade.