Recolonization by the mountain galaxias Galaxias olidus of a montane stream after the eradication of rainbow trout Oncorhynchus mykiss

The introduced salmonid Oncorhynchus mykiss was eradicated by use of the piscicide rotenone from a section of small montane stream upstream of an impassable barrier. Recolonization of the stream both above and below the barrier by the native Galaxias olidus was monitored annually for four successive years. Following trout eradication, G. olidus recolonized the trout-free stream section above the barrier but was never detected below the barrier where trout still occurred. Initial colonization was by juvenile G. olidus but a successful breeding population had established three years after trout eradication. The implications of the use of barriers and targeted eradication programmes are discussed for the management of small, threatened fish species.