FRAGMENTED POPULATION-STRUCTURE IN A NATIVE NEW-ZEALAND FISH - AN EFFECT OF INTRODUCED BROWN TROUT

We conducted a survey of 198 sites in eight catchments in the Taieri River drainage of the South Island, New Zealand, and found a strong negative association between the distributions of introduced brown trout and native Galaxias vulgaris. Although habitat degradation due to agricultural and forestry practices was associated with lower densities of fish, these reductions were found in all fish species and did not help to explain the trout/galaxiid pattern. Statistical analyses incorporating a variety of physical, chemical and biological variables showed that presence and abundance of G. vulgaris were best predicted by the absence of trout. In most cases, G. vulgaris were only found above waterfalls that were large enough to inhibit trout migration. We suggest that predation is the most likely mechanism for the observed disjunct distributions between the native fish and trout. Moreover, the strong predation effect is probably a result of New Zealand's comparative paucity of native piscivorous fishes, resulting in a native fish fauna having no evolved defenses.