Control of freshwater fish and crayfish community structure in Taranaki, New Zealand: dams, diadromy or habitat structure?

1. Freshwater fish and crayfish communities were surveyed along elevational gradients in streams radiating from Mount Taranaki, New Zealand. Six of the 38 streams surveyed had darns or weirs and 32 of the 85 sites were above these barriers. 2. Of the 15 fish and one crayfish species captured, 14 were native. The number of species declined with increasing elevation and distance from the sea. Species richness was also lower above darns even when effects of elevation and distance from the sea were accounted for. 3. Linear regression models using sites without dams were constructed to predict the effect of elevation on fish and crayfish community structure and to allow the effect of dams to be evaluated. The number of species predicted by these models was consistently higher than the number of species found at the above-dam sites. 4. Four fish community groups were classified using two-way indicator species analysis (TWINSPAN). One high elevation group of sites consisted of short steep streams on the west side of the mountain and a second contained longer, lower gradient streams on the east side. The other two groups (3 and 4) consisted of mid-elevation sites and low-elevation sites, respectively. 5. Discriminant analysis was used to predict biotic group membership using the environmental data. Overall, 80% of sites were classified correctly. Correlation of environmental variables with axis scores in the canonical variate analysis revealed that distance from the sea, site elevation and presence of dams were most strongly associated with fish distribution patterns.