Factors affecting the distribution of blue duck Hymenolaimus malacorhynchos on New Zealand rivers

Abstract

Benthic invertebrates and/or physical habitat data were collected along 37 sections of river to help develop criteria for selecting sites for the establishment of new populations of blue duck Hymenolaimus malacorhynchos. Sections of river where blue duck pairs were present had significantly greater physical stability (range of Pfankuch scores = 61–103), ch channel gradient (> 12 m km −1) and elevation (82–1050 m a.s.l.), narrower widths (8–60 m), coarser bed substrata (particularly more boulders), and more riparian native forest (>50% of vegetation on average) than sections where blue duck were absent. Abundance and taxonomic richness of the invertebrate faunas (major foods of blue duck) were not significantly different between sections with and without birds, but invertebrate faunas of sections where blue duck pairs were present had significantly greater proportions of stoneflies, more stonefly taxa, and lower proportions of dipteran larvae. Density of blue duck at the time of the survey was significantly correlated with physical stability of the river such that more stable sections had more pairs per kilometre. Physical stability, channel gradient, and the composition of riparian vegetation and bed substrata are recommended as useful criteria for selecting sections of river for the establishment of new populations of blue duck in the future.