Effects of hydraulic conditions and larval size on the microdistribution of Hydrobiosidae (Trichoptera) in two New Zealand rivers

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Abstract
Hydraulic conditions utilised by common taxa of the caddisfly family Hydrobiosidae were characterised for the Mangles River, South Island, and the Tongariro River, North Island, New Zealand. Costachorema spp. (predominantly C. xanthopterum and C. callistum) and Hydrobiosis parumbripennis larvae were collected from a wide range of water depths (10–155 cm) and velocities (8–178 cm s−1) and from predominantly small gravel to large cobble substrates. Overall densities of Costachorema spp. were significantly (P < 0.01) and positively correlated with water velocity, Froude number, boundary layer Reynolds number, and inferred shear velocity at both sites. Similar relationships were evident for total densities of H. parumbripennis in the Mangles River, whereas a significant inverse correlation was detected between density of this species and water depth and Reynolds number in the Tongariro River. Strongest correlations with densities of Costachorema spp. or H. parumbripennis were detected using water depth, velocity, or Froude number. Almost all (≥ 90%) C. xanthopterum larvae with head widths > 0.60 mm and most (72%) C. callistum larvae with head widths > 0.91 mm were found in fast water (> 90 cm s−1). There was also a tendency for proportionately more large H. parumbripennis larvae (head widths > 1.2 mm) to be found in faster water. Some possible mechanisms behind the observed microdistribution patterns in relation to hydraulic factors and larval size are discussed.