Comparison of fish and macroinvertebrate standing stocks in relation to riparian willows(Salixspp.) in three New Zealand streams

Abstract

The distribution of fish and benthic invertebrates in relation to riparian willow (Salix spp.) concentrations was investigated in three New Zealand streams. In each of the streams, representative sites were sampled quantitatively in willowed and non‐willowed sections. The abundance (m^‐2) and biomass (g m^‐2) of brown trout (Salmo trutta L.) were significantly greater in the willowed than in the non‐willowed sections in two streams; the third stream showed the same trend but any differences were not significant. Mean size of trout was greater in the willowed than in the non‐willowed sections, but size of fish varied greatly and the differences were not significant. Eels (Anguilla spp.) showed no consistent pattern in population characteristics between streams; however, their abundance in one river and biomass in another were significantly greater in the willowed than in the non‐willowed sections. Distribution of other fish (e.g., Galaxias spp. and Gobiomorphus spp.) appeared not related to riparian willows. The abundance and biomass of several benthic invertebrate taxa (e.g., Ephemeroptera, Trichoptera, Diptera) differed significantly in relation to riparian willow concentrations. In all three streams, species richness and diversity of the benthic invertebrates were greater in the willowed than the non‐willowed sections, with moderate degrees of similarity between the communities. Moderately‐willowed riparian zones are beneficial to trout and food organisms in streams and small rivers.