PREY AVAILABILITY AND DIET OF JUVENILE BROWN TROUT (SALMO-TRUTTA) IN RELATION TO RIPARIAN WILLOWS (SALIX SPP) IN 3 NEW-ZEALAND STREAMS

Macroinvertebrate benthos and drift and the diet of juvenile brown tro ut (Salmo trutta) were investigated in willowed and non-willowed secti ons of three New Zealand streams during summer. The abundance and biom ass of the principal macroinvertebrate taxa in the benthos and daytime drift differed in relation to riparian willow (Salix spp.) density, b ut the trends were not consistent across all three streams. Crustacea, Ephemeroptera, Trichoptera, and Diptera comprised a major proportion of the diet of juvenile trout. Diets of trout from non-willowed and wi llowed sections were dissimilar in three of five pair-wise comparisons . In all three streams, juvenile trout fed selectively, with the amphi pod Paracalliope fluviatilis, ephemeropteran Deleatidium, and trichopt erans Aotenpsyche and Hydrobiosidae being the preferred prey. Their av oidance of the gastropod Potamopyrgus antipodarum, trichopteran Oxyeth ira albiceps, coleopteran Hydora, and drift of terrestrial origin appe ared to occur in all streams. Total abundance and biomass of invertebr ates drifting during the day described the potential prey available to juvenile brown trout better than did the abundance and biomass of ben thic invertebrates.