ASPECTS OF JUVENILE RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) DIET IN RELATION TO FOOD-SUPPLY DURING SUMMER IN THE LOWER TONGARIRO RIVER, NEW-ZEALAND

Invertebrates were collected from the benthos, drift, and stomachs of juvenile rainbow trout (Oncorhynchus mykiss) in different flow environ ments and at different times of day in the lower Tongariro River in De cember 1992, to investigate spatial and diel patterns in prey abundanc e and diet. The benthic and drift communities were dominated numerical ly by Diptera (both 71%), Oligochaeta (22-23%), and Trichoptera (5% an d 2% in the benthos and drift, respectively). Terrestrial invertebrate s comprised 3% of the drift. The most common prey in the stomachs of j uvenile rainbow trout (44-130 mm fork length) were Diptera (74%), Tric hoptera (19%), Ephemeroptera (6%), and some terrestrial organisms. Rel ative abundances of different invertebrate taxa in the benthos, drift, and stomachs of juvenile rainbow trout were all significantly interco rrelated. Juvenile trout fed selectively on Trichoptera (particularly emerging adults), the ephemeropteran Deleatidium spp., and some Dipter a (mostly Maoridiamesa and Aphrophila neozelandica), and avoided Oligo chaeta. The stomach fullness index was similar during three periods be tween dawn and dusk, indicating that feeding activity was continuous. The proportion of Diptera in the diet of small fish was higher than in larger fish and the reverse was observed for the proportion of Tricho ptera. Densities of most benthic invertebrate taxa favoured by juvenil e trout were highest in medium or fast flowing habitats, suggesting th at maintenance of such conditions is important for food production.