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 environments and at different times of day in the lower Tongariro River in December 1992, to investigate spatial and diel patterns in prey abundance and diet. The benthic and drift communities were dominated numerically by Diptera (both 71%), Oligochaeta (22–23%), and Trichoptera (5% and 2% in the benthos and drift, respectively). Terrestrial invertebrates comprised 3% of the drift. The most common prey in the stomachs of juvenile rainbow trout (44–130 mm fork length) were Diptera (74%), Trichoptera (19%), Ephemeroptera (6%), and some terrestrial organisms. Relative abundances of different invertebrate taxa in the benthos, drift, and stomachs of juvenile rainbow trout were all significantly intercorrelated. Juvenile trout fed selectively on Trichoptera (particularly emerging adults), the ephemeropteran Deleatidium spp., and some Diptera (mostly Maoridiamesa and Aphrophila neozelandica ), and avoided Oligochaeta. The stomach fullness index was similar during three periods between 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 Trichoptera. Densities of most benthic invertebrate taxa favoured by juvenile trout were highest in medium or fast flowing habitats, suggesting that maintenance of such conditions is important for food production.