M. Dedual et Kj. Collier, ASPECTS OF JUVENILE RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) DIET IN RELATION TO FOOD-SUPPLY DURING SUMMER IN THE LOWER TONGARIRO RIVER, NEW-ZEALAND, New Zealand Journal of Marine and Freshwater Research, 29(3), 1995, pp. 381-391
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.