Annual contribution of terrestrial invertebrates to a New Zealand trout stream

Whole-stream drift from stream reaches enclosed by 1 mm mesh barriers was sampled to estimate the potential contribution of terrestrial invertebrates to trout production in a high-country pasture stream in New Zealand. Invertebrates were classified into two activity groups: winged or wingless. Biomass of winged insects was significantly related to temperature but not discharge. Biomass of wingless invertebrates was not related to temperature or discharge; however, abundance was significantly related to discharge but not temperature. These results suggest that mode of entry differed between activity groups. Winged insects apparently entered the stream as a function of overall activity, whereas wingless invertebrates entered the stream passively-possibly a-result of fluctuations in discharge. An empirical model based on these relationships indicated that total input of terrestrial invertebrates may support as much as 5% of annual production by resident brown trout.