Studies of invertebrate production in trout streams have often shown p
rey production to be insufficient to support trout production while si
multaneously providing a reasonable surplus of prey (Alien paradox). T
he Alien paradox was explained by analyzing a comprehensive production
budget for a trout stream in New Zealand. Budget compartments include
d primary production, production by brown trout and surficial and hypo
rheic macroinvertebrates, input of terrestrial invertebrates, and cann
ibalism by trout. Ecological efficiencies from the literature were use
d to estimate food demands. Although much of the food demand by trout
was apparently derived from surficial macroinvertebrates, a balanced b
udget was obtained only when all other prey sources were included. The
budget indicated that surplus production by benthic macroinvertebrate
s was nil. However, given the uncertainty of budget statistics, surplu
ses as high as similar to 10-20% would probably not be detected. Secon
dary production by primary consumers required only similar to 20% of t
otal primary production, indicating strong top-down control of herbivo
ry by trout. Results from this study, other recent studies showing str
ong effects of trout on stream food webs, and the long tradition of th
e Alien paradox suggest that in productive streams (>100 kg wet mass t
rout ha(-1) yr(-1)) trout may consume most (>80%) benthic prey product
ion.