AN APPRAISAL OF THE ALLEN PARADOX IN A NEW-ZEALAND TROUT STREAM

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.