An appraisal of the Allen paradox in a New Zealand trout stream

Studies of invertebrate production in trout streams have often shown prey production to be insufficient to support trout production while simultaneously providing a reasonable surplus of prey (Alien paradox). The Alien paradox was explained by analyzing a comprehensive production budget for a trout stream in New Zealand. Budget compartments included primary production, production by brown trout and surficial and hyporheic macroinvertebrates, input of terrestrial invertebrates, and cannibalism by trout. Ecological efficiencies from the literature were used to estimate food demands. Although much of the food demand by trout was apparently derived from surficial macroinvertebrates, a balanced budget was obtained only when all other prey sources were included. The budget indicated that surplus production by benthic macroinvertebrates was nil. However, given the uncertainty of budget statistics, surpluses as high as similar to 10-20% would probably not be detected. Secondary production by primary consumers required only similar to 20% of total primary production, indicating strong top-down control of herbivory by trout. Results from this study, other recent studies showing strong effects of trout on stream food webs, and the long tradition of the Alien paradox suggest that in productive streams (>100 kg wet mass trout ha(-1) yr(-1)) trout may consume most (>80%) benthic prey production.