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The influence of natural flow and temperature and introduced brown trout on the temporal variation in native fish abundance in a “reference” stream

Understanding what drives variation in fish abundance at reference sites provides perspective for assessing the effects of human alterations to river flow and land use. We examined temporal variation in fish community abundance in a headwater tributary of a large river in the upper South Island (New Zealand) over 10 years. We were interested in the influence of natural flow variation and temperature on native fish abundance within the context of potential competition/predation pressure from juvenile introduced trout. Results from biannual sampling highlighted the dominating influence of floods on fish population dynamics, overriding biotic effects. We found no evidence for adverse effects of trout on native fish, and flow-related habitat performed more poorly in explaining variation in fish abundance than low- and median-flow statistics. Differences in temporal variation in abundance between species were largely consistent with life histories that provide resistance and (or) resilience to flood flows and relative insensitivity to low flows. Long-term data (≥10 years) are needed for detecting meaningful trends and quantifying the effects of human activities on fish community abundance.