Effects of a severe flood on instream habitat and trout populations in seven New Zealand rivers

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Abstract
The effects of a major flood on instream habitat and trout numbers in 7 rivers in the South Island, New Zealand, were determined by drift diving (snorkeling) sections of each of the rivers before and after the event. A general coarsening of the substrate was noted in runs and pools but not in riffles. In most of the rivers there was significant scouring of pools but little change of run and riffle habitats. In 6 of the 7 rivers, brown trout (Salmo trutta) abundance decreased significantly, with small fish (10–20 cm fork length) being reduced by 90–100%, medium fish (20–40 cm FL) by 62–87%, and large fish (> 40 cm FL) by 26–57 %. Rainbow trout (Salmo gairdneri) occurred in 3 of the rivers but were not present in 2 of these after the flood. In the third river the abundance of largera in bow trout increased substantially whereas that of small fish decreased by 94%. The percentages of trout remaining in the surveyed reaches after the flood were highly correlated with river gradient and instream cover but not with relative severity of the flood.

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