The image depicts the mean habitat change for adult brown trout, when comparing the natural flows (no abstractions) with the current state consented abstractions. Abstractions (takes) are depicted as “.” in the figure, and for each river segment (NZReach) the physical habitat change for brown trout was simulated, using simulated mean daily flows. Hence river segments that are coloured in green indicate that brown trout has better habitat due to abstractions, whereas river segments that are coloured red indicate that abstractions are causing a reduction in habitat for brown trout.
So, abstractions (taking water from the river) can lead to an improvement in physical habitat, as the river might have been too high or too fast without any abstractions, and taking water 'shifts' the flow environment closer to the optimum conditions for brown trout. (Dave Allen)