Potential Effects of Reduced Flows on Fish Habitats in a Large Braided River, New Zealand

Abstract

To predict the effects of reduced flows on potential rearing habitat for fish in a large braided river (the lower Rakaia River on South Island, New Zealand), water depth, velocity, and substrate data were collected along transects in two representative reaches at five discharges ranging from 69 to 146 m3/s during 1981–1982. From these measurements and habitat-suitability curves, weighted usable area (WUA) was computed for each of six species of fish. Within both reaches WUA declined with decreasing discharge for each of the native fish species, although variably so in space and time (2–33%), whereas for the two exotic salmonid species there was no consistent and appreciable change. The overall decrease in WUA was attributed primarily to the loss of habitat in minor channels (<1 m3/s). Weighted usable area ranged from approximately 20,000 m2/km of river for juvenile chinook salmon Oncorhynchus tshawytscha, which prefer relatively deep, slack water with cobble-bedded substrates, to 80,000 m2/km for the longfinned eel Anguilla diefenbachii, which inhabits a wide range of water depths and velocities, probably because of its ability to live among the interstices of bed sediments. Inclusion of substrate size distribution in the data analysis significantly lowered the amount of WUA for all species (range 15–65%), but had no appreciable effect on the rate of change of WUA with decreasing discharge. For all the branch channels measured (ranges 0.01–76 m3/s and 3–140 m wide), average WUA for six fish species combined was positively correlated with channel discharge and channel width, suggesting that WUA for the whole river would continue to decline with decreasing river discharge below 69 m3/s. Unlike single-thread rivers, braided rivers tend to show (within the general range of base flows) relatively little change in WUA with decreasing discharge, because the loss of habitat in minor channels is offset by the gain of habitat in major channels. Received August 26, 1983 Accepted November 10, 1984