The effects of decreasing discharge on the behaviour and distribution of O+ brown trout (Salmo truttaL), were studied in the Silverstream, a tributary of the Taieri River, Otago. The preferred habitat is runs, where the behaviour is territorial, but with decreasing discharge most of the population moved into pools and adopted shoaling behaviour. Analysis of the physical changes indicated that current velocity was the significant factor, with a threshold value of 0.30 m s‐1for poolward movement. The discharge giving a mean run velocity of 0.30 m s‐1was compared with empirical criteria for minimum satisfactory flows for fish, and very close agreement was found.