DROUGHT, DIFFERENTIAL MORTALITY AND THE COEXISTENCE OF A NATIVE AND AN INTRODUCED FISH SPECIES IN A SOUTH EAST AUSTRALIAN INTERMITTENT-STREAM

The longitudinal distribution of Salmo trutta and Galaxias olidus, a s mall salmoniform fish, was mapped over four summers (1985-1988) in the upper reaches of the Lerderderg River, an intermittent stream in cent ral Victoria. Over the four successive summers of the study, the distr ibution of S. trutta expanded upstream. Coincident with the expansion of S. trutta was a contraction in the distribution of G. Olidus upstre am. In the summer of 1988, an extended dry spell resulted in high leve ls of S. trutta mortality at the upstream limits of their distribution . This suggested that the upstream distribution of S. trutta could ult imately be constrained by an inability to tolerate prolonged periods o f low stream flow and high temperatures that tended to occur in the he adwaters of the stream. A study of summer mortality was conducted over three successive summers (1989-1991) at three reaches along the river . The two downstream reaches contained only S. trutta, the upstream re ach contained both S. trutta and G. olidus. Salmo trutta mortality was consistently higher at the most up stream reach compared with lower r eaches. High levels of S. trutta mortality at the two downstream reach es were only observed in 1991, a particularly dry year. In contrast, G . olidus survived in scattered small pools throughout the upper reache s of the stream. The results of the study suggest a shifting pattern o f S. trutta and G. olidus distribution determined by the duration of l ow stream flow periods and summer high temperatures.