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 small salmoniform fish, was mapped over four summers (1985-1988) in the upper reaches of the Lerderderg River, an intermittent stream in central Victoria. Over the four successive summers of the study, the distribution of S. trutta expanded upstream. Coincident with the expansion of S. trutta was a contraction in the distribution of G. Olidus upstream. In the summer of 1988, an extended dry spell resulted in high levels of S. trutta mortality at the upstream limits of their distribution. This suggested that the upstream distribution of S. trutta could ultimately be constrained by an inability to tolerate prolonged periods of low stream flow and high temperatures that tended to occur in the headwaters 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 reach contained both S. trutta and G. olidus. Salmo trutta mortality was consistently higher at the most up stream reach compared with lower reaches. High levels of S. trutta mortality at the two downstream reaches were only observed in 1991, a particularly dry year. In contrast, G. olidus survived in scattered small pools throughout the upper reaches of the stream. The results of the study suggest a shifting pattern of S. trutta and G. olidus distribution determined by the duration of low stream flow periods and summer high temperatures.