Salvelinus malma (Dolly Varden charr) and S. leucomaenis (white-spotted charr) are morphologically similar and closely related fishes in the family Salmonidae. The two species are found together in many streams on Hokkaido Island in Japan, but Dolly Varden are distributed at higher altitudes (further upstream) than white-spotted charr, with a zone of overlap at intermediate altitudes. In streams where one species happens to be absent, the other expands its range, indicating that the distributions may be maintained by competition (i.e. each species suffers, and is thus excluded from certain sites, in the presence of the other species). Water temperature, an abiotic factor with profound consequences for fish ecology (discussed already in Section 2.4.4), increases downstream.
By means of experiments in artificial streams, Taniguchi and Nakano (2000) showed that when either species was tested alone, higher temperatures led to increased aggression. But this effect was reversed for Dolly Varden when in the presence of white-spotted charr (Figure 8.1a). Reflecting this, at the higher temperature, Dolly Varden were suppressed from obtaining favorable foraging positions when white-spotted charr were present, and they suffered lower growth rates (Figure 8.1b, c) and a lower probability of survival.
Thus, the experiments lend support to the idea that Dolly Varden and white-spotted charr compete: one species, at least, suffers directly from the presence of the other. They coexist in the same river, but on a finer scale their distributions overlap very little. Specifically, the white-spotted charr appear to outcompete and exclude Dolly Varden from downstream locations in the lat-ter's range. The reason for the upper boundary of white-spotted charr remains unknown as they did not suffer from the presence of Dolly Varden at the lower temperature.
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