Food habit divergence between white-spotted charr and masu salmon in Japanese mountain streams: circumstantial evidence for competition

The diet and foraging microhabitat of white-spotted charr, Salvelinus leucomaenis, were compared between mountain stream reaches where it occurs with (sympatric) and without (allopatric) masu salmon, Oncorhynchus masou masou, a potential competitor, to examine the evidence for interspecific competition between these fish, which commonly co-occur in Japanese mountain streams. In three streams examined, the similarity between the diets of allopatric charr and salmon was much greater than that between the diets of sympatric charr and salmon. Both allopatric charr and sympatric salmon intensively utilized terrestrial invertebrates among stream drifts (52%-65% of the diet), whereas this prey category made up only an intermediate portion of sympatric charr diets (11%-29%). Examination of available prey composition in stream drifts showed that the consumption of terrestrial invertebrates by allopatric charr and sympatric salmon was approximately twice as much as that by sympatric charr. The presence of salmon, a potential competitor, may alter the diet of charr in the sympatric reaches. Charr holding focal points closer to the streambed were considered less efficient than sympatric salmon in their utilization of terrestrial invertebrates drifting primarily on the stream surface, although the foraging microhabitat of the charr was not influenced by the salmon. The mechanisms responsible for the dietary divergence between sympatric charr and salmon are probably the consequence of scramble competition over terrestrial invertebrates drifting on the stream surface.[PUBLICATION ABSTRACT]