1. Foraging behaviour, diet and interference competition were examined for two morphologically similar charrs, Salvelinus malma (Dolly Varden) and S. leucomaenis (white-spotted charr), under varying food resource conditions over four summers in a Japanese mountain stream. Data were used to test predictions from a mechanistic model of resource partitioning developed from an earlier field experiment.
2. The charrs adopted one of two distinct foraging modes, ambushing drifting invertebrates from relatively fixed foraging positions or actively searching for benthic prey over large areas. The proportion of benthos foragers increased markedly when drifting prey declined, and was much greater in S. malma than S. leucomaenis when drift was lean, upholding predictions made from our earlier experiment.
3. For drift foragers of both species, frequency of foraging attempts decreased as drift rate declined, and aggressive encounters increased. In contrast, for benthos foragers of both species the frequency of foraging attempts was essentially constant across the range of benthos biomass measured, and aggressive encounters remained low.
4. Salvelinus leucomaenis ate larger drifting prey than S. malma, even though the former charr were smaller. In contrast, S. malma foraged on benthic prey at a higher rate than S. leucomaenis, although there was no difference in prey mass. Thus, the optimal point to shift to benthic foraging is at a higher drift threshold for S. malma than S. leucomaenis, most probably due to differences in jaw morphology. Moreover, because dominance for favourable drift foraging positions was based on size alone, S. malma shifted to benthos foraging at a larger size than S. leucomaenis, as predicted by a simple model.
5. Charr consumed distinct prey types according to their foraging mode. Drift foragers primarily ate terrestrial invertebrates, whereas benthos foragers ate mainly chironomid larvae. Consequently, diet overlap was high when drift was abundant and both species were drift foragers, but declined as drift declined and S. malma shifted to benthos foraging. Therefore, species-specific differences in foraging mode shifts across the resource gradient explained the flexible resource partitioning we observed, and probably account for the coexistence of these congeneric charrs in zones of sympatry in northern Japan.