Experimentally induced foraging mode shift by sympatric charrs in a Japanese mountain stream

Foraging mode shifts may allow animals flexibility to adapt to a variety of ecological conditions. Theory holds that ectotherms such as fishes should shift from ambush to active search modes when prey density declines, to maintain a minimum encounter rate. Salvelinus malma (Dolly Varden) sympatric with S. leucomaenis (white-spotted charr) in a northern Japan mountain stream were observed to shift from ambushing drifting invertebrates at fixed focal points to picking benthic invertebrates from the substrate along circuitous routes throughout pools, as drift declined throughout summer. Experimentally reducing drift in two pools during early summer when all charr were drift foragers caused subordinate malma to make marked shifts from drift to benthic foraging within 1 h, whereas nearly all leucomaenis remained drift foragers. Some charr of both species responded to reduced drift by emigrating from pools. Combined results of three replicate experiments revealed that malma shifted to benthic foraging when their rate of drift forays was reduced below a specific threshold, thereby upholding predictions of the theory. Adaptive foraging mode shifts may promote coexistence of these congeneric charrs in reaches of Japanese mountain streams.