Pulsed supplies of small fish facilitate short-term intraguild predation in salmon stocked streams

Abstract Pulsed supplies of prey organisms generally increase predator food intake. However, whether this holds true when predators and pulsed prey are in same guild (i.e., intraguild [IG] predators and prey) is unclear. IG prey may increase IG-predator food intake by providing a food source, but they may decrease food intake through competition. To test these hypotheses, we compared the food intake of white-spotted charr (Salvelinus leucomaenis) (IG predator) in streams that were stocked or unstocked with hatchery-reared masu salmon (Oncorhynchus masou) fry (IG prey) in streams in Hokkaido, Japan. One day after stocking, masu salmon fry occupied nearly 60% of the stomach contents by wet weight of white-spotted charr in stocked streams, and mean stomach content weight was six-times higher than in unstocked streams. However, predation of white-spotted charr on stocked fry was rare on other days. Acquisition of predator-avoidance behavior by stocked fry and/or a lack of accommodation by white-spotted charr to the sudden emergence of a new prey source (e.g., the charr in study sites were basically insectivorous, and their foraging behavior might have been ineffective for piscivory) may explain this time-limited intraguild predation. In days other than the first day post-stocking, food intake by white-spotted charr did not differ between stocked and unstocked streams. No effects of interspecific competition on white-spotted charr food intake were observed; this may be due to the body-size advantage of white-spotted charr and/or the low density of stocked masu-salmon fry.