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

Pulsed supplies of prey generally increase predator food intake. However, it is unclear whether this holds true when predators and pulsed prey are in same guild (i.e., intraguild [IG] predators and prey). IG prey may increase IG-predator food intake through predation, 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 masu salmon (Oncorhynchus masou) fry (IG prey) in streams in Hokkaido, Japan. One day after stocking, mean stomach content weight of charr was six-times higher than in unstocked streams due to fry consumption. In particular, large charr showed intense piscivory. However, predation on fry was rare on other days. Decreasing small fry may be partially responsible for the short-term occurrence of predation. In addition, acquisition of predator-avoidance behavior by fry and/or a lack of accommodation by charr to the sudden emergence of a new prey source may explain this time-limited intraguild predation. In days other than the first day post-stocking, food intake by charr did not differ between stocked and unstocked streams. No effects of interspecific competition on charr food intake were observed.