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.