Trophic cascading effects of predatory fish on leaf litter processing in aJapanese stream

A manipulative field experiment to test for trophic cascading effects of pr edatory fish on detritus processing by benthic invertebrates was performed in stream channels running through a wetland forest in northern Japan, To c ontrol for fish effects on benthic invertebrates, two simple treatments (fi sh-present and fish-absent) were established for 4 weeks, with two common p redatory fish, rainbow trout (Oncorhynchus mykiss) and freshwater sculpin ( Cottus nozawae), being introduced into and excluded from stream cages. At t he end of experiment, the biomass of the dominant detritivore, an amphipod (Jesogammarus jezoensis), was significantly less in the fish-present treatm ent (0.56 g m(-2) in dry mass on average) than that in the fish-absent trea tment (1.32 g m(-2)), there being no significant treatment effect evident f or the second-dominant detritivore, coleopteran larvae (Optioservus kubotai ). The loss of oak leaves (Quercus crispla) from titter bags in the fish-pr esent treatment (0.31 g week(-1) in dry mass on average) was significantly less than in the fish-absent treatment (0.54 g week(-1)). Predator-induced lower biomass and likely lowered foraging activities of the J. jezoensis we re responsible for the suppression of litter processing efficiency. In cont rast, the standing crop of fine particulate organic matter did not differ s ignificantly between the treatments. The experimental results revealed that the predatory fish had an indirect but significant effect on leaf litter p rocessing in the stream.