| Summary: | The conditions under which fish have a large impact on benthic invertebrate
community structure in streams are poorly defined. To better understand the
mechanisms by which fish affect lotic invertebrate community structure, a series of
experiments were performed in a fishless and fish-bearing stream in the University of
British Columbia Malcolm Knapp Research Forest in the Coast Mountains of British
Columbia.
The first experiment examined the effect of predation by prickly sculpin (Cottus
asper) and coho parr (Onchorhynchus kisutch) on invertebrates associated with tile and
gravel substrate placed in enclosures installed in pools and riffles in fishless Mayfly
Creek.. Effects of fish were most pronounced on tile substrate and in riffle habitat, and
least pronounced on gravel substrate in pool habitat. Fish predation resulted in decreased
density of larger-bodied (>6mm) herbivores (primarily the mayflies Ameletus and Baetis)
and increased abundance of smaller (< 3mm) herbivorous invertebrates (primarily
Orthocladiinae chironomids and nemourid stoneflies) and algae. The effects of fish
predation in riffles in fish-bearing Jacobs Creek were similar to those observed in Mayfly
Creek, except that there was no increase in algal biomass in the presence of fish.
The indirect increase in algal biomass and density of small herbivores observed
under fish predation was reproduced in experimental stream channels by excluding large
invertebrate herbivores (tailed frog tadpoles (Ascaphus) and Ameletus), confirming that
direct effects of fish on large-bodied grazers released algae and smaller herbivores from
competitively dominant grazers.
Manipulation of coho densities in upper enclosure sections in a fourth experiment
indicated that upstream predation could have a small but measurable indirect effect on
algal biomass in downstream patches.
The abundance of detritivores in Mayfly and Jacobs Creek appear to be largely
controlled by bottom-up forces (resource limitation), while herbivores are more strongly
influenced by top-down effects (fish predation). A model for effects of fish predation on
invertebrate community structure is proposed which predicts stronger predation effects
in algal-based food chains than detrital-based food chains, both of which occur at a
hierarchy of spatial scales in streams. === Science, Faculty of === Zoology, Department of === Graduate
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