Sections of a small coastal British Columbia stream that had previously been cleaned of large woody debris (LWD) were compared with sections where most debris was left and with others where debris had been relatively undisturbed for at least 40 yr. Three sections where debris had been removed had simple habitat that was less sinuous, wider, and shallower and had less pool volume and overhead cover than four sections with more complex habitat where debris was retained. Habitat in four relatively undisturbed sections was generally similar to complex sections. Most pools in all sections were scour or plunge pools formed by LWD or large roots oriented perpendicular to the flow or angled downstream. Standing crop (kolograms per hectare) and individual weights of age 1 + and older coho salmon (Oncorhynchus kisutch) and cutthroat trout (O. clarki) were significantly greater (P < 0.02) in complex than in simple sections. Biomass of age 1 + and older salmonids was closely related to section pool volume (r2 = 0.92, P = 0.0006). Projections based on this model and average habitat conditions suggest that during 1990 a total of 8.0 kg of salmonid biomass, 5 times the current standing crop, was forgone in the 332-m simple reach due to prior debris removal.

We investigated the effect of turbidity on the foraging behaviour of juvenile chinook salmon (Oncorhynchus tshawytscha) in the laboratory. We assessed visual ability by measuring the reaction distance of juvenile chinook to planktonic adult Artemia prey. Chinook exhibited a log-linear decline in reaction distance with increasing turbidity. These results were similar to those obtained by workers for other species. We also determined the effect of turbidity on the foraging rate of juvenile chinook for surface (Drosophila), planktonic (Artemia), and benthic (Tubifex) prey across a range of turbidity levels (< 1,18, 35, 70, 150, 370, 810 NTU). Foraging rates were reduced at higher turbidities for all three prey. However, for surface and benthic prey, foraging rates were also low in clear water, while highest rates were attained at intermediate turbidity levels (35–150 NTU). We suggest that increased feeding rate in turbid conditions reflects a reduced potential risk to predators.

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The role of fish in regulation of the structure and function of freshwater ecosystems (lakes and streams) is examined with respect to effects resulting directly from their feeding activity, indirectly from size or other forms of prey selectivity, and from influences on nutrient cycling and transport. That fish have long had a legitimate place in limnological research is documented by a review of limnological books, texts, and journals. Recognition of their "top-down" effects, started largely from Czechoslovakian research (Hrbáček and others) in the mid-1950's, was popularized by American and German research (Brooks, Jacobs, and others) in the mid-1960's and has enjoyed a remarkable upsurge in the 1970's, continuing into the 1980's. Although size-selective predation has been an important phenomenon in stimulating widespread research on effects of fish on freshwater communities and their function, the importance of other mechanisms in such regulation is considered.