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The relationship of invertebrate drift abundance to the standing crop of benthic organisms in a small stream Peterson, G. Ross


The object of this study was to test the influence of benthic invertebrate drift in a stream on the benthic standing crop; whether it operated only as an agent for removal of surplus organisms from the standing crop or in a more indiscriminate manner, leading to depletion of these organisms in upstream areas. It was hypothesised that drift acts as a benthic population regulator and therefore would not influence a standing crop at optimum density. This was tested by comparing standing crop changes in two sections at a small stream differing in numbers of drift predators, coho salmon underyearlings ( Oncorhynchus kisutch (Walbaum) ), and steelhead trout underyearlings and juveniles (Salmo gairdnerii Richardson). It was found that these two salmonid species derived a significant proportion of their diet from groups of invertebrates which frequently occur in the drift. It is inferred therefore that predation by the fish contributed to observed differences in drift abundance between areas with high and low fish population density. However, it was not possible to quantify the consumption of drift accurately because it could not be determined whether an organism found in the stomach of a fish came from the drift or directly from the benthos. It was assumed that all means of accrual and removal of organisms to and from the standing crop other than drift and predation by salmonids were equal in the two stream sections. Benthic standing crop measurements showed no differences in density between stream sections that could be attributed to the drift difference. It was expected that some decrease would have resulted in the high fish density section relative to the low density section as more direct predation on the benthos would have occured in that section. It was found, however, that the standing crop increased in the high fish density section relative to the low fish density section, suggesting a greater influence on the standing crop by the unmeasured factors of accrual and removal than the measured drift and predation factors.

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