Linking drift and benthic density along fishless to fish transitions in Rocky Mountain streams

Abstract

The influence of predators on drift and benthic density is important for understanding prey community structure in freshwater streams. Drift activity of stream invertebrates in fishless streams does not differ between day and night, but when visual drift-feeding trout are present, stream invertebrates drift nocturnally with peaks just after sunset. Drift and benthic collections from a fishless to fish transition zone in two Rocky Mountain streams were used to evaluate the hypothesis that 1) daytime and nighttime drift remain constant above the fish barrier and 2) benthic density is greater downstream of the fish barrier. Two drift samples (day and night) and six benthic samples were taken within a 100 m reach along the fishless to fish transition. Drift density showed no difference between day and night when sampling just above the fish barrier. Benthic densities were highly variable and no difference was found between the upstream and downstream reach or between sample sites. Average residence time was calculated using the number of Baetis bicaudatus drifting per day and the number of Baetis bicaudaus in a 50 reach downstream and upstream of the barrier. Average residence time in both the upstream and downstream reach of a fish barrier was approximately 6 to 12 days. The influence of an introduced predator downstream of a fish barrier may influence the decision of Baetis to drift. Future research should consider the distance traveled and size of invertebrates drifting in order to determine how transitioning into a fish stream affects the drift propensity of these organisms.

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