Date of Award

Winter 1992

Project Type


Program or Major


Degree Name

Doctor of Philosophy

First Advisor

Peter F Sale


Some benthic insects in streams disperse by drifting short distances downstream in the water column. While drifting they are at risk from predators, such as trout, that feed in the water column. I examined prey selection by brook trout (Salvelinus fontinalis L.) in Stoney Brook, New Hampshire, U.S.A., and effects of trout on drift dispersal and benthic density of five prey taxa (mayflies: Ephemeroptera).

Effects of trout on mayflies were tested by field manipulation of trout density in replicate 35 m long sections of stream. Trout consumed all five mayflies in roughly similar numbers. Feeding rates of trout (by weight) were not significantly influenced by time of day, or by trout density. However, comparison of prey consumed to prey available indicated that selection for mayflies, and for larger prey was greater during the day than at night. Benthic densities of the two mayflies drifting most frequently, Baetis and Paraleptophlebia, were reduced at high trout densities, whereas densities of the other three mayflies were unaffected by trout. Daytime drift of none of the five mayflies was affected by trout. However, nighttime drift (both drift density (no.m$\sp{-3}$water) and departure from 0.5 m$\sp2$ patches of substratum) of Baetis and Paraleptophlebia was increased at high trout density. Effects of trout on the density of Baetis were due primarily to increased drift dispersal from stream sections, whereas both direct predation and increased drift contributed to the reduction in density of Paraleptophlebia by trout. Nighttime drift (departure from 0.5 m$\sp2$ patches of substratum) of Ephemerella was reduced in the presence of trout, but this reduction was not detected at a larger scale (35 m stream sections).

Trout influenced the size distribution of two of the five mayflies. The mean size of benthic Ephemerella decreased as trout density increased, whereas the mean size of benthic Paraleptophlebia was increased where trout were present. The mechanism for these shifts in size distribution was uncertain, though the effect of trout of Paraleptophlebia was most likely due to size-specific alteration of drift dispersal.

Influences of trout on mayfly populations were thus complex. However, influences of trout on drift dispersal were important in determining the effects of trout on benthic populations, and can explain some variation among prey taxa in the effect of predators.