Influence of turbidity on brook trout reactive distance and foraging success

Past research has focused on the effects of sediment action on stream morph ology and the habitat of brook trout Salvelinus fontinalis. Throughout the Appalachian Mountains, the watersheds in which brook trout reside are bring influenced by timber harvesting and related road construction. Although th ese streams may have gradients steep enough to prevent deleterious sediment deposition, elevated stream turbidity is nevertheless common. An understan ding of the sublethal effects of increased sedimentation and turbidity is e ssential to further our knowledge of the effects of increased sediment load ing on stream fish production and how these effects differ among species li ving in sympatry. The specific objectives of this study were to determine ( 1) the effects of turbidity on the reactive distance of brook trout, (2) ho w turbidity affects encounter rates between brook trout and their prey, and (3) how turbidity affects brook trout's foraging success. We used videogra phic techniques to study brook trout foraging behavior in an artificial str eam. Three brook trout were tested during each sampling period, creating a competitive situation under which a more accurate measure of reactive dista nce could be made. Treatment turbidity levels ranged from 0 to 43 nephelome tric turbidity units. The reactive distance of brook trout decreased curvil inearly with increasing turbidity. The probability of a brook trout's react ing to a given prey item was correlated with reactive distance and also dec reased with turbidity. However, turbidity had no influence on the probabili ty of attack given a reaction, the probability of capture given an attack, or the probability of ingestion given capture. In natural streams, inverteb rate drift densities typically increase with turbidity as Rows increase, bu t this increase may not be enough to compensate for the decreased ability o f brook trout to detect drifting prey.