Influence of Turbidity on Brook Trout Reactive Distance and Foraging Success

Past research has focused on the effects of sediment action on stream morphology and the habitat of brook trout Salvelinus fontinalis. Throughout the Appalachian Mountains, the watersheds in which brook trout reside are being influenced by timber harvesting and related road construction. Although these streams may have gradients steep enough to prevent deleterious sediment deposition, elevated stream turbidity is nevertheless common. An understanding of the sublethal effects of increased sedimentation and turbidity is essential to further our knowledge of the effects of increased sediment loading on stream fish production and how these effects differ among species living in sympatry. The specific objectives of this study were to determine (1) the effects of turbidity on the reactive distance of brook trout, (2) how turbidity affects encounter rates between brook trout and their prey, and (3) how turbidity affects brook trout's foraging success. We used videographic techniques to study brook trout foraging behavior in an artificial stream. Three brook trout were tested during each sampling period, creating a competitive situation under which a more accurate measure of reactive distance could be made. Treatment turbidity levels ranged from 0 to 43 nephelometric turbidity units. The reactive distance of brook trout decreased curvilinearly with increasing turbidity. The probability of a brook trout's reacting to a given prey item was correlated with reactive distance and also decreased with turbidity. However, turbidity had no influence on the probability of attack given a reaction, the probability of capture given an attack, or the probability of ingestion given capture. In natural streams, invertebrate drift densities typically increase with turbidity as flows increase, but this increase may not be enough to compensate for the decreased ability of brook trout to detect drifting prey.