Exploring the Persistence of Stream-Dwelling Trout Populations under Alternative Real-World Turbidity Regimes with an Individual-Based Model

—We explored the effects of elevated turbidity on stream-resident populations of coastal cutthroat trout Oncorhynchus clarkii clarkii using a spatially explicit individual-based model. Turbidity regimes were contrasted by means of 15-year simulations in a third-order stream in northwestern California. The alternative regimes were based on multiple-year, continuous monitoring in two streams. Turbidity affected model fish by reducing both their risk of predation and their reactive distance to drifting prey. It did not affect their ability to locate nondrifting food, such as invertebrates on the stream bottom. Under a calibration scenario that assumed trout predominantly consume drifting prey, the less-turbid real-world regime produced relatively stable abundance across years (similar to field observations) whereas the more-turbid regime (under otherwise identical physical conditions) resulted in extinction within the 15-year simulation period. Additional simulations revealed sensitivity to the relative amounts of prey available via drift versus search feeding and showed that seasonal variation in food availability or strong positive relationships between streamflow and food concentration would not prevent extinction in the high-turbidity regime under a drift-feeding-based food calibration. Extinction of predominantly drift-feeding trout populations in our simulations contrasts with field observations of salmonid populations that have persisted in moderately turbid regimes. The results highlight the need for better understanding of patterns in the availability of food under turbid conditions and the capability of stream salmonids to use nonvisual cues in feeding. Logging, road building, and other human activities can increase the levels of suspended sediment in streams, with direct consequences for individual stream salmonids, including death (Bozek and Young 1994). Observed sublethal effects of elevated suspended sediment on stream salmonids include impaired respiration (Berg and Northcote 1985), increased physiological stress (Redding et al. 1987), lower feeding success because of reduced reactive distance to drifting prey (Barrett et al. 1992; Sweka and Hartman 2001a), and lower growth rates in short-term experiments (Shaw and Richardson 2001; Sweka and Hartman 2001b). For drift-feeding fishes such as salmonids, the effect of turbidity on prey capture success would appear to be of particular concern because it occurs at modest levels of turbidity. In contrast to some well-established effects of elevated turbidity on individual fish, the populationlevel effects of elevated but sublethal suspended sediment concentrations are less certain (but see Rowe et al. 2000). Addressing this issue for salmonid populations is challenging because (1) many factors other than suspended sediment influence their abundance; (2) significant spatiotemporal variation in suspended sediment characterizes most natural streams; (3) turbidity can have positive effects on fish by reducing predation risk (Gregory and Levings 1998); and (4) our knowledge of the dependence of overall feeding success on turbidity remains incomplete. We are unaware of any empirical observations on stream fish that quantify population-level consequences of elevated suspended sediment where fish cannot avoid elevated suspended sediment by way of habitat