Local-Habitat, Watershed, and Biotic Features Associated with Bull Trout Occurrence in Montana Streams

—We evaluated the association of local-habitat features, large-scale watershed factors, the presence of nonnative brook trout Salvelinus fontinalis, and connectivity to neighboring populations with patterns of occurrence of threatened bull trout S. confluentus in 112 first-order to fourthorder streams in the Bitterroot River drainage in western Montana. Species presence or absence was estimated via single-pass electrofishing, local-habitat features were measured in 500-m sampling reaches, watershed variables were obtained from topographic maps, and potential demographic support from nearby bull trout populations occupying larger main-stem streams was estimated from electrofishing data records. We defined a set of nine candidate models that represented various combinations of these four main factors and used an information-theoretic approach to evaluate the relative plausibility of competing models. Models combining local habitat (width, gradient, and woody debris) with brook trout presence and the main-stem abundance of bull trout and a global model (all variables) were the best approximating models. In contrast, watershed models based on elevation, basin area, and tributary slope and models with local-habitat or biotic variables alone were poor predictors of bull trout occurrence. Bull trout occurrence was positively associated with channel width, large woody debris, and the presence of a ‘‘strong’’ neighboring main-stem population and negatively associated with channel gradient and the presence of brook trout. Our findings suggest that bull trout have increased resistance to invasion by brook trout in streams with high habitat complexity and connectivity. Consideration of abiotic and biotic factors at multiple scales, along with a means for ranking their relative importance, is needed to perform more comprehensive assessments of landscape and local influences on species distribution patterns. A growing body of literature suggests that the patterns of distribution for many fishes are the result of both local-habitat conditions and largerscale biotic and abiotic processes (e.g., Rabeni and Sowa 1996; Dunham et al. 1997; Schrank et al. 2001). The physical characteristics of streams at the small scale of individual habitat units or stream reaches have often been associated with variation in fish density (e.g., Rabeni and Sowa 1996; Watson and Hillman 1997). However, large-scale watershed or landscape features such as stream size, basin area, spatial geometry, and stream temperature or surrogates for climate (Bozek and Hubert * Corresponding author: [email protected] 1 Present address: Alaska Department of Fish and Game, 333 Raspberry Road, Anchorage, Alaska 99518, USA. 2 Present address: U.S. Geological Survey, Arkansas Cooperative Fish and Wildlife Research Unit, Department of Biological Sciences, University of Arkansas, Fayetteville, Arkansas 72701, USA. Received August 1, 2002; accepted March 10, 2003 1992; Fausch et al. 1994; Rieman and McIntyre 1995; Dunham et al. 1997, 1999; Harig and Fausch 2002) as well as biotic factors such as the presence of nonnative species and degree of isolation from other populations (Osborne and Wiley 1992; Dunham and Rieman 1999; Schrank et al. 2001) have also been implicated. Large-scale processes are likely to be important because landscape features influence (1) local-scale habitat features (Rabeni and Sowa 1996), (2) species interactions or individual physiological responses (Fausch et al. 1994), and (3) dispersal, colonization, and gene flow within and among local populations (Luttrell et al. 1999; Rieman and Dunham 2000). Patterns of the occurrence of bull trout Salvelinus confluentus provide a good illustration of the influence of both local and large-scale factors on species distribution. Several studies have linked bull trout density in stream reaches to local-habitat features such as pool frequency, amount of large wood and fine sediments, water temperature, and the presence of groundwater (Saffel and Scarnecchia 1996; Watson and Hillman 1997; Jakober et