Effects of biotic and abiotic factors on the distribution of trout and salmon along a longitudinal stream gradient Ernesto A. de la Hoz Franco & Phaedra Budy USGS Utah Cooperative Fish and Wildlife Research Unit, Department of Aquatic, Watershed and Earth Resources, 5210 Old Main Hill, Utah State University, Logan, UT 84322-5210, U.S.A. (e-mail: phaedra.budy@usu.edu) Received 16 October 2003 Accepted 14 July 2004 Key words: cutthroat trout, exotics, competition, temperature, distribution, conservation Synopsis We examined the influence of biotic and abiotic factors on the distribution, abundance, and condition of salmonid fishes along a stream gradient. We observed a longitudinal change in fish distribution with native cutthroat trout, Oncorhynchus clarki utah, and introduced brown trout, Salmo trutta, demonstrating a distinct pattern of allopatry. Cutthroat trout dominated high elevation reaches, while reaches at lower elevations were dominated by brown trout. A transition zone between these populations was associated with lower total trout abundance, consistent changes in temperature and discharge, and differences in dietary preference. Variation in cutthroat trout abundance was best explained by a model including the abundance of brown trout and diel temperature, whereas variation in brown trout abundance was best explained by a model including the abundance of cutthroat trout and discharge. These results suggest the potential for condition-mediated competition between the two species. The results from our study can aid biologists in prioritizing conservation activities and in developing robust management strategies for cutthroat trout. Introduction The spatial arrangement of biotic and abiotic habitat components may influence not only the distribution and abundance of individual fish spe- cies across different spatial scales (Bozek & Rahel 1991, Grossman et al. 1995, Rahel & Nibbelink 1999) but also community-level properties such as species richness and production (Hawkes et al. 1986, Hughes & Gammon 1987, Rahel & Hubert 1991). Biotic factors that affect fish populations and communities include inter- and intraspecific com- petition (e.g., Fausch & White 1981), disease (e.g., de la Hoz Franco & Budy 2004), predation (e.g., Gilliam & Fraser 2001), and food availability (e.g., Bowlby & Roff 1986); abiotic factors that often affect fish distribution and abundance in streams include temperature and discharge (Jackson et al. 2001, Lobon-Cervia 2003). How- ever, the interaction of these biotic and abiotic factors can also determine the distribution of spe- cies arranged along longitudinal or altitudinal gradients in streams (Fausch et al. 1994). An allopatric pattern of species distribution is often observed in streams that provide a longitu- dinal gradient of habitat for salmonids, where the species that is present at high-elevation reaches overlaps little with another species that occupies reaches at low elevations (e.g., Fausch 1989). Models which attempt to understand this distri- bution, when physical barriers are missing, include explanations of restricted upstream movement based on physical limitations (Moyle and Light 1996) and those based on some combination of species interactions and environmental variables (Dunson & Travis 1991). Limitations in the Environmental Biology of Fishes (2005) 72: 379–391 Ó Springer 2005