Length Variation in Age-0 Westslope Cutthroat Trout at Multiple Spatial Scales

—Phenotypic diversity provides ecological and evolutionary functions, stabilizing populations in variable environments. Although benefits of larger body size in juvenile fishes are well documented, size variation may have value as well. We explored the distribution of length and length variation in age-0 westslope cutthroat trout Oncorhynchus clarkii lewisi at three spatial scales: area (10 km), stream (10 km), and site (10 km). In addition, we examined relationships between length variables (mean length and interquartile range of length) and instream (temperature and conductivity) and landscape (aspect, elevation, headwater distance, and valley width) variables that were expected to be associated with fish size. Conductivity was included as a surrogate for productivity. Most variation in mean length and interquartile range of fish length was found among areas (62.2% and 62.6%, respectively). Mean length also varied among streams and sites (21.9% and 15.8%, respectively). Similarly, interquartile range of fish length varied among streams and sites (19.1% and 18.3%, respectively). Both length variables were associated with temperature and elevation. Mean fish length was also associated with conductivity, but the association between interquartile length range and conductivity was weak. We conclude that the conservation of variation in phenotypic attributes, such as length, in westslope cutthroat trout may require conservation of viable populations across broad areas and across environmental gradients that are associated with growth. Phenotypic variation—the intraspecific variation in life history, morphology, and behavior—plays a critical part in many evolutionarily and ecologically essential functions (Skulason and Smith 1995) and therefore may be important to conserve. Processes driven by phenotypic variation include the ability to adapt to local conditions and disturbance regimes, population differentiation that may lead to speciation, and the ability to reduce intraand interspecific competition through niche differentiation (e.g., Utter 1981; Skulason and Smith 1995). Diversity in some traits may allow adjustment to variable environments and compensation in processes that are closely linked to survival and fitness (Caswell 1983; Hilborn et al. 2003). Demographic modeling suggests that variation among individuals reduces extinction risk for small populations (Kendall and Fox 2002); variation among populations may contribute to the persistence of larger systems (Hilborn et al. 2003). Causes, consequences, and evolutionary potential of phenotypic variation are important questions in the ecology and conservation of organisms in variable environments (Schlichting 1989). The processes responsible for generating phenotypic variation in cutthroat trout Oncorhynchus clarkii and other migratory salmonids (Gresswell et al. 1994) and the extent and spatial distribution of variables associated with those processes are poorly understood. Successful fish conservation and management may require recognition and understanding of phenotypic variation and processes generating that variation (Gresswell et al. 1994; Beechie et al. 2006). Organism size has been well recognized as an important phenotypic characteristic that mediates various biological processes, including metabolism, growth, production rate, reproductive condition and commitment, and constraints on body function (Peters 1983). Through these relationships, body size has important effects on organism fitness. The benefits of large juvenile size may include increased first-year survival, intraspecific competitive advantage, and reduced vulnerability to predation (Sogard 1997). However, there are also advantages to small size and associated feeding behaviors. Predators have been shown to prefer larger prey (Litvak and Leggett 1992). Actively feeding and satiated individuals may * Corresponding author: [email protected] 1 Current address: Department of Environmental and Forest Biology, College of Environmental Sciences and Forestry, State University of New York, 1 Forestry Drive, Syracuse, New York 13210, USA Received July 12, 2007; accepted April 8, 2008 Published online October 16, 2008 1529 North American Journal of Fisheries Management 28:1529–1540, 2008 Copyright by the American Fisheries Society 2008 DOI: 10.1577/M07-120.1 [Article]