Selection of Spawning Sites by Coho Salmon in a Northern California Stream

—We assessed the relative importance of various factors contributing to spawning site use by a population of threatened coho salmon Oncorhynchus kisutch in Freshwater Creek, California, and created a predictive model of spawning habitat selection based on logistic regression analysis. We excluded sampling sites that previous studies had established as unsuitable on the basis of depth and substrate criteria and asked why fish chose particular locations and not others in seemingly suitable habitat. We evaluated surface water velocity, depth, substrate size composition, gravel inflow rates, vertical hydraulic gradient, geomorphic channel units, hyporheic water physicochemistry, cover, and proximity to other redds not in sampling sites during the 2004–2005 spawning season. In univariate comparisons with unused sites, coho salmon selected sites with a smaller median particle diameter, a larger percentage of gravel–pebble substrate, and higher gravel inflow rates. Based on multivariate logistic regression, the probability of a site’s being used for spawning was best modeled as a positive function of the gravel–pebble fraction of the substrate, location at a pool or run tail, and the presence of existing redds in close proximity to the site. This model explained 38% of the variation in the data and was a better predictor of spawning habitat use than a more traditional model based on depth, velocity, and substrate. Our results highlight the potential importance of social behavior in contributing to habitat selection by spawning salmonids. Habitat use by salmonids in streams is typically described on the basis of water depth, water velocity, and substrate composition. These three variables are widely used in traditional models, such as the physical habitat simulation system (PHABSIM) (Milhous et al. 1989), and their importance has been demonstrated in several studies of spawning site selection by salmonids (e.g., Reiser and Wesche 1977; Witzel and MacCrimmon 1983; McHugh and Budy 2004), as well as in the predictive model of Knapp and Preisler (1999). However, some studies have suggested that depth, velocity, and substrate alone may underestimate or overestimate the area suitable for spawning, and PHABSIM has been shown to have limited accuracy in predicting the use of spawning habitat. For example, Shirvell (1989) found that 70% of the spawning area used by a population of Chinook salmon Oncorhynchus tshawytscha was predicted to be unusable, and 87% of the area predicted to be usable never had spawning activity. This result suggests that spawning site selection may be more complex than traditional