Polymorphic mountain whitefish (Prosopium williamsoni) in a coastal riverscape: Size class assemblages, distribution, and habitat associations

Fluvial mountain whitefish (Prosopium williamsoni) offer a rare example of trophic polymorphism among stream dwelling salmonids in temperate North American river systems. Trophic polymorphism is a form of resource-based phenotypic diversification that occurs when exploitation of under-utilized resources necessitates specific morphological characteristics. Studies of trophic polymorphism in the species are few, focusing on diet, foraging behavior, genetic variation, and snout morphology. Pinocchio whitefish develop an elongated snout used to overturn substrate while foraging for benthic invertebrates, whereas normal whitefish have an even sloping head shape and feed primarily on drifting prey. I hypothesized that (1) morphotypes would be associated with distinctly different habitat features based on known feeding behavior and diet, and (2) these differences in habitat association would segregate morphotypes at channel unit and reach scales. In this study, I first compared the spatial distributions of morphotypes and size classes. I then assessed the assemblage structure of morphotypes and size classes and their relationships with aquatic habitat. Finally, I quantified the associations between specific morphotypes and longitudinal variation in aquatic habitat. Spatially continuous sampling was conducted over a broad extent (29 km) in the Calawah River, WA (USA). Whitefish were enumerated via snorkeling in three size classes: small (10--29 cm), medium (30--49 cm), and large (≥ 50 cm). Spatial distributions were compared among size classes of morphotypes by relating cumulative abundance to distance upstream. Assemblage structure and relationships with aquatic habitat were assessed with non-metric multidimensional scaling (NMS). Unit-scale differences between morphotypes in pool and non-pool habitats were quantified with Wilcoxon rank sum tests. Associations between specific morphotypes and habitat were quantified at the 1-km reach scale with generalized additive models (GAMs) using thin-plate regression splines. Large size classes of both morphotypes were distributed downstream of small and medium size classes, and normal whitefish were distributed downstream of pinocchio whitefish. Normal whitefish size classes were associated with higher gradient and depth, whereas pinocchio whitefish size classes were positively associated with pool area, distance upstream, and depth. Normal whitefish relative density did not differ between habitat types at the unit scale (p = 0.34, W = 3085), but pinocchio whitefish were more dense in pool habitat than in non-pool habitat (p < 0.0001, W = 1084). Reach-scale GAMs indicated that normal whitefish were associated with larger substrate size in downstream reaches (R 2 = 0.64), and pinocchio whitefish were associated with greater stream depth in the reaches farther upstream (R2 = 0.87). Results at the reach scale suggested broad scale spatial segregation (1--10 km), particularly between larger and more phenotypically extreme individuals, whereas unit-scale results indicated spatial overlap between morphotypes within individual pools. These results provide the first perspective on spatial distributions and multi-scale habitat relationships of polymorphic mountain whitefish within a coastal riverscape.