Sympatric Mountain Whitefish and Trout in a Coastal Riverscape: Factors Influencing Ecological Patterns at a Hierarchy of Spatial Scales

Mountain whitefish (Prosopium williamsoni) are widely distributed and relatively abundant among temperate rivers in western North America; however, their role in stream ecosystems remains largely unknown. In particular, small coastal drainage basins of western Washington (USA) that support mountain whitefish represent a unique portion of their geographic distribution, yet little is known about these populations. The species has been considered a competitor of trout, but sympatric populations have only been examined in terms of diet and micro-habitat use, with no examples of competitive interactions with trout. Mountain whitefish exhibit trophic polymorphism in northern temperate rivers, including the Olympic Peninsula in Washington. However, studies of morphological differences have not been examined within the context of other sympatric species or habitat use at intermediate spatial scales. The Calawah River (western Olympic Peninsula) supports coastal cutthroat trout (Oncorhynchus clarki), coastal rainbow/steelhead trout (O. mykiss), and a morphologically diverse population of mountain whitefish. This study investigates patterns of distribution and habitat use among whitefish morphotypes and sympatric trout species over a broad extent (30 km) at three spatial scales: reach (1-2 km), channel unit (20-200 m), and sub-unit (<10 m). In addition, differences in microhabitat use at the sub-unit scale are paired with behavioral observations of interference competition among all three species.