Complementation of Habitats for Bonneville Cutthroat Trout in Watersheds Influenced by Beavers, Livestock, and Drought

—Multiple age-classes of Bonneville cutthroat trout Oncorhynchus clarkii utah throughout two Rocky Mountain watersheds were influenced by interactions among geomorphology, land use, activity by beavers Castor canadensis, and drought. Age-0 trout were present in a limited portion of the watersheds, and their distribution became increasingly restricted as drought conditions developed over a 3-year period. The Coal Creek watershed (including Huff Creek) produced the most age-0 trout in the first 2 years of the drought, lacked beaver activity, and was affected by land use, suggesting that spawning habitat was determined by geomorphology rather than land use or beaver activity. However, the high abundance of age-0 cutthroat trout in Huff Creek did not result in a high abundance of juvenile and older age-classes of fish in subsequent years, most likely because of the lack of complementary habitats providing refuge for older fish. A nearby watershed and its major stream, Water Canyon, had less spawning habitat and produced fewer age-0 fish during the first 2 years of the study but had more trout in the juvenile and adult age-classes, most likely because of a higher degree of habitat complementation. In Water Canyon, less-intense livestock grazing and the presence of beavers allowed for the development of pools and woody riparian vegetation that provided cover for older trout. Water Canyon was also the only stream to produce age-0 trout during the most severe year of drought, suggesting that streams with more natural habitat may provide a spawning refuge during low-flow conditions that occur periodically in the region. These results demonstrate that habitat complementation is important for the coexistence of multiple age-classes of fish and that the adjacency of spawning habitat and refugia is crucial for the persistence of fish in the face of environmental stress associated with drought. Habitat conditions that influence the distribution and abundance of stream fishes are often the result of interactions among geomorphologic processes, climatic events, and land use patterns. At large spatial scales, basin geomorphology and geochemistry determine the range of conditions possible for smaller scale factors such as gradient, substrate types, pool–riffle development, biological productivity, and temperature regimes (Modde et al. 1991; Baxter and Hauer 2000; Isaak and Hubert 2001a). Climate conditions, such as drought or prolonged cold periods, contribute to a natural range of stream temperature and flow variability that characterizes most stream systems (Poff et al. 1997). Land use practices often modify stream habitat conditions and, in extreme cases, may push streams beyond their range of natural variability (Magee and McMahon 1996; Isaak and Hubert 2001b; Marchetti and Moyle 2001). Understanding how basin geomorphology, land use patterns, and climatic conditions interact to influence stream habitat conditions is especially challenging for the management of fish populations in areas designated for multiple uses such as livestock grazing, timber harvest, mining, and recreation. Land use practices that have relatively minor impacts on stream habitats in some geomorphologic settings can be quite harmful in other settings (Nelson et al. 1992). Furthermore, efforts to improve habitat conditions by adding physical structures to streams or by altering grazing practices in the watershed may be confounded by natural climate fluctuations (Binns and Remmick 1994; Gowan and Fausch 1996). Maintaining healthy fish populations requires maintaining access to the set of complementary habitats needed by various life history stages (Schlosser 1995; Rosenfeld et al. 2000). In a general sense, complementation can be thought of as the use of nonsubstitutable resources by individual organisms moving between habitat patches in a landscape (Dunning et al. 1992). Fish populations may be negatively affected by anthropogenic structures such as dams or road culverts that disrupt the connectivity between complementary habitats (e.g., Fausch et al. 2002; Schrank and Rahel 2004). Fish populations may also be negatively affected by land use activities that diminish the quality of habitats needed by particular life history stages, even if the habitats needed by other * Corresponding author: [email protected] 1 Present address: Oregon Cooperative Fish and Wildlife Research Unit, Department of Fisheries and Wildlife, Oregon State University, 104 Nash Hall, Corvallis, Oregon 97331, USA. Received September 8, 2006; accepted November 30, 2007 Published online May 1, 2008 881 Transactions of the American Fisheries Society 137:881–894, 2008 Copyright by the American Fisheries Society 2008 DOI: 10.1577/T06-207.1 [Article]