Annual Intrabasin Movement and Mortality of Adult Bonneville Cutthroat Trout among Complementary Riverine Habitats

—We evaluated annual movement and mortality patterns of native Bonneville cutthroat trout Oncorhynchus clarkii utah in the Smiths Fork–Bear River watershed, which is part of the Great Basin in the western USA. Our objectives were to identify complementary habitats within the watershed, identify sources of mortality for Bonneville cutthroat trout during seasonal movements, and determine whether anthropogenic structures disrupt movement of adult Bonneville cutthroat trout within this system. Bonneville cutthroat trout migrated upstream during spring runoff (median distance 1⁄4 37.1 km) and experienced a seasonal mortality rate of 43% during this period. After spawning in the headwater streams, fish moved downstream during the summer–autumn period (median distance 1⁄4 11.6 km) and experienced a seasonal mortality rate of 16%. Whereas upstream movement in the spring was fast and highly directed, downstream movement during summer–autumn was slower and less directed. During winter, fish remained generally sedentary (median movement upstream1⁄4 0.1 km) and the seasonal mortality rate was 11%. No anthropogenic structures blocked fish movement throughout the watershed. However, an irrigation canal entrained 9% of the fish that moved past its headgate, suggesting that the canal may act as an ecological trap. Our results provide empirical support for conceptual models that emphasize the importance of habitat complementarity as the basis for annual longdistance movement patterns in riverine fishes. Managing migratory species such as the Bonneville cutthroat trout will require maintaining river connectivity and minimizing ecological traps so that fish can move among widely separated habitats to meet their life history requirements. Movement between different areas is common in many species of fish (Lucas and Baras 2001). This movement provides fish an opportunity to exploit new resources, escape inhospitable conditions, or avoid predation. To understand the reasons for such movements, conceptual models have emphasized the importance of habitat complementarity—the idea that no single habitat can satisfy all of the ecological and life history requirements of a species (Schlosser 1991; Dunning et al. 1992; Northcote 1997). The basic premise of these models is that optimal habitat for a species varies (1) across life history stages, (2) with seasonal changes in abiotic conditions or resource availability, or (3) in response to interactions with other species. Thus, to maximize survival, growth, and reproduction, fish species often need to move among contrasting habitats. Often, movements are over short distances that are considered to be within the home range of the individual (Dingle 1996). Such movements allow fish to exploit spatially variable food resources or minimize interactions with competitors or predators (Gowan and Fausch 2002). In addition to short-distance movements, there can be directed movements that cover longer distances that extend beyond the home range. These long-distance movements are often associated with spawning events and may result in movement to areas with different habitat conditions compared with where the fish normally resides. These directed, undistracted movements exhibited by many riverine fishes are known as migrations (Dingle 1996). Depending on the species and system, movement among complementary habitats can have ecological tradeoffs. Anadromous and semelparous fish perish after spawning in freshwater systems, and even iteroparous fish can have high mortality rates associated with spawning migrations (e.g., Vinyard and Winzeler 2000; Narum et al. 2008). Additionally, anthropogenic breaks in the riverscape (e.g., dams or seasonal dewatering in the system) may disrupt movement among habitats (Schlosser 1995; Fausch et al. 2002), potentially negating the benefits associated with a migratory life history. As research and management paradigms shift from emphasizing small areas and short time periods to considering multiple scales that include large geographical areas and long time periods, concepts such as connectivity, landscape patchiness, and large-scale ecological changes become important for understanding the complex life history * Corresponding author: [email protected] 1 Present address: Minnesota Department of Natural Resources, Section of Fisheries, 1601 Minnesota Drive, Brainerd, Minnesota 56401, USA. Received January 20, 2009; accepted May 3, 2010 Published online August 9, 201