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

Abstract

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 = 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 = 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 upstream = 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 long-distance 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.