Seasonal Movements, Habitat Use, Aggregation, Exploitation, and Entrainment of Saugers in the Lower Yellowstone River: An Empirical Assessment of Factors Affecting Population Recovery

Abstract

AbstractMigratory barriers, habitat loss, entrainment in irrigation canals, and overexploitation, especially at times of aggregation, have been suggested to explain the failure of Yellowstone River saugers Sander canadensis to return to historical abundances after a late‐1980s decline that was attributed to drought. These factors are thought to affect saugers throughout their range and migratory large‐river fishes in general. We characterized the seasonal movement patterns, habitat use, and aggregation of saugers and estimated movement, exploitation, and irrigation canal entrainment rates to test these hypotheses. Saugers aggregated near spawning areas in spring and subsequently dispersed 5–350 km to upstream home locations, where they remained for the rest of the year. Upstream movement was not overtly restricted by low‐head diversion dams. During the spawning period, terrace and bluff pools, which are unique geomorphic units associated with bedrock and boulder substrate, were positively selected, while all other habitat types were avoided. Tributary spawning was rare. After moving to home locations, saugers used most habitat types in proportion to their availability but selected reaches in specific geologic types that allowed formation of deep, long pools. Exploitation occurred primarily in early spring and late autumn, was low annually (18.6%), and was not related to aggregation. Annual survival was high (70.4%). Entrainment in one irrigation diversion accounted for more than half of all nonfishing mortality. Therefore, habitat loss and overexploitation probably did not prevent sauger recovery, as the absence of migratory barriers allowed adult saugers unrestricted access to widely separated and diverse habitats and did not induce artificial aggregation. In other systems, population declines attributed to overexploitation during periods of aggregation may therefore have been caused fundamentally by migration barriers that created artificial aggregations.