An empirical assessment of factors precluding recovery of Sauger in the lower Yellowstone River : movement, habitat use, exploitation, and entrainment
Jaeger, Matthew Edward.
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Sauger (Sander canadensis) were designated as a critically imperiled Species of Special Concern in Montana because of declines in distribution and abundance. Migratory barriers, habitat loss, entrainment in irrigation canals, and overexploitation, especially at times when sauger were aggregated, were suggested to explain the failure of Yellowstone River sauger to return to historical abundances. I characterized seasonal movement patterns, habitat use, and aggregation of sauger and estimated movement, exploitation, and Intake Canal entrainment rates to test these hypotheses. Seasonal movement, aggregation, and habitat use were investigated by telemetering and tracking 30 fish in 2001, 31 fish in 2002, and 30 fish in 2003. Exploitation and entrainment rates were assessed by tagging 957 sauger with reward tags. Tag-shedding rate (2.1%) was estimated by double-tagging and non-reporting rate (61.5%) was estimated using postcards as tag surrogates. Sauger aggregated near spawning areas in spring and subsequently dispersed 5 to 350 km to upstream home river locations where they remained for the rest of the year. 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 use during spawning was rare. Following movement to home river locations, sauger used most habitat types in proportion to their availability but selected reaches of specific geologic types. Exploitation occurred primarily in early spring and late autumn. Annual survival was high (61.3%). Entrainment in irrigation diversions may have accounted for more than half of non-fishing mortality. Migratory barriers, habitat loss, and overexploitation of adult sauger likely are not preventing sauger recovery, but the effects of these factors may be more pronounced for juvenile sauger. Habitat alteration and interactions with non-native walleye and smallmouth bass may also preclude recovery.