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

—Migratory 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. Saugers Sander canadensis are among the most widely distributed of North American fishes; however, declines in distribution and abundance have resulted from rangewide habitat fragmentation and degradation (Rawson and Scholl 1978; Hesse 1994; Pegg et al. 1997; McMahon and Gardner 2001). Impoundment and subsequent flow management isolate saugers from important spawning and rearing habitats, reduce turbidity and temperature, and alter the timing and magnitude of the * Corresponding author: [email protected]. Received September 20, 2004; accepted July 11, 2005 Published online November 4, 2005 hydrograph from the natural regime in which saugers evolved (Hesse 1994; McMahon and Gardner 2001). Channelization reduces off-channel rearing habitats (Gardner and Berg 1980; Hesse 1994). Unrestricted access to widely separated and diverse habitat types throughout the year is critical to riverine fishes in general (Schlosser 1991; Fausch et al. 2002). The sauger’s highly migratory nature (Collette et al. 1977; Penkal 1992; Pegg et al. 1997), propensity to spawn in only a few areas (St. John 1990; Penkal 1992), and reliance on a wide variety of habitats with natural temperatures and turbidities throughout their life history (Penkal 1992; Hesse 1994; Amadio et al. 2005) combine 1551 YELLOWSTONE RIVER SAUGERS FIGURE 1.—Map of the lower Yellowstone River and major tributaries, diversion dams, and underlying geologic types showing the spawning and home locations of telemetered saugers during 2001–2003. Where overlap of location points occurs, outlying spawning and home locations are displayed to allow full representation of each distribution. to make this percid species one of the most sensitive to habitat fragmentation and alteration (McMahon 1999). Rangewide declines in sauger abundances have also been attributed to overexploitation of aggregations (Nelson 1969; Hesse 1994; Pegg et al. 1996); however, most previous studies have implied that the apparent propensity of saugers to aggregate makes them susceptible to overexploitation. Effective management and mitigation of anthropogenic influences requires an understanding of the ecology of a species under natural conditions. However, almost all major river systems where saugers occur are altered to such an extent that research directed at achieving such an understanding is no longer possible. The Yellowstone River retains a near-natural hydrograph and relatively pristine main-stem habitats (White and Bramblett 1993), making it the most suitable large river in which to examine the movements, habitat use, and other factors affecting sauger population abundance under natural conditions. Sauger abundance in the Yellowstone River declined to historical lows after a regionwide drought during 1987–1990 (McMahon and Gardner 2001). However, the failure of the sauger population to recover after five out of the seven subsequent years (1991–1997) displayed aboveaverage spring–summer discharge suggested that factors in addition to discharge affected abundances (McMahon and Gardner 2001). Many of the same factors believed to affect saugers throughout their range (i.e., migratory barriers, habitat loss, and overexploitation during times of aggregation) were suggested to explain the failure of Yellowstone River saugers to return to historical abundances (McMahon and Gardner 2001). Entrainment in irrigation canals was also thought to affect recovery; over 67,000 saugers are entrained each year in the Intake Diversion Canal (Figure 1; Hiebert et al. 2000), the largest of six major diversions on the Yellowstone River. However, a lack of information regarding sauger exploitation and entrainment rates, specifically as they related to equally unknown seasonal movement and aggregation patterns and habitat use, made it difficult to assess the validity of these hypotheses and to effectively manage saugers in the Yellowstone