Telemetry was used to evaluate seasonal and diel movement patterns, general habitat use, survival, and spatial distributions of hatchery‐reared juvenile pallid sturgeon Scaphirhynchus albus stocked in the Missouri River below Fort Randall Dam, South Dakota and Nebraska. Sampling occurred at about 2‐week intervals during spring through fall. Of 22 ultrasonically tagged fish, 13 were intensively followed to assess hourly diel movement rates. A total of 258 relocations were made for the 22 tagged fish; 19 fish were relocated at least once, and 16 were relocated multiple times. At least 68% of tagged fish survived for 2.5 years. In 2000, fish were found mainly upstream of the stocking site, whereas in 2001 and 2002 the entire reach was used. Juveniles were found primarily in the main channel at depths exceeding 80% of the maximum channel cross‐section depth, where bottom current velocities ranged from 0.1 to 0.9 m/s. Clusters of juvenile pallid sturgeon (i.e., two or more fish) within 750 m of each other were observed on 15 dates, and clusters of juveniles within 500 m of each other were observed on 10 dates. Clusters were observed during all seasons. Mean (±2 SE) range of movement was significantly less in 2000 (8.3 ± 5.8 km) than in 2001 (29.3 ± 8.8 km) and 2002 (28.9 ± 15.0 km). We detected no significant differences in movement rates among years, seasons, or diel periods (i.e., dawn, daytime, dusk, and night). Our study indicates that range of movements of juvenile pallid sturgeon based on shorter studies (i.e., ≤1 year) should be interpreted cautiously because hatchery‐reared fish may not have acclimated to the riverine environment until the second year after stocking. Based on survival and distribution of juvenile pallid sturgeon throughout the study area, we conclude that this reach is suitable for recovery efforts.

Summary The Pallid Sturgeon is listed as federally endangered under the Endangered Species Act in the United States. When the species was listed in 1990 it was considered extremely rare and was poorly understood. Habitat alteration, commercial harvest, environmental contaminants, and other factors were identified as threats. Today our scientific understanding of the species and its life history requirements have increased greatly as summarized below.

We used ultrasonic telemetry to determine the distribution, seasonal migration patterns, and site fidelity of adult paddlefish Polyodon spathula in a main‐stem Missouri River impoundment. Thirty‐two adult paddlefish collected from two different spring congregation areas in Lake Francis Case, South Dakota, were surgically implanted with ultrasonic tags and relocated monthly from March to November 1994–1996. Paddlefish tagged in the Big Bend tailwaters were captured in this area and in another possible spring congregation (the White River) during prespawning and spawning periods, whereas paddlefish tagged in the White River were never located in the Big Bend tailwaters or in areas above the White River. Male and female paddlefish used reservoir habitats similarly except during the prespawning period, when male paddlefish used the White River more than would be expected by chance and all females implanted at the White River site remained below the White River. In both the postspawning and winter periods, all paddlefish moved downstream and congregated in the lower reservoir reaches. Up to 62% of males and 36% of females returned to their original capture site during the presumed spawning period at least 1 of the 2 years after tagging. Thirty‐one percent of males at large for 2 years returned to their initial spring capture location each of the following years, but only 9% of females returned. Our results indicate that some paddlefish exhibited site fidelity to the areas where they were captured (i.e., the White River) and presumably attempted to spawn, although winter distribution was similar among all tagged paddlefish. Restoration and stocking efforts may need to focus on habitat management at the locations where paddlefish may spawn, given adequate substrate.

Juvenile pallid sturgeon Scaphirhynchus albus raised in hatcheries and stocked in the wild are used to augment critically imperiled populations of this federally endangered species in the United States. For pallid sturgeon in recovery priority management area 2 (RPMA 2) of the Missouri River and lower Yellowstone River where natural recruitment has not occurred for decades, restoration programs aim to stock an annual minimum of 9000 juvenile pallid sturgeon for 20 years to re-establish a minimum population of 1700 adults. However, establishment of this target was based on general guidelines for maintaining the genetic integrity of populations rather than pallid sturgeon-specific demographic information because data on the historical population size was lacking. In this study, information from a recent population estimate (158 wild adults in 2004, 95% confidence interval 129-193 adults) and an empirically derived adult mortality rate (5%) was used in a cohort population model to back-estimate the historic abundance of adult pallid sturgeon in RPMA 2. Three backestimation age models were developed, and assumed that adults alive during 2004 were 30-, 40-, or 50-years old. Based on these age assumptions, population sizes [±95% confidence intervals; (CI)] were back-estimated to 1989, 1979, and 1969 to approximate size of the population when individuals would have been sexually mature (15 years old) and capable of spawning. Back-estimations yielded predictions of 344 adults in 1989 (95% CI 281-420), 577 adults in 1979 (95% CI 471-704), and 968 adults in 1969 (95% CI 790-1182) for the 30-, 40-, and 50-year age models, respectively. Although several assumptions are inherent in the back-estimation models, results suggest the juvenile stocking program for pallid sturgeon will likely re-establish an adult population that equals in the short-term and exceeds in the long-term the predicted population numbers that occurred during past decades in RPMA 2. However, re-establishment of a large population in RPMA 2 that exceeds populations present 40+ years ago should be considered conservatively, as this strategy will increase the number of reproductive adults and thereby increase the likelihood for natural recruitment in this recruitment-limited system.

The ability to distinguish captive-bred and natural-origin individuals in the wild is critical for evaluating the impact of captive breeding programs on natural populations. Continued persistence of endangered pallid sturgeon (Scaphirhynchus albus) in the Missouri River is largely dependent on captive breeding efforts that spawn natural-origin adults in fish hatcheries and release their progeny into the wild. Prior to release, hatchery-origin individuals are physically marked so they can be distinguished from natural-origin individuals when recaptured. During the years 2004-2006, 24 unmarked juvenile pallid sturgeon tissue samples were collected in the Missouri River downstream of Gavins Point Dam, South Dakota, USA that were presumed natural-origin. However, these individuals were similar in size to hatchery-origin fish released in this area raising concerns that these individuals were actually hatchery-origin fish with lost or malfunctioning tags. We used microsatellite based parentage analysis to determine if the unmarked fish were members of hatchery families that had been released in this area. This retrospective genetic tagging approach revealed that 23 of 24 unmarked fish were indeed hatchery-origin. The origin of the remaining individual remains unknown because genetic samples were not available from all of the families released below the dam and the unassigned fish may have originated from one of these un-sampled families. These results provide important insight into the conservation status of endangered pallid sturgeon as well as provide data important for guiding management decisions. Our results also demonstrate the efficacy of using genetic tags as an alternative or complimentary approach to physically marking individuals.