We examined the seasonal food habits and diet overlap of juvenile pallid sturgeon (Scaphirhynchus albus) and adult shovelnose sturgeon (S. platorynchus) in the Missouri River downstream of Fort Randall Dam, South Dakota. Seasonal trends were found for both sturgeon species as chironomids were consumed in the greatest numbers and dry weights during early summer, ephemeropterans dominated during late summer, and trichopterans were most abundant in early spring and late fall diets. However, diet overlap between juvenile pallid sturgeon and adult shovelnose sturgeon was low based on a Schoener index value of 0.53 in 2003 and 0.21 in 2004. Juvenile pallid sturgeon consumed a greater proportion of fish in 2003 and Ephemeroptera in 2004 compared to shovelnose sturgeon. Shovelnose sturgeon consumed a greater proportion of Chironomidae in both years compared to juvenile pallid sturgeon. These results illustrate that shovelnose sturgeon is not an acceptable surrogate for the endangered pallid sturgeon based on food habits.

The pallid sturgeon (Scaphirhynchus albus) is an endangered species native to the Missouri and Mississippi rivers. To date, recovery efforts have focused on stocking juvenile fish, but little is known about ontogenetic changes in diet composition. Although diet composition for pallid sturgeon is believed to change from macroinvertebrates to fish, it is unclear at what size and ⁄ or age these ontogenetic diet shifts occur. To evaluate diet composition, 29 hatchery-stocked pallid sturgeon (range 356-720 mm fork length [FL]; mean = 549; SE = 23) were collected from the Missouri River downstream of Fort Randall Dam, South Dakota and Nebraska during summer 2006. The majority of pallid sturgeon (72%) were captured within a large delta region formed by the Niobrara River in the headwaters of Lewis and Clark Lake. Predominant prey of pallid sturgeon based on percent occurrence was Ceratopogonidae (81%), Isonychiidae (67%), Chironomidae (52%), and fishes (24%). Percent composition by wet weight showed that diets were composed of fishes (68%), Ephemeroptera (23%), Decapoda (6%), and Diptera (3%). Graphical analysis of combined data showed that mayflies, particularly Isonychiidae, were an important component of pallid sturgeon diets. Nonetheless, the percent composition of fishes in the diet increased with pallid sturgeon body size; for fish > 600 mm FL (5-7 years of age) diets were composed primarily of fish prey (66%, mostly johnny darters Etheostoma nigrum). These findings highlight the importance of ontogenetic changes in diet composition for pallid sturgeon. Moreover, the unique habitat formed in the delta region is characterized by higher fish and invertebrate densities that may enhance foraging opportunities and thus improve recovery efforts for stocked pallid sturgeon.

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 We synthesized wild and stocked pallid sturgeon Scaphirhynchus albus capture data collected in four recovery areas of the Missouri River during 1998–2007, providing the first basin‐wide analysis of size structure, growth, and condition. Proportional size distribution (PSD) values ranged from 20 to 33 and were indicative of past stockings given the continued lack of natural recruitment. A new weight‐length regression derived from 2268 captured wild (8%), hatchery‐stocked (75%), and unknown origin (16%) pallid sturgeon had a significantly lower slope and intercept from a previously published model that used only 214 wild fish and a truncated size range. Relative condition (Kn) declined after stocking throughout the basin but stabilized at 0.94 within 3 years. Spatially, Kn of juvenile pallid sturgeon (330–629 mm) was generally highest in the reaches of the Missouri River with large tributaries. In accordance with the latitudinal counter gradient growth hypothesis, similar absolute growth increments (both length and weight) of tagged age‐1 pallid sturgeon in the Upper and Lower Missouri River indicated upstream fish grew at faster rates given the 1.3 fold difference in growing season length. From North to South, von Bertalanffy growth coefficients (k) showed a latitudinal increase while L∞ decreased, providing additional support of the latitudinal counter gradient growth hypothesis. However, growth rates of tagged juvenile pallid sturgeon aged 2–6 years were highest at the two most downstream reaches showing an increased influence of growing season length. In the Missouri River, pallid sturgeon (≤ ages 1–9 or 10 years) exhibited linear growth in the two most upstream reaches (Montana and North Dakota), exhibited logistic growth in the inter‐reservoir reach in South Dakota/Nebraska while a power function best described growth downstream of the lowermost dam (Nebraska, Iowa, Kansas, and Missouri). Differing growth models among reaches highlights the confounding affect of habitat fragmentation by dams in the Missouri River on growth as well as potential latitudinal affects on sexual maturation. Relative condition and growth of pallid sturgeon appears adequate throughout the Missouri River indicating success of past stocking efforts to forestall extirpation.

Summary We compared the effectiveness of passive gill nets, hoop nets, set lines, and drifted trammel nets, towed beam trawls and otter trawls to develop criteria to best determine the mean catch per unit effort (CPUE) for juvenile pallid sturgeon (Scaphirhynchus albus) based on selectivity and seasonal efficiency in various habitats of the Missouri River downstream of Fort Randall Dam, South Dakota. Sampling occurred from April to November in 2003 and 2004 and from March to November in 2005. We captured 29 juvenile pallid sturgeon in a total of 498 overnight gill net sets, 55 in 870 drifted trammel nets, 19 on 1683 set lines, and six in 166 otter trawl tows. No pallid sturgeon were captured in 515 beam trawl tows or 520 overnight hoop net sets. Seasonal trends in mean CPUE were found and the relative precision was the greatest in October and November for gill nets, in August for trammel nets, in April for set lines, and in October for otter trawls. A higher proportion of pallid sturgeon captures for gill nets were in the inside bend macrohabitat generally associated with lower water velocities, trammel nets over sand substrate and in the outside bend macrohabitat typically associated with higher water velocities and greater depths, set lines in lower water velocities, and otter trawls in depths >2.5 m and over sand substrate. Although we found trends among seasons and habitats for gill nets, trammel nets, set lines, and otter trawls, the catch rates were low and annual point estimates of relative abundance are not adequate to detect changes in relative abundance of juvenile pallid sturgeon in this reach of the Missouri River. Independently, gill nets, trammel nets, and otter trawls likely captured the size structure of the population of pallid sturgeon in the Missouri River downstream of Fort Randall Dam. Based on our results, a standardized protocol can now be established to systematically monitor juvenile pallid sturgeon, an essential element for determining responses to recovery efforts in the Missouri River.