Free embryos of wild pallid sturgeon Scaphirhynchus albus were released in the Missouri River and captured at downstream sites through a 180-km reach of the river to examine ontogenetic drift and dispersal processes. Free embryos drifted primarily in the fastest portion of the river channel, and initial drift velocities for all age groups (mean=0.66-0.70 ms −1 ) were only slightly slower than mean water column velocity (0.72 ms −1 ). During the multi-day longdistance drift period, drift velocities of all age groups declined an average of 9.7% day −1 . Younger free embryos remained in the drift upon termination of the study; whereas, older age groups transitioned from drifting to settling during the study. Models based on growth of free embryos, drift behavior, size-related variations in drift rates, and channel hydraulic characteristics were developed to estimate cumulative distance drifted during ontogenetic development through a range of simulated water temperatures and velocity conditions. Those models indicated that the average free embryo would be expected to drift several hundred km during ontogenetic development. Empirical data and model results highlight the long-duration, long-distance drift and dispersal processes for pallid sturgeon early life stages. In addition, results provide a likely mechanism for lack of pallid sturgeon recruitment in fragmented river reaches where dams and reservoirs reduce the length of free-flowing river available for pallid sturgeon free embryos during ontogenetic development.

Prior to anthropogenic modifications, the historic Missouri River provided ecological conditions suitable for reproduction, growth, and survival of pallid sturgeon Scaphirhynchus albus. However, little information is available to discern whether altered conditions in the contemporary Missouri River are suitable for feeding, growth and survival of endangered pallid sturgeon during the early life stages. In 2004 and 2007, nearly 600 000 pallid sturgeon free embryos and larvae were released in the upper Missouri River and survivors from these releases were collected during 2004-2010 to quantify natural growth rates and diet composition. Based on genetic analysis and known-age at release (1-17 days post-hatch, dph), age at capture (dph, years) could be determined for each survivor. Totals of 23 and 28 survivors from the 2004 and 2007 releases, respectively, were sampled. Growth of pallid sturgeon was rapid (1.91 mm day )1 ) during the initial 13-48 dph, then slowed as fish approached maximum length (120-140 mm) towards the end of the first growing season. The diet of youngof-year pallid sturgeon was comprised of Diptera larvae, Diptera pupae, and Ephemeroptera nymphs. Growth of pallid sturgeon from ages 1-6 years was about 48.0 mm year )1 . This study provides the first assessment of natural growth and diet of young pallid sturgeon in the wild. Results depict pallid sturgeon growth trajectories that may be expected for naturally produced wild stocks under contemporary habitat conditions in the Missouri River and Yellowstone River.

Nearly 430,000 larval pallid sturgeon Scaphirhynchus albus (5-13 d posthatch) were released in the Missouri River and allowed to free-drift for 5.7 km and then were sampled at a river bend by boats positioned on the inside-bend, inside-bend channel border, mid-channel, and outside-bend locations of the channel. Boats were rigged with conical nets and rectangle nets designed to sample for drifting pallid sturgeon larvae along the bottom and at the surface of the river to address three objectives: (1) examine the vertical drift behavior of larvae; (2) compare numbers, concentrations, and lengths of larvae between conical and rectangle nets; and (3) examine the lateral distribution of free-drifting larvae. The vertical distribution of larvae was not uniform as nearly all larvae (98%) were sampled in nets fished on the bottom, and larval concentrations (expressed as larvae/m 3 ) were greater in bottom-fished nets. Large-opening rectangle nets (0.375 m 2 ) fished on the bottom sampled greater numbers of larvae than small-opening conical nets (0.196 m 2 ) fished on the bottom; however, larval concentrations were similar between the two net designs. Numbers and concentrations of larvae differed among lateral sampling locations and increased from the inside-bend location to the outside-bend location. Lengths of sampled larvae were similar between net designs and vertical locations in the water column, but larvae sampled at the mid-channel location were slightly smaller than larvae sampled at other lateral locations. These results serve as a guide for sampling larval pallid sturgeon in large rivers. Specifically, sampling adjacent to the bottom in the high-velocity thalweg of the channel maximizes the likelihood of collecting larvae and quantifying numbers of drifting larvae of this federally endangered species.

We sought to develop sperm cryopreservation methods for the pallid sturgeon Scaphirhynchus albus, a federally listed endangered species. Males were injected with synthetic luteinizing hormone releasing hormone at 50 μg/kg of body weight. After 24 h, sperm were collected, diluted at a ratio of 1:4 (sperm : extender) with Hanks' balanced salt solution (HBSS; diluted to 100 milliosmoles/kg), and kept refrigerated until use. Methanol was used as a cryoprotectant at concentrations of 5, 10, and 15% (volume per volume) and was mixed 1:1 with HBSS before the experiment to reduce effects of initial mixing. Sperm were mixed with the cryoprotectant, loaded into 0.5‐mL straws, packed into goblets (5 straws/goblet), and placed in the lower position on aluminum canes. Motility was estimated before freezing to determine the effects of cryoprotectant toxicity; there was no significant difference in motility at the concentrations tested (P = 0.4828). After a 2‐min equilibration period, the canes were lowered into a nitrogen vapor shipping dewar. The cooling rate of −22°C/min was recorded by thermocouples inserted into 0.5‐mL straws filled with extender and cryoprotectant. After 1 year of storage in liquid nitrogen, straws were thawed in a 40°C water bath for 9 s and motility was estimated. Postthaw motility did not differ among the cryoprotectants tested (P = 0.4880). Each sample was used to fertilize approximately 150 eggs, which were incubated at 21°C. Sperm that was cryopreserved with 5% or 10% methanol produced eggs with significantly higher hatch rates did sperm cryopreserved with 15% methanol (P < 0.0001). The development of techniques for cryopreserved sperm of pallid sturgeon allows for the creation of germplasm repositories that will aid in the recovery of this endangered species.

We released nearly 1.0 million 1-day post-hatch (dph) and 5-dph pallid sturgeon (Scaphirhynchus albus) free embryos in the Missouri River on 1 July 2019 and sequentially captured survivors at multiple sites through a 240-km river reach to quantify daily growth and survival rates during the early life stages. Genetic analysis was used to assign captured fish to released family lots and known ages. Growth rate was similar (0.74–0.75 mm day−1) between the 1- and 5-dph age groups during the 3–4-day dispersal period when water temperature averaged 16.8 °C. Daily survival rate was 0.64 during 1–4 dph for the original 1-dph age group and 0.80 during 5–7 dph for the original 5-dph age group. Total survival during free embryo dispersal (hatch to 9 dph) was estimated as 0.0437. The transition from dispersing as free embryos to settling as benthic larvae was verified for fish originally released as 5 dph. Growth of settled larvae was quantified with a Gompertz model through 75 dph (9 September; 112 mm) when water temperature was 18.8–21.0 °C in the rearing areas. Settled larvae had an estimated daily survival rate of 0.96, and estimated total survival during 9–75 dph was 0.0714. This study provides the first empirical survival estimates for pallid sturgeon early life stages in natural settings and is one of few studies reporting similar information for other sturgeon species. Applications of this work extend to pallid sturgeon restoration programs where population models are being developed to predict recruitment potential and population responses to river management alternatives.