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.
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.
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.
A multiweek standardized sampling regime during 2004–2016 in a 60-km reach of the Upper Missouri River assessed reproduction and catch rates for Sturgeon Chub Macrhybopsis gelida and Sicklefin Chub Macrhybopsis meeki. We sampled age-0 Macrhybopsis (primarily Sturgeon Chubs, but potentially including Sicklefin Chubs) all years to indicate successful reproduction, but noted an inverse correlation of catch per unit area (CPUA) with year. There was an inverse correlation for CPUA of age-1+ Sturgeon Chubs with year. There was no correlation for CPUA of age-1+ Sicklefin Chubs with year, but we noted a depression in CPUA during 2010 and 2012. The study reach includes restoration directives for federally endangered Pallid Sturgeon Scaphirhynchus albus, with 245,000 hatchery-origin Pallid Sturgeon (HOPS) stocked since 1998 to supplement the declining wild stock. Pallid Sturgeon longer than 350 mm transition to piscivory and are known to prey on Sturgeon Chubs and Sicklefin Chubs. We examined the hypothesis that mass additions of HOPS to the existing predator community could have population-level effects on the two chub species. Population modeling for the stocked HOPS through time yielded estimates of nearly 1,300 piscivore-sized HOPS in 2004, an increase to 26,000 HOPS in 2012, and decreasing numbers through 2016 (14,500). Candidate variables that also included discharge and water temperature best supported a negative correlation between HOPS abundance and age-0 Macrhybopsis CPUA. We found an inverse correlation for CPUA of age-1+ Sturgeon Chubs and estimated HOPS abundance, and there was also evidence of an inverse association between age-1+ Sicklefin Chub CPUA and HOPS in the study area. Results for a 60-km reach of the Upper Missouri River suggest declining CPUA for age-0 Macrhybopsis and Sturgeon Chubs during 2004–2016 and modest recovery of Sicklefin Chubs after 2012. Although causative factors driving CPUA changes through time are not known, correlative analyses suggest that large numbers of HOPS added to the Missouri River predator community potentially influence CPUA of Sturgeon Chubs and Sicklefin Chubs in the study area. Testing this hypothesis will require expanded quantification of chub populations and HOPS numbers through time.
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