Population characteristics of pallid sturgeon (Scaphirhynchus albus (Forbes & Richardson, )) in the Lower Missouri River

Abstract

Summary Population characteristics of pallid sturgeon Scaphirhynchus albus in the lower Missouri River are relatively unknown. Therefore, data collected from the Nebraska Game and Parks Commission Pallid Sturgeon Population Assessment Program was synthesized to (i) document the population structure of pallid sturgeon by origin (hatchery‐reared or wild), gender, and reproductive readiness, (ii) document the minimum size and age‐at‐maturity by gender, and (iii) document the fecundity rates of the fish that were successfully spawned in the hatchery. During this 4‐year study (2008–2011), relative abundance for wild and hatchery‐reared pallid sturgeon collected with gill nets did not significantly change whereas relative abundance for wild fish using trot lines declined significantly. The proportion of hatchery‐reared pallid sturgeon increased annually, with the population being composed primarily of hatchery‐reared fish. The proportion of reproductively ready females to non‐reproductively ready females was 1 : 2.0, compared to male ratios at 1 : 0.9. Minimum fork length‐at‐maturity was estimated for females at 788 mm and for males at 798 mm. Minimum age‐at‐maturity for hatchery‐reared released fish was age‐9 for females and age‐7 for males. Highest relative fecundity, based on the ovosomatic index, was 10% with an overall mean of 7%. The number of eggs per ml (egg size) was not correlated with fork length (P = 0.0615) or weight (P = 0.0957). Relative condition factor (Kn) for females was significantly different by reproductive condition (P = 0.0014) and Kn for males did not differ between reproductive conditions (P = 0.2634). Detecting shifts in population characteristics are essential not only to understand population dynamics since hatchery inputs and natural perturbations continue to change the population structure but also to assess species recovery efforts to ensure long‐term species sustainability.

Assessment of pallid sturgeon relative condition in the upper channelized Missouri River

“…Hatchery-reared fish in the lower Missouri River were growing (Shuman et al 2011), exhibiting migratory behavior and reaching sexual maturity DeLonay et al 2012;Steffensen et al 2013a). The numbers of reproductive fish in the lower Missouri River was increasing prior to 2013; however, the proportion of reproductive fish collected annually continues to decrease.…”

“…There is also some evidence that fish may not be maturing at the same approximate size as they were prior to 2011. Prior to 2011, Steffensen et al (2013a) determined that the minimum length-of-maturity was approximately 800 mm, but over the past two years the minimum length of reproductive females sampled has been 895 mm. Several known hatchery-reared pallid sturgeon from 2001 and 2002 year classes were captured this past year and were determined to be still immature and yet to reach their first maturation cycle (J.…”

“…Blackwell et al (2000) suggested that fish in good condition have more energy available for gamete production thus increasing relative fecundity. The relationship between condition and fecundity for pallid sturgeon from the lower Missouri River is unknown, but fork length and weight displayed a positive correlation with the number of eggs released (Steffensen et al 2013a). However, this information needs to be assessed cautiously as the sample size was very limited (n D 11).…”

Population prediction and viability model for pallid sturgeon (Scaphirhynchus albus , Forbes and Richardson, ) in the lower Missouri River

“…Steffensen et al. () described the gender ratio, spawning intervals, age‐at‐maturity, and fecundity estimates observed during annual intensive broodstock collection efforts from 2008–2011 for an 80.5 rkm reach of the lower Missouri River. Maximum age was set at 41 years (Keenlyne et al., ).…”

Using the robust design framework and relative abundance to predict the population size of pallid sturgeon Scaphirhynchus albus in the lower Missouri River

“…Scaphirhynchus albus currently inhabit the riverine reaches of the Missouri River and the Mississippi River below the confluence with the Missouri River, but will also select large tributaries (Bailey & Cross, ). Similar to other sturgeon species, S. albus are long‐lived and mature late with intermittent spawning cycles (Keenlyne et al ., ; Billard & Lecointre, ; Secor et al ., ; Steffensen et al ., ). The factors affecting natural reproduction and recruitment of S. albus in the lower Missouri River [Gavins Point Dam, river km (rkm) 1305·2 downstream to the confluence of the Missouri and Mississippi Rivers (rkm 0·0)] are not completely understood, but are probably affected by the low level of reproductively‐ready fish, poor fitness to achieve reproductive readiness and produce healthy gametes, hybridization with the sympatric S. platorynchus , lack of spawning cues or aggregations, insufficient spawning habitats and water quality.…”