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Factor E:

Other Natural or Manmade Factors Affecting its Continued Existence

Potential new threats identified subsequent to the 5-year review (USFWS 2007) or new information has resulted in additional evaluation of: 1) energy development, 2), hybridization, and 3) invasive species/aquatic nuisance species.

Energy Development

In-channel Power Generation

Hydrokinetic power generation (i.e., projects using an array of in-river turbines to generate electricity) could pose a threat to pallid sturgeon. Individually, an in-river turbine may have minimal affects, but cumulatively, hydrokinetic projects could have adverse effects if they 
become widely developed and are built in a manner or location that is inconsistent with the life history requirements of pallid sturgeon.

According to the Federal Energy Regulatory Commission, valid preliminary permits for hydrokinetic projects on the Mississippi River have been issued. Preliminary permits only allow the permitted party to study or evaluate projects at the described site.

At the writing of this revision, no licensed hydrokinetic projects exist in the range of the pallid sturgeon. However, as the project sites mentioned above are evaluated, some may prove to be feasible and licensing pursued. Concerns over placement of these structures, relative to pallid sturgeon, include potential effects on movement patterns and habitat alterations needed to maintain flow to these structures. More information will be needed to assess this potential threat.

Gas and Oil Exploration

Exploration of natural gas and oil deposits occurs in portions of the pallid sturgeon’s range.

Preliminary assessment of the impacts of seismic air guns, a tool used for exploration, suggests that they may have negative effects on larval pallid sturgeon (Kretnz in litt. 2010). Additional research is necessary to fully evaluate the extent and magnitude of these effects.

Summary of Impacts From Energy Development

Increased demand for energy resources has led to an increased interest in new technology development and exploration. Of particular interest will be the potential effects on all life-stages associated with placement and quantity of hydrokinetic power generators within the habitats occupied by pallid sturgeon. Additionally, oil and gas exploration techniques have the potential to take pallid sturgeon yet the ability to evaluate these takings will be nearly non-existent given the nature of the river systems these fish live in. Strict adherence to existing environmental laws will be necessary to minimize effects and more data will be needed to adequately evaluate and monitor impacts related to energy development. 


The original version of this recovery plan (USFWS 1993) identified hybridization as a threat to pallid sturgeon. This was, in part, based on limited observations of sturgeon (N=12) collected from the middle Mississippi River that appeared morphologically-intermediate to shovelnose and pallid sturgeon (Carlson and Pflieger 1981; Carlson et al. 1985) and the belief that hybridization was contemporary (i.e., post 1960 and influenced by anthropogenic changes to habitat). Subsequent genetic and morphological studies have been conducted to explore hybridization between pallid and shovelnose sturgeon ( Phelps and Allendorf 1983; Carlson et al 1985; Campton et al. 2000; Tranah et al. 2001 and 2004; Kuhajda et al. 2007; Ray et al. 2007; Murphy et al. 2007a). Below is a brief review of the current literature regarding the treatment of intermediate-character sturgeon and putative pallid/shovelnose hybridization in the Mississippi River basin.

Carlson et al. (1985) used principal components analysis based on morphometric measures described in Bailey and Cross (1954) and found that morphologically-intermediate specimens fell in between the pallid and shovelnose sturgeon groups leading to their hybridization origin hypothesis. Efforts to confirm hybridization used a suite of allozyme markers (Phelps and Allendorf 1983). These results neither supported nor refuted the hybridization origin hypothesis and only suggested that pallid and shovelnose sturgeon share close taxonomic affinities. Tranah et al. (2004) assessed the genetic origins of 10 morphologically intermediate sturgeon collectedfrom the Atchafalaya River. These results were consistent with the hypothesis that hybridization occurs between pallid and shovelnose sturgeon. However, this study simply demonstrated that morphologically-intermediate fish had intermediate genotypes. Schrey (2007) analyzed 529 Scaphirhynchus samples from the upper Missouri, lower Missouri, middle Mississippi, and Atchafalaya rivers using sixteen microsatellite loci. Like Tranah et al. (2004), the author also found that genetically-intermediate fish tended to also be morphologically-intermediate. 

While there are competing hypotheses that may explain morphologically intermediate fish (Murphy et al. 2007a; Ray et al. 2007), there appears to be a positive correlation between genotype and phenotype (Tranah et al. 2004; Schrey 2007). The latest genetic analysis confirms introgressive hybridization between pallid and shovelnose sturgeon occurs and likely has been occurring for several generations, perhaps as many as 60 years (Schrey et al. 2011). However, the significance of hybridization as a factor in the status of pallid sturgeon is poorly understood. Hybridization between two species could result in the eventual loss of one or both parental forms (Arnold 1992; Allendorf et al. 2001; Rosenfield et al. 2004). Conversely, hybridization has been postulated to have played a role in sturgeon speciation (Birstein et al. 1997; Vasil’ev 1999; Robles et al. 2005), indicating that hybridization may have always been a process occurring in the evolution of these species and it can lead to the creation of new species (Arnold 1992). Understanding the benefits and threats that hybridization may impose on pallid sturgeon recovery remains uncertain because the mode and rate of Scaphirhynchus hybridization are difficult to assess.

Summary of Impacts Related to Hybridization

While we know that artificial mating of pallid sturgeon with shovelnose sturgeon can produce living offspring (Kuhajda et al. 2007), accurate assessment of hybridization in the evolution of Scaphirhynchus and its perceived threats to pallid sturgeon recovery will require statistically testing the hypothesis of hybridization against alternatives. Since hybridization is accepted as a process occurring in Scaphirhynchus and likely has been occurring for many decades (Schrey et al. 2011), it is important to determine the cause (i.e., historical/natural or contemporary) and extent of hybridization. Once these processes are elucidated, simulation studies can address the actual risks associated with pallid/shovelnose hybridization. If it is determined that alteration of habitats has influenced temporal or spatial reproductive isolating mechanisms resulting in increased rates of hybridization, addressing this threat will likely rely on both site-specific and ecosystem improvement efforts; many of which are identified in the Recovery Outline/Narrative 
section below. 

Invasive Species/Aquative Nuisance Species

Although not a threat specifically identified in the pallid sturgeon listing package (55 FR 36641-36647), the potential impact of invasive and aquatic nuisance species can be applied to Listing Factor A- The present or threatened destruction, modification, or curtailment of its habitat or range and Listing Factor C- Disease or Predation. Several species with the potential for impacting pallid sturgeon have become established in parts of the species’ range. These include the Asian carps (common (Cyprinus carpio), grass (Ctenopharyngodon idella), silver (Hypophthalmichthys molitrix), bighead (Hypophthalmichthys nobilis) and black (Mylopharyngodon piceus)) as well as the zebra mussel (Dreissena polymorpha). Populations ofAsian carp appear to be expanding exponentially in parts of the Mississippi River basin as the range of the zebra mussel continues to expand (Kolar et al. 2005). 

According to the American Fisheries Society (AFS Policy 15), potential negative impacts by nonnative species have been categorized into five broad categories: habitat alteration, trophic alteration, spatial alteration, gene pool deterioration and disease transmission. Documenting these impacts in large river ecosystems is especially difficult. Few studies have documented the impacts from these species in the Mississippi Basin. However, data are available from other watersheds that shed insight into potential effects from invasive species.

If food resources were limited from the presence of large populations of planktivores (e.g., Asian carps), early life-stage pallid sturgeon could face increased competition with native planktivorous fishes such as gizzard shad (Dorosoma cepedianum), bigmouth buffalo (Ictiobus cyprinellus) and paddlefish (Kolar et al. 2005). Several authors have expressed concern that, because nearly all fish feed on zooplankton as larvae and juveniles, Asian carps have high potential to impact native fishes of the Mississippi Basin (Laird and Page 1996; Chick and Pegg 2001; Chick 2002). The diets of bighead and silver carp have significant overlap with those of gizzard shad and bigmouth buffalo (Sampson et al. 2009). In addition to directly competing for food resources, Asian carps also could affect recruitment by predation on pallid sturgeon eggs or drifting larvae. Miller and Beckman (1996) have documented white sturgeon eggs in the stomachs of common carp. Additionally, disease or parasites can be spread by Asian Carp.

Goodwin (1999) noted that channel catfish became infested with anchorworm when cultured with bighead carp. Heckmann et al. (1986 and 1995) reported that this tapeworm was spread to two endangered species when baitfishes were released into Lake Mead, Arizona and Nevada. Currently, the Asian tapeworm is known to infest native fishes in five States; however, none are in the Mississippi River drainage (Kolar et al. 2005).

Zebra mussel colonization has occurred in areas occupied by pallid sturgeon but data are limited on direct effects. In other sturgeon species, data demonstrate reduce foraging effectiveness of juvenile lake sturgeon (Acipenser fulvescens), and mussel presence resulted in avoidance of those areas by study fish more than 90% of the time (McCabe et al. 2006).

Summary of Impacts From Invasive and Aquatic Nuisance Species

Potential threats from invasive or aquatic nuisance species include increased predation on eggs, larval, or juvenile life stages, competition for food in the case of the carps, exclusion of native species from preferred habitats, spread of diseases or parasites, and alteration of habitat quality. Further study is needed to fully qualify and quantify the magnitude of this probable threat to pallid sturgeon.

Factor E Summary

Energy development and invasive species are two threats that may have substantial deleterious effects on pallid sturgeon populations. Strict adherence to existing environmental laws will be necessary to minimize effects from these threats and more data will be needed to adequately evaluate the extent and magnitude of these effects.