Factor A:

Present or Threatened Destruction, Modification or Curtailment of it’s Habitat or Range

The following known and potential threats that affect the habitat or range of pallid sturgeon are discussed in this section, and include:

1. Large river habitat alterations, including river channelization, impoundment, and altered flow regimes 
2. Water quality 
3. Entrainment 
4. Climate change

River Channelization, Bank Stabilization, Impoundment, and Altered Flow Regimes

Modification and curtailment of pallid sturgeon habitat and range are attributed to large river habitat alterations, including river channelization, bank stabilization, impoundment, and altered flow regimes. Following is a brief summary of these activities by river system. 

Missouri River

Historically, the Missouri River was dynamic, ever-changing, and composed of multiple channels, chutes, sloughs, backwater areas, side channels, and migrating islands and sandbars. As early as 1832, Congress endorsed an act approving the removal of snags from the river (Funk and Robinson 1974). In 1884, the Missouri River Commission was formed to improve navigation on the river (Funk and Robinson 1974). The Commission used revetments of woven willow and rock to stabilize banks, and dikes to narrow the channel and close off chutes during its 18 years of development and maintenance of the Missouri River. However, the need for barge traffic declined with the expansion of railroad networks, and in 1902 the Missouri River Commission was dissolved and responsibility for the Missouri River was given directly to the U.S. Army Corps of Engineers (COE) (Funk and Robinson 1974). In 1912, Congress approved a navigation channel 1.8 m (6 ft) in depth from Kansas City, Missouri to the mouth near St. Louis, Missouri. Subsequently, the Rivers and Harbors Act of 1945 authorized an increase in channel depth to 2.7 m (9 ft) and width to 91.4 m (300 ft) from Sioux City, Iowa to the mouth and by 1967 a self-scouring channel was largely completed (Funk and Robinson 1974).

During the last century, the Missouri River was altered as a result of the Flood Control Act of 1944 to address societal needs. The most obvious habitat changes were the installation of dams in the upper Missouri River and some tributaries as well as channelization/stabilization of the lower Missouri River for navigation. These anthropogenic modifications to the river restrict the life cycle requirements of pallid sturgeon by blocking movements to spawning and feeding areas, decreasing turbidity levels by trapping sediment in reservoirs, reducing distances available for larvae to drift, modifying temperatures, destroying spawning areas, altering conditions and flows of potential remaining spawning areas, and possibly reducing food sources by lowering productivity (Hesse et al. 1989; Keenlyne 1989; USFWS 2000a; Bowen et al. 2003).

Water levels in the reservoirs impounded by Fort Peck Dam (Fort Peck Reservoir), Montana and Garrison Dam (Lake Sakakawea), North Dakota  may be impediments to larval pallid sturgeon survival. In laboratory studies, Scaphirhynchus larvae were found to passively drift upon hatching (Kynard et al. 2002). Subsequent work was conducted with larval pallid sturgeon released downstream of Fort Peck Dam as part of a larval drift study. Combined, these data indicate that pallid sturgeon larvae can drift 245 to 530 km (152 to 329 mi) depending on water column velocity and temperature (Kynard et al. 2002; Braaten et al. 2008). This drift distance would likely result in naturally-spawned larvae from the two occupied reaches of the upper Missouri River drifting into the headwaters of either Fort Peck Reservoir or Lake Sakakawea where survival is believed to be minimal. Braaten et al. (2008) speculate that differences in larval drift rates found between shovelnose and pallid sturgeon might explain why the two species experience different recruitment levels in the upper Missouri River. As part of this study, pallid sturgeon fry of various ages (in days) were released. Subsequently, in 2005 4 recaptured pallid sturgeon were genetically traced back to the 11- to 17-day-old fry released in 2004 (Patrick Braaten, USGS Columbia Environmental Research Center, unpublished data). This indicates that fry released between 11 and 17 days post-hatch are able to survive to age-1 in the Missouri River between Fort Peck Dam and Lake Sakakawea supporting the Kynard et al. (2007) hypothesis that implicates total drift distance as a limitation on natural recruitment in thisreach of the Missouri River. 

In addition to limiting drift distance and inundating habitat, an altered hydrograph also affects downstream temperature profiles and reduces sediment transport. Cold water releases have been attributed to spawning delays in several native riverine fishes and changing fish community composition downstream (Wolf 1995; Jordan 2000). Canyon Ferry, Hauser, and Holter dams are upstream of Great Falls, Montana. Though they do not impose any migratory barriers for pallid sturgeon, these structures, like the main-stem Missouri River dams, can affect sediment and nutrient transport and maintain an artificial hydrograph. Thus, the main-stem and tributary dams upstream of Fort Peck Dam affect downstream reaches by reducing both sediment input and transport. The results are a reduction of naturally occurring habitat features like sandbars. Discharge and sediment load, together with physiographic setting, are primary factors controlling the morphology of large alluvial rivers (Kellerhals and Church 1989). Seasonally high turbidity levels are a natural component of pre-impoundment ecological processes. Reduced sediment transport and the associated decrease in turbidity could affect pallid sturgeon recruitment and feeding efficiency. The relationship between high turbidity levels and larval pallid sturgeon survival is unclear.

Increased predation on white sturgeon yolk sac larvae at low turbidity levels suggest that high turbidity levels associated with a natural hydrograph and natural sediment transport regimes may offer concealment for free drifting sturgeon embryos and larvae (Gadomski and Parsley 2005). Given that the diet of pallid sturgeon is generally composed of fish and aquatic insect larvae with some preference for piscivory as they mature (see Life History section, above), higher pre- impoundment turbidity levels may have afforded improved foraging effectiveness by providing older juveniles and adults some level of concealment. From Garrison Dam, North Dakota to Gavins Point Dam, South Dakota, the Missouri River retains little of its historical riverine habitat; most of this reach is impounded in reservoirs. However, within these reservoirs, pallid sturgeon have been documented in the more riverine reaches, some of which have been identified as useful for recovery efforts (Erickson 1992; Jordan et al. 2006; Wanner et al. 2007). Despite these data, most of these inter-reservoir reaches are poorly understood and further research is needed to evaluate and define their significance to species’ recovery.

The Missouri River downstream of Gavins Point Dam is over 1,296 Rkm (800 Rmi) in length, is unimpeded by dams, and is biologically and hydrologically connected with the Mississippi River. However, this reach is not immune from past and present anthropogenic modifications. For example, in the unchannelized reach extending from Gavins Point Dam downstream for approximately 95 Rkm (59 Rmi) side channel and backwater habitats have changed (Yager et al. 2011). Changes include a decrease of 77% and 37%, respectively, in total and mean area of side channels, as well as decrease of 79% and 42%, respectively, in total and mean length of side channels (Yager et al. 2011). Channelization of the Missouri River downstream from this reach has reduced water surface area by half, doubled current velocity, decreased habitat diversity, and decreased sediment transport (Funk and Robinson 1974; USFWS 2000a).

Although the Missouri River downstream of Gavins Point Dam is not impounded, it is influenced by the operation of upstream and tributary dams. Major effects include a modified hydrograph and reduced sediment transport. Damming and channelizing the Missouri River adversely affects pallid sturgeon (USFWS 2000a and 2003). 

Missouri River Tributaries

At the time of listing, few observations of pallid sturgeon occurred in waters outside of the main-stem Mississippi, Missouri, and Yellowstone rivers; tributary observations were attributed to special circumstances associated with high-flow conditions (55 FR 36641-36647). While historical captures of pallid sturgeon occurred near the mouths of tributaries or within close proximity to tributary confluences with the Missouri River, more recent observations indicate that Missouri River tributaries may be more important than originally recognized when the species was listed. These habitats appear to be important to the pallid sturgeon during certain times of the year or perhaps during certain life stages. Tributaries identified below are based on documented observations of pallid sturgeon and should not be list may be revised if new data become available.

Yellowstone River

The Yellowstone River is the largest tributary to the Missouri River. While often referred to as “the last undammed river,” this descriptor is a misnomer. At about the same time that Forbes and Richardson (1905) were describing pallid sturgeon as a species, the first and lowermost of six low-head diversion dams was being constructed across the river approximately 115 Rkm (71 Rmi) from the confluence with the Missouri River. This structure, Intake Dam, was constructed by the Bureau of Reclamation (BOR) and effectively blocks upstream movements of pallid sturgeon (Bramblett and White 2001) and increases entrainment from the system through the irrigation delivery canals (Jaeger et al. 2005).

Adult pallid sturgeon use the lower Yellowstone River (Bramblett 1996). Aggregations of pallid sturgeon during spawning season strongly suggest that spawning occurs in the lower Yellowstone River (Bramblett 1996). Recent evidence of spawning success in the lower Yellowstone River has been documented (Fuller et al. 2007), but detectable levels of recruitment remain absent.

Upstream movements of both adult and juvenile pallid sturgeon are blocked by Intake Dam. This barrier appears to be limiting adult fish from accessing upstream habitats which may be suitable for spawning (Bramblett and White 2001; Jaeger et al. 2005). Additionally, about half of juvenile study fish stocked upstream of Intake Dam did not emigrate during the study period, suggesting suitable habitats for juvenile fish exist upstream of Intake Dam (Jaeger et al. 2005).

Naturally-produced larvae in the lower Yellowstone River will drift into Lake Sakakawea as long as spawning occurs downstream of Intake Dam ( Braaten et al. 2008). This information indicates that available drift distance for larvae is artificially truncated by Intake Dam on the upstream end and water levels in Lake Sakakawea at the downstream end. This lack of drift 
distance is an ongoing threat limiting recruitment from the Yellowstone River.

Pallid sturgeon also have been entrained in the irrigation canal associated with Intake Dam (Jaeger et al. 2004). In 2012, a new canal water headworks was completed that incorporates fish screens intended to eliminate entrainment losses. To date, upstream passage has not been provided at Intake Dam. Available data indicate that providing fish passage at Intake Dam remains necessary to facilitate pallid sturgeon recovery.

Yellowtail Dam on the Bighorn River and Tongue River Dam on the Tongue River, both major tributaries to the Yellowstone River, have altered sediment transport and flows into the lower Yellowstone River. Other anthropogenic modifications on the Yellowstone River include bank stabilization projects to protect private property and transportation infrastructure, as well as municipal, industrial, and agricultural water withdrawal projects.

Milk River

The Milk River is ecologically important to the Missouri River downstream of Fort Peck Dam as it contributes flows, sediment, and warmer water temperatures. The Milk River is subject to irrigation diversions that can substantially alter the hydrograph in this system.

Correspondingly, several barriers would effectively block migrations within this system. The lowermost is Vandalia Diversion Dam located near Rkm 188 (Rmi 117). In 2004, a wild adult pallid sturgeon was documented in the Milk River approximately 4 Rkm (2.5 Rmi) above the confluence with the Missouri River (Braaten and Fuller 2005; Fuller in litt., 2011).

Subsequently in 2011, 4 males and 1 female migrated into the Milk River; the furthest upstream location was approximately 57.9 Rkm (36 Rmi) (Fuller in litt., 2011).

Marias River

Historically, the Marias River influenced the Missouri River downstream from their merger. During 1805 when the Lewis and Clark Expedition arrived at the confluence with the Marias River, they could not decide which river was the Missouri River (Moulton 1987). The influence of the Marias River on the Missouri River is not only limited to physical features but also affects the fish communities. Several large migratory species such as paddlefish (Polyodon spathula), blue sucker (Cycleptus elongatus), and shovelnose sturgeon presently or historically were known to migrate up the Marias River presumably to spawn (Gardner and Jensen 2007). It is possible that in the past, pallid sturgeon also may have migrated up the Marias River to spawn.

Operations of Tiber Dam on the Marias River at Rkm 132 (Rmi 82) have now altered the natural flow and sediment regime of the Marias River and may have affected its use by fish species including pallid sturgeon (Gardner and Jensen 2007). While historical data documenting occupation by pallid sturgeon are absent, hatchery-reared pallid sturgeon recently have been captured in the lower 1 Rkm (0.6 Rmi) (Gardner 2010).

Niobrara River

Wild pallid sturgeon were documented in the Niobrara River around the mid-1900s (Mestl in litt., 2011). Since that time, the lower reach of the Niobrara River has been affected by rapid aggradation due to the siltation at the head of Lewis and Clark Lake on the Missouri River. Approximately 2.2 to 2.8 m (7.5 to 9.5 feet) of aggradation observed since the 1950s has changed the lower Niobrara River from a “relatively deep, stable channel with large, bank- attached braid bars to a relatively shallow aggrading channel with braid bars” (Skelly et al. 2003). It is not known to what degree channel aggradation has affected habitats for pallid sturgeon.

Pallid sturgeon habitat in the lower Niobrara River also may be affected by water withdrawals. The Nebraska Department of Natural Resources has declared a portion of the lower Niobrara River as fully appropriated (Nebraska 2007). Although habitat suitability has changed substantially over the last five decades, the Niobrara River still retains braided channels with shifting sand bars representative of pre-channelization conditions of rivers throughout the pallid sturgeon’s historical range (Peters and Parham 2008). Recently, three hatchery-reared pallid sturgeon released in the Missouri River were documented in the Niobrara River; two were approximately 1.6 Rkm (1 Rmi) upstream of the confluence with the Missouri River while the other was approximately 9.6 Rkm (6 Rmi) upstream of the confluence (Wanner et al. 2010).

Big Sioux River

The Big Sioux River is a north to south flowing prairie river that originates in South Dakota and drains into the Missouri River downstream of Gavins Point Dam, the lowermost dam on the Missouri River. Historical observations of pallid sturgeon in this system are absent. However, one tagged pallid sturgeon moved upstream 21.1 Rkm (13.1 Rmi) into this river from the Missouri River (DeLonay et al. 2009).

Platte River

The Platte River is a Missouri River tributary downstream of Gavins Point Dam. Increasing numbers of both hatchery-reared and presumed-wild pallid sturgeon have been observed in the Platte River since the species was listed. One observation was approximately 158 Rkm (98.4 Rmi) upstream from the Missouri River confluence (Snook et al. 2002; Swingle 2003; Peters and Parham 2008; Hamel in litt., 2009, 2010). Additionally, limited data indicate that the lower Platte River (defined as the Platte River from the confluence with the Missouri River upstream to the Elkhorn River) could be used for spawning (Swigle 2003). These data indicate the Platte River provides suitable habitat and can help support multiple life stages of the species.

Although not developed as a navigation corridor, the Platte River has been influenced by anthropogenic alterations that likely affect pallid sturgeon habitat. Water demands for industrial, municipal, and agricultural purposes led to construction of low-head diversion dams on the upper Platte River and large impoundments on the Platte River and its tributaries. Eschner et al. (1983) state that the Platte River and its tributaries “…have undergone major changes in hydrologic regime and morphology since 1860.” These authors describe a process where islands eventually attached to the floodplain, became vegetated, and eventually fixed in place resulting in decreased channel widths. These authors attribute many of these changes in channel morphology to water development and diversions. Similarly, Rodekohr and Englebrecht (1988) noted the Platte River is more constricted than it was in 1949. Despite some of these changes, there appears to be sufficient beneficial qualities within the lower Platte River, such that pallid sturgeon will occupy it (Swigle 2003; Peters and Parham 2008). Sampling within the Missouri River near the confluence of the Platte River results in substantially more pallid sturgeon captures when compared against other Missouri River sampling sites downstream to the Kansas River confluence ( Steffensen and Hamel 2007 and 2008). This suggests that the Platte River also provides some positive benefits to pallid sturgeon habitat in the Missouri River.

Kansas River

The Kansas River has anthropogenic alterations that likely affect some aspects of pallid sturgeon life history. Bowersock Dam (Rkm 82, Rmi 51) near Lawrence Kansas was constructed in the 1870s. Because this barrier was installed prior to pallid sturgeon being identified as a species, there is little historical occupancy data for reaches upstream. The Johnson County Weir is another potential barrier to pallid sturgeon movement in the lower Kansas River (Rkm 23.7, Rmi 14.7). This structure was built in 1967 to maintain sufficient water delivery for municipal purposes. To date, 15 pallid sturgeon, most confirmed to be of hatchery origin (Niswonger, in litt., 2011), have been collected from the lower Kansas River. All known hatchery fish were originally stocked in the Missouri River.

Osage River

The Osage River is one of the larger Missouri River tributaries in Missouri. Pallid sturgeon have been documented near the confluence of the Osage and Missouri rivers, including three hatchery-reared pallid sturgeon in the lower Osage River between Lock and Dam #1 (Rkm 19.4; Rmi 12.1) and the confluence with the Missouri River in 2010 (USFWS 2010).

Grand River

The Grand River is a turbid tributary that was highly channelized during the same period that pallid sturgeon were likely declining. However, this system supports a fish assemblage that is largely riverine with species such as lake sturgeon occasionally being captured. While historical data documenting occupation by pallid sturgeon are absent, hatchery-reared pallid sturgeon have been captured in the lower 3 Rkm (1.8 Rmi) (DeLonay 
et al. 2009).

Mississippi River

The Mississippi River is often divided into an upper, middle and lower reaches. Like the Missouri River, the Mississippi River has been anthropogenically altered, beginning during the early portions of the 18th century as the French began to settle along the Mississippi River (Cowdrey 1977). These earlier efforts were generally localized and limited in scope. It was not until the 19th century that large-scale efforts to improve navigation and flood control began to have more substantial impacts. Snagging (removing dead trees from the river) was one of the first efforts to facilitate using the river as a transportation corridor. In the early 1800s with Federal appropriations, snag boats removed large woody debris from the middle and lower Mississippi River between St. Louis, Missouri and New Orleans, Louisiana (Simons et al. 1974; Cowdrey 1977). 

The next major efforts to improve navigation involved maintaining navigable channels. In the mid-1800s, creation of jetties and dredging provided the first successful large-scale reduction of sediment deposition and the subsequent forming of sandbars that blocked shipping routes (Cowdrey 1977). Flood control became an increasingly important focus of Congress as more people settled in the Mississippi River valley and the human costs of flood damage increased. Small and localized levee systems were in existence in the 1700s; however, it was not until the 19th and 20th centuries that levee networks increased in size and scope. As the levee system was completed, flood stages increased resulting in the need to shunt flood waters from the river (Cowdrey 1977). The Flood Control Act of 1928 included provisions for strengthening and raising existing levees and including floodways and spillways (Cowdrey 1977); examples include the Bonnet Carré spillway, the Morganza floodway, and the Old River Control Complex.

In addition to the dams on the upper Missouri River, flows into the middle and lower Mississippi River also are influenced by a series of locks and dams in the upper Mississippi River. The earliest lock and dam structures were constructed in 1867 at the Keokuk Rapids, Iowa. By 1940, the locks and dams from Minneapolis, Minnesota down to Alton, Illinois, were in place and operational. Finally, revetments and various structures have been used to reduce erosion and restrict flows in many areas. Willow mattresses and cypress pilings, later replaced by articulated concrete mats and rock riprap, were used to prevent loss of riparian land and control flow patterns (Cowdrey 1977). This reduction in river bank erosion has reduced the amount of sediments and large woody debris entering the system. Subsequent loss of connectivity and channel sinuosity occurred as habitats were channelized and off-channel habitats became isolated from normal riverine flow. Modifications to the Mississippi River occurred largely from construction of the locks and dams, levees, and channel maintenance structures.

Upper Mississippi River

The upper Mississippi River (UMR), as it relates to pallid sturgeon, is defined as being upstream of the confluence of the Missouri and Mississippi rivers to Lock and Dam 19 near Keokuk, Iowa. This reach is approximately 260 Rkm (162 Rmi) in length. The lower most lock and dam (Lock and Dam 26 near Alton, Illinois) is located approximately 8 Rkm (5Rmi) upstream of the Missouri-Mississippi river confluence (Figure 5). Although fish passage through the six lock and dam structures is impeded for many species, it can occur through the lock chamber or the dam gates during flood events. Historical pallid sturgeon observations in the upper Mississippi River near Keokuk, Iowa (Coker 1929) are reported as “dubious” (Bailey and Cross 1954), although pallid sturgeon have been documented to move from into the pools of the upper Mississippi River (Herzog in litt., 2009; Herzog 2010). However, the extent of use within this impounded reach of the UMR is poorly understood, and further research is needed to assess its role in species recovery. 

Middle Mississippi River

The middle Mississippi River (MMR) is defined as the Missouri-Mississippi river confluence near St. Louis, Missouri to the Mississippi-Ohio river confluence near Cairo, Illinois. This reach is approximately 313 Rkm (195 Rmi) in length.

In 1881, Congress approved plans to regulate the MMR, and by 1973 this reach of the Mississippi River had experienced levee construction, > 160 km (100 mi) of revetments, and installation of more than 800 dikes to maintain a minimum navigation channel depth of 2.7 meters (9 feet) (Simons et al. 1974). Lock and Dam 27, (Chain of Rocks dam and canal) is located at Rkm 298.5 (Rmi 185.5) near Granite City, Illinois. The canal structure was completed to facilitate navigation around the shallow bedrock that occurred in this reach. Large quantities of rock were dumped over the existing bedrock to create a low-head dam necessary to make the lock canal navigable. Although no pallid sturgeon have been documented in the canal, both pallid and shovelnose sturgeon concentrate below the Chain of Rocks dam during fall and winter low-flow events (Killgore et al. 2007a).

The cumulative effects of these alterations include an average reduction in river width, river bed degradation, a slight increase in the maximum river stage, a reduction in minimum river stage, and a constricted flood plain (Simons et al. 1974).

Lower Mississippi River

The lower Mississippi River (LMR) is defined as the Mississippi River from the Mississippi-Ohio rivers confluence to the Gulf of Mexico. This reach is 
approximately 1,541 Rkm (958 Rmi) in length, and based on proportion of river size to anthropogenic alterations, represents one of the least modified reaches within the pallid sturgeon range.

Between 1929 and 1942, bendway cutoffs shortened the LMR by 245 Rkm (152 Rmi) over a 809 km (503 mile) reach (Winkley 1977). The LMR was reduced an additional 88.5 Rkm (55 Rmi) between 1939 and 1955 by constructing artificial channels that bypassed natural river meanders (Winkley 1977). This channel length reduction resulted in the river entrenching in steeper gradient reaches and eroding large amounts of material from the channel banks and bed. Deposition of this material in the lower gradient reaches resulted in a semi-braided channel, and by the 1970s, the river was attempting to reestablish a meandering condition (Winkley 1977). Dikes and bank armoring have been employed in the LMR to stabilize the channel and direct flows to reduce the need for dredging.

Levee construction initially began in the New Orleans area in the 1700s. Today, excluding a few tributary mouths, levees line the west side of the river and fill in low areas between natural bluffs on the east side (Cowdrey 1977; Baker et al. 1991). These levees are estimated to have reduced the floodplain area by as much as 90% depending on flood magnitude (Baker et al. 1991). Although the LMR channel has been enclosed by levees, numerous and extensive sandbars, vegetated and seasonal islands, and secondary channels remain, equating to a 1.6 million acre floodplain that retains floodplain backwaters and sloughs that are seasonally connected to the river (Schramm et al. 1999).

Mississippi River Tributaries

As previously stated, data post-listing indicate that main-stem tributaries and tributary confluences are more important than previously recognized. Several captures of pallid sturgeon have occurred within tributaries, near the mouth of tributaries, and within close proximity to tributary confluences with the Mississippi River. These habitats appear to be important to the pallid sturgeon during certain times of the year or perhaps during certain life stages.

Arkansas River

The Arkansas River confluences with the Mississippi River near Rkm 933 (Rmi 580). To date, three pallid sturgeon have been documented entering the lower reaches of the Arkansas River. All observations were downstream from the Wilbur D. Mills Dam in the late-winter through spring (February – April) (Kuntz in litt., 2012). Additional efforts are ongoing to better understand usage of this tributary by pallid sturgeon and what role this tributary serves for the recovery of pallid sturgeon.

Saint Francis River

The Saint Francis River flows through south-east Missouri into Arkansas where it confluences with the Mississippi River. In 1994 hatchery fish were documented in the lower Saint Francis River (Graham in litt., 1994) downstream from the W. G. Huxtable Pumping Plant. Additional data are necessary to better understand use of this river by pallid sturgeon and what role this river serves in pallid sturgeon recovery efforts.

Meramec River

This tributary to the MMR, located near Rkm 254 (Rmi 158), is a large river within Missouri that contains transitional habitats within its lower reaches. There are no historical accounts of pallid sturgeon in this river; however, pallid sturgeon have been documented in the Mississippi River near the Meramec River confluence (Koch et al. 2006a). It is not known whether pallid sturgeon historically migrated within this system, and additional data are necessary to determine what role this tributary serves for the recovery of pallid sturgeon. 

Kaskaskia River

The Kaskaskia River is located near Rkm 188 (Rmi 117) near Chester, Illinois. This is Illinois’ second largest river system at 515 Rkm (320 Rmi) long draining about 10% of the State. Several pallid sturgeon have been documented at the confluence with the Mississippi River (Koch et al. 2006a), although movement into the Kaskaskia River by pallid sturgeon has not been documented. However, these movements are likely impeded because of a lock and dam near the mouth. In addition, the watershed of the Kaskaskia River has been modified over the last 100 years by urbanization, channelization, and levee and dam construction. It is unknown whether pallid sturgeon historically migrated within this system, and additional data are needed to determine if this tributary serves any role for the recovery of pallid sturgeon.

Ohio River

The Ohio River is the largest tributary to the Mississippi River system within the range of pallid sturgeon. However, there are no recent reports of pallid sturgeon and no confirmed records of presence in this system, and additional data are needed to determine if this tributary serves any role for the recovery of pallid sturgeon.

Atchafalaya River

The Atchafalaya River is a distributary of the lower Mississippi River that begins just south of Cochie, Louisiana and extends downstream to Morgan City, Louisiana (Rkm 180/Rmi 112), where it flows into the lower Atchafalaya River and ultimately to the Gulf of Mexico. At approximately Atchafalaya River Rkm 156 (Rmi 97), the Wax Lake Outlet was constructed in 1942, providing a shorter route for flood waters to leave the Atchafalaya River. Prior to 1859, the Atchafalaya River received Mississippi River water from overbank flooding. Snagging and channel excavation in support of navigation during the late 19th and early 20th centuries resulted in channel enlargement and increased flows into the Atchafalaya River from the Mississippi and Red rivers. By the 1950s the Atchafalaya River threatened to capture most of the lower Mississippi River flow, and in 1963 the COE constructed the Old River Control Complex to prevent this capture by regulating flows into the Atchafalaya River.

The Old River Control Complex (i.e., Low Sill, Overbank, and Auxiliary) at approximately Mississippi Rkm 505 (RM 314) can carry a combined maximum discharge of 700,000 cfs. Since the completion of the Sidney A. Murray, Jr. Hydroelectric Station in 1990, just upstream of the Old River Control Complex, the flows are now split between the hydroelectric station and the Old River Control Complex structures with flows released to maximize hydro-power production. The Old River Control Complex, in coordination with the hydro-power plant, carries 30% of the combined discharge from the Mississippi and Red rivers, maintaining Mississippi River discharge into the Atchafalaya River at levels comparable to the 1950s. The Atchafalaya River has been leveed to prevent flooding of communities and agricultural lands from Rkm/Rmi 0 to Rkm 85 (Rmi 53). Downstream of Rkm 85, the river levees only contain flows less than the average annual discharge; all greater discharges flow overbank. Most pallid sturgeon reported from this river have been captured immediately below the Old River Control structures where almost all sampling occurs (Reed and Ewing 1993). However, pallid sturgeon use of the middle and lower Atchafalaya River has been documented (Constant et al. 1997; Schramm and Dunn 2007, Herrala and Schramm 2011).

There is no evidence that pallid sturgeon occupied the Atchafalaya River distributary prior to the mid-20th century capture of Mississippi River flows. To date, hatchery fish (2 specimens) released in the Mississippi River below Natchez, Mississippi, and above Memphis, Tennessee (1 specimen), have been captured in the Atchafalaya River, indicating that pallid sturgeon can be entrained from the Mississippi River into the Atchafalaya River. It is possible that many of the pallid sturgeon observations in the Atchafalaya River are the result of entrainment from the Mississippi River; the magnitude of which has not been quantified.

Summary of Impacts from River Channelization, Bank Stabilization, Impoundment, and Altered Flow Regimes

The species was essentially extirpated from approximately 28% of the historical range due to impoundment, and the remaining unimpounded range has been modified by channelization and bank stabilization, or is affected by upstream impoundments that alter flow regimes, turbidity, and water temperatures (Hesse et al. 1989; Keenlyne 1989; USFWS 2000a). River 
channelization, bank stabilization, impoundment, altered flow regimes, and their effects are document throughout the range of the pallid sturgeon and each can negatively affect pallid sturgeon life-history requirements. The most obvious effects to habitat are associated with the six main-stem Missouri River dams. These dams and their operations have: 1) truncated drift distance of larval pallid sturgeon (Kynard et al. 2007; Braaten et al. 2008), 2) created physical barriers that block normal migration patterns, 3) degraded and altered physical habitat characteristics, 4) greatly altered the natural hydrograph (Hesse et al. 1989), and 5) reulted in subtle changes in river function that influence both the size and diversity of aquatic habitats, connectivity (Bowen et al. 2003), and benthos abundance and distribution (Morris et al. 1968). Moreover, these large impoundments have replaced large segments of riverine habitat with lake conditions. Damming of the upper Missouri River has altered river features such as channel morphology, current velocity, seasonal flows, turbidity, temperature, nutrient supply, and paths within the food chain (Russell 1986; Unkenholz 1986; Hesse 1987). In addition to the main- stem Missouri River dams, important tributaries like the Yellowstone River, Platte River, and Kansas River have experienced similar affects due to dams and water resource development in their respective watersheds. Other issues that have influenced habitat formation and maintenance are associated with maintaining navigation channels on portions of the Missouri River as well as efforts to control flooding. The Mississippi River has received a substantial amount of anthropogenic modification through time, and some changes resulting from those modifications have likely been detrimental to pallid sturgeon. These anthropogenic habitat alterations likely adversely affect pallid sturgeon by altering the natural form and functions of the Mississippi River (Simons et al. 1974; Baker et al. 1991; Theiling 1999; Wlosinski 1999). Anthropogenic alterations to tributaries may have contributed to habitat degradation in the Mississippi River as well. Impoundment of major tributaries reduced sediment delivery to the main channel (Fremling et al. 1989) resulting in channel degradation and reduction in shallow water habitats (Simons et al. 1974; Bowen et al. 2003). Thus dams, bank stabilization, and channelization activities, individually and cumulatively when implemented within the range of 
pallid sturgeon, should be considered threats to the species.

Water Quality

Contaminants /Pollution

Much of the information we have regarding the likely effects to pallid sturgeon from contaminants comes from information obtained for shovelnose sturgeon, which can be used as a surrogate species to evaluate environmental contaminant exposure. Shovelnose sturgeon are considered a suitable surrogate species for pallid sturgeon in that they live for 20 years or longer, inhabit the same river basins, spawn at similar intervals and locations, and accumulate similar inorganic and organic contaminants (Ruelle and Keenlyne 1994; Buckler 2011). However, while inferences can be drawn from data related to shovelnose sturgeon, limitations of using this species as a surrogate for pallid sturgeon are based on life history differences between the two species. Pallid sturgeon have a longer life-span, attain a larger size, are more piscivorous, and contain a higher percentage of body fat (Ruelle and Keenlyne 1994). These differences may contribute to different contaminant effects or pathways; pallid sturgeon may be at greater risk than shovelnose sturgeon to contaminants that bioaccumulate and cause reproductive impairment because they have a more piscivorous diet, greater maximum life-span, and a longer reproductive 
cycle than shovelnose sturgeon.

Contaminants detected in shovelnose sturgeon throughout the Missouri, Mississippi, Platte, and Atchafalaya rivers include: organochlorines, metals, aliphatic hydrocarbons, polycyclic aromatic hydrocarbons, polychlorinated biphenyls (PCBs), and elemental contaminants (Allen and Wilson 1991; Welsh 1992; Welsh and Olson 1992; Ruelle and Henry 1994b; Palawski and Olsen 1996; Conzelmann et al.1997; Coffey et al. 2003; Schwarz et al. 2006).

A few field studies have included shovelnose sturgeon health assessments in an effort to evaluate environmental contaminant exposure and effects to pallid sturgeon (Coffey et al. 2003; Schwarz et al. 2006). Organochlorine pesticides and PCBs were detected at concentrations of concern in Mississippi River shovelnose sturgeon tissue samples. Adverse health problems observed included abnormal reproductive biomarkers and enlarged livers (Coffey et al. 2003). A similar evaluation in the lower Platte River identified PCBs, selenium, and atrazine as contaminants that may adversely affect sturgeon reproduction (Schwarz et al. 2006).

Shovelnose sturgeon collected from the Platte, lower Missouri and Mississippi rivers have exhibited intersexual characteristics (having both male and female gonad tissue) (Harshbarger et al. 2000; Wildhaber et al. 2005; Koch et al. 2006b; Schwarz et al. 2006). Intersexual shovelnose sturgeon from the middle Mississippi River were found to have higher concentrations of organochlorine compounds when compared to male shovelnose sturgeon (Koch et al. 2006b). One pallid sturgeon exhibited both male and female reproductive organs (DeLonay et al. 2009). Although the effects of intersex on sturgeon reproduction are unknown, intersex in other fish species has been linked to decreased gamete production, lowered sperm motility, and decreased egg fertilization (Jobling et al. 2002). Koch et al. (2006b) observed reduced numbers of spermatozoa in highly contaminated and intersexual shovelnose sturgeon that may suggest limited reproductive success.

Laboratory studies also have evaluated environmental contaminant exposure and effects to shovelnose sturgeon. Papoulias et al. (2003) injected unhatched shovelnose sturgeon larvae with PCB 126 and Tetrachlorodibenzo-p-dioxin (TCDD). They found yolk sac and pericardial swelling, hemorrhaging of the eyes and head, shortened maxillaries, and delayed development. However, the experimental exposure concentrations of PCB 126 was at levels beyond what might be found in the wild, but the negative effects from TCDD exposure concentrations were at levels that are conceivable in the Mississippi River (Papoulias et al. 2003)

To date, few studies have measured environmental contaminant concentrations in pallid sturgeon. Tissue samples from three Missouri River pallid sturgeon and 13 other pallid sturgeon, mostly collected from the Mississippi River had metals (mercury, cadmium, and selenium), PCBs, and organochlorine pesticides (chlordane, DDT, DDE, and dieldrin) at concentrations of concern (Ruelle and Keenlyne 1993; Ruelle and Henry 1994a). In addition to the previously mentioned reports on contaminants in pallid sturgeon, raw contaminants data for pallid sturgeon from North Dakota, Illinois, and Louisiana are currently being compiled.

Point-source discharges may adversely affect pallid sturgeon and their habitat. Wastewater treatment plant effluent can contain hormonally active agents. Endocrine disruption in fish exposed to estrogenic substances discharged by wastewater treatment plants is well documented (Purdom et al. 1994; Routledge et al. 1998; Cheek et al. 2001; Schultz et al. 2003). In addition to wastewater treatment plants, drinking water treatment plants also are a concern. In April 2004, several radio-tagged pallid sturgeon were repelled from the mouth of the Platte River immediately following a milky discharge from a drinking water treatment facility upstream (Parham et al. 2005). Further investigation found that the facility was not in compliance with its discharge permit which expired in 1993, and that the discharge likely contained several toxic irritants including ferric sulfate, calcium oxide, hydrofluosilicic acid, chlorine, and ammonia.

Several fish consumption advisories within the range of pallid sturgeon are attributable to contaminants (Buckler 2011). The State of Tennessee closed commercial fishing on portions of the Mississippi River because of concerns over chlordane and other contaminants (Tennessee 2008 a and b). The Missouri Department of Health and Senior Services has issued a “do not eat” advisory for shovelnose sturgeon eggs and recommends consuming no more than one shovelnose sturgeon per month because of concerns over PCB, mercury, and chlordane levels (Missouri 2010). Illinois issued a sturgeon consumption advisory due to PCBs and chlordane levels on the Mississippi River between Lock and Dam 22 to Cairo, Illinois (Illinois 2010). The Kansas Department of Health and Environment (2010) has issued a consumption advisory for bottom- feeding fish, including sturgeon, due to PCB levels in the Kansas River downstream of Bowersock Dam to Eudora. Fish consumption advisories have been issued for the Missouri River from Omaha to Rulo, Nebraska (Nebraska 2010). Although fish consumption advisories are for the protection of human health, river segments with such designations also have been associated with adverse health effects in the shovelnose sturgeon themselves, including enlarged livers, abnormal ratios of estrogen to testosterone, and intersexual characteristics (Coffey et al. 2003; Schwarz et al. 2006). Further assessments should be targeted in these areas to evaluate the exposure and effects of the impairing contaminants on pallid sturgeon and their reproductive 
physiology.

Additionally, Injuries resulting from chance encounters with discarded human-made objects like gaskets and rubber bands have been documented in the Mississippi River; approximately 5% of shovelnose sturgeon and 9% of pallid sturgeon exhibit scars or deformities from such injuries (Murphy et al. 2007b). Mortalities have not been reported or estimated.

Dissolved Oxygen

While the tolerances of low dissolved oxygen concentrations have not been quantified for all life stages of pallid sturgeon, data from other sturgeon species are insightful. In general, sturgeon are not as tolerant of hypoxic conditions as are other fishes (Secor and Gunderson 1998; Niklitschek and Secor 2005). Temperature and dissolved oxygen levels can affect sturgeon survival, growth and respiration with early life stages being more sensitive than adults (Secor and Gunderson 1998).

Like many sturgeon species, pallid sturgeon are primarily benthic (living on or near the bottom) organisms within 10-12 days post hatch (Kallemeyn 1983; Kynard et al. 2007). This benthic life history strategy can result in sturgeon encountering areas with low oxygen levels (hypoxia). Like most organisms that encounter unsuitable habitats, juvenile and adult sturgeon have some ability to avoid unfavorable environmental conditions via migration (Auer 1996). In reservoirs, white sturgeon will avoid those areas where riverine features become more lake like (transition zone) and oxygen levels approach 6 mg/l (Sullivan et al. 2003). Under hypoxic conditions, juvenile Atlantic sturgeon will move upward in the water column to access more oxygen-rich water (Secor and Gunderson 1998).

Anthropogenic changes within the range of pallid sturgeon that affect dissolved oxygen concentrations could be affecting survival and recruitment. Dissolved oxygen levels of 3 mg/l and water temperatures of 22-26 oC (71.6-78.8 oF) appeared lethal for juvenile Atlantic sturgeon, and shortnose sturgeon (Secor and Gunderson 1998; Campbell and Goodman 2004). Reduced growth was observed in Atlantic sturgeon at lower non-lethal levels (Secor and Gunderson 1998). In the upper Missouri River basin, larval pallid sturgeon are likely transported into or through reservoir transition areas. Because they are weak swimmers at this early life stage (Kynard et al. 2007), they are less able to migrate away from any encountered hypoxic conditions. Study efforts have been initiated to better evaluate the effects of riverine to reservoir transition areas on pallid sturgeon survival.

Summary of Impacts related to Water Quality

Overall water quality can have both immediate and long-term effects on the species. New information, post-listing suggests that water quality can affect individuals during many life phases and localized and/or regionally poor or degraded water quality should be viewed as a threat to the species. More information is needed to evaluate the exposure and effects of environmental contaminants to pallid sturgeon. In response to this need, in 2008, a basin-wide contaminants review for pallid sturgeon was initiated. To date, this investigation has identified pesticides, metals, organochlorines, hormonally active agents, and nutrients as contaminants of concern throughout the species’ range (See Appendix A). However, additional data are needed to quantify and qualify the magnitude of this threat. 

Entrainment

Another issue that can cumulatively have negative consequences for pallid sturgeon range-wide is entrainment loss. The loss of pallid sturgeon associated with cooling intake structures for power facilities, dredge operations, irrigation diversions, and flood control points of diversion has not been fully quantified, but entrainment has been documented for both pallid and shovelnose sturgeon.

Adult shovelnose sturgeon (and likely adult pallid sturgeon) exhibit relatively high prolonged swimming speeds (Adams et al. 1997; Parsons et al. 2003) and would be at lower entrainment risk than young fish. Juvenile pallid and shovelnose sturgeon exhibit comparable swimming abilities (Adams et al. 2003). They are not strong swimmers relative to other species and are at greater risk of entrainment (Adams et al. 1999a), but they also exhibit a variety of complex swimming behaviors which may increase their ability to resist flow (Hoover et al. 2005). Scaphirhynchus larvae are weak swimmers and experience high rates of mortality under simulated propeller entrainment and high rates of stranding under simulated vessel-induced drawdown (Adams et al. 1999b; Killgore et al. 2001).

Water Cooling Intake Structures: Preliminary data on the Missouri River indicate that these structures may be a threat that warrants more investigation. Initial results from work conducted by Mid-America at their Neal Smith power facilities located downstream of Sioux City, Iowa, found hatchery-reared pallid sturgeon were being entrained (Burns & McDonnell Engineering Company, Inc. 2007a and 2007b). Over a 5-month period, four known hatchery-reared pallid sturgeon were entrained, of which two were released alive and two were found dead.

Dredge Operations:

The COE has initiated work to assess dredge entrainment of fish species and the potential effects that these operations may have on larval and juvenile Scaphirhynchus. Available data indicate that shovelnose sturgeon can be entrained, and this entrainment is relatively lethal (Ecological Specialists, Inc 2010). Thus, dredging in locations where pallid sturgeon congregate could result in entrainment and mortality. Small pallid sturgeon likely are at risk of being entrained in dredges and additional data for escape speed, position-holding ability, orientation to the current and response to noise, and dredge flow fields are being used to develop a risk assessment model for entrainment of sturgeon by dredges (Hoover et al. 2005).

Irrigation Diversions:

Entrainment of hatchery-reared pallid sturgeon has been documented in the irrigation canal associated with the BOR’s Lower Yellowstone Project Intake Diversion Dam on the Yellowstone River where some of these fish are believed to have perished (Jaeger et al. 2004).

Flood control points of diversions:

Two hatchery-reared juvenile pallid sturgeon released in the Mississippi River and one adult hatchery-reared pallid sturgeon released in either the lower Missouri or middle Mississippi river were entrained by the Old River Control Complex as they subsequently collected in the Atchafalaya River. During May and June 2008, 14 pallid sturgeon were collected behind the Bonnet Carre’ spillway (Reed in litt., 2008; USFWS 2009), indicating that entrainment occurs at this facility during the rare occasions when flood waters need to be shunted from the Mississippi River to Lake Pontchartrain. Additional smaller structures exist or are planned for diverting water and sediments from the Mississippi River for marsh enhancementand hurricane protection in coastal Louisiana. Pallid sturgeon entrainment potential and significance is unknown. 

Summary of Impacts of Entrainment

Entrainment has been documented to occur in the few instances it has been studied. Thus it is a greater threat than anticipated in the original version of this plan. The overall effects from entrainment are variable and will be dependent on population demographics, exposure time, quantity of un-screened diversion points, and duration of diversion point usage, (i.e., year-round versus seasonal or sporadic operation). Further evaluation of entrainment associated with dredging operations, water diversion points, and commercial navigation is necessary across the pallid sturgeon’s range to adequately evaluate and quantify this threat.

Climate Change

Although not a threat specifically identified in the pallid sturgeon listing package (55 FR 36641-36647), our analyses under the Endangered Species Act include consideration of ongoing and projected changes in climate. The terms “climate” and “climate change” are defined by the Intergovernmental Panel on Climate Change (IPCC). “Climate” refers to the mean and variability of different types of weather conditions over time, with 30 years being a typical period for such measurements, although shorter or longer periods also may be used (IPCC 2007). The term “climate change” thus refers to a change in the mean or variability of one or more measures of climate (e.g., temperature or precipitation) that persists for an extended period, typically decades or longer, whether the change is due to natural variability, human activity, or both (IPCC 2007). Various types of changes in climate can have direct or indirect effects on species. These effects may be positive, neutral, or negative and they may change over time, depending on the species and other relevant considerations, such as the effects of climate interactions with other variables (e.g., habitat fragmentation) (IPCC 2007). In our analyses, we use our expert judgment to weigh relevant information, including uncertainty, in our consideration of various aspects of climate change. Both the Intergovernmental Panel on Climate Change and U.S. Global Change Research Program (GCRP) identify that the trend in global climate patterns is one of warming; average temperatures in the United States are at least 1.1oC (2oF) higher than they were 50 years ago (IPCC 2007; GCRP 2009).

Within the range of pallid sturgeon, predicted affects appear to be shifts in runoff patterns: discharge peaks are anticipated to occur earlier and potentially be larger, late season river flows may be reduced, and water temperatures may rise (IPCC 2007). These changes to the water cycle are anticipated to affect water use (GCRP 2009), which may alter existing reservoir operations. Broadly, these potential effects to pallid sturgeon could be altered spawning behavior (i.e., movement and timing) and reduced late-season habitat suitability due to reduced flows and presumably warmer temperatures. Another predicted outcome is increased or prolonged periods of drought (IPCC 2007; GCRP 2009). Increased water demand coupled with reduced late-season flows could significantly affect in-channel habitats which in turn may affect other species that are food items for pallid sturgeon.

These effects would likely occur first, or be most pronounced, in the more northern portion of the pallid sturgeon range; the IPCC (2007) study suggests that in general, temperature increases correlate with latitude. Thus, higher northern latitudes appear to have relatively higher predicted warming trends. However, reduced annual runoff predicted in the Missouri River basin may be offset by the anticipated increased runoff in the upper Mississippi River basin (GCRP 2009) resulting in minimal effects within the middle and lower Mississippi River basins. 

Summary of Impacts of Climate Change

At this time, it is difficult to evaluate long-term effects from climate change as there have been many anthropogenic influences across the species’ range. Assessing this potential threat will be difficult to tease out relationships associated with climate change without careful consideration of other already confounding factors.

Factor A Summary

While there are several programs and efforts established to mitigate anthropogenic affects to pallid sturgeon, the present or threatened destruction, modification or curtailment of its habitat or range, remains a threat. However, the magnitude of this threat varies across the species range, due in part to restoration efforts and proportion of perturbations relative to the volume of habitat available. For example, the effects from dams (i.e., altered hydrographs and temperature profiles, altered ecologic processes, habitat fragmentation, and conversion of riverine reaches to reservoir) may be the single greatest factor affecting the species in the upper Missouri River basin. While in the middle and lower Missouri River as well as the middle Mississippi River, water quality, entrainment, and maintenance of the channel for navigation purposes and the associated impacts are not to be discounted. Additionally, the effects from other listing factors may be more limiting to the species in these areas. The same applies to the lower Mississippi River. Currently main-stem riverine habitat is not fragmented by dams and many natural ecological processes can still create a diversity of physical habitats believed important for the species. However, data are limited related to water quality and the potential effects from hydrokinetic energy development described in Factor E below.