7
PALLID STURGEON

This chapter addresses the question, Do current habitat conditions in the lower Platte River affect the likelihood of survival and limit (adversely affect) the recovery of the pallid sturgeon? The segment of the lower Platte River of interest in this question extends downstream from the confluence of the Elkhorn River with the lower Platte to the mouth of the lower Platte at its confluence with the Missouri River.

Sturgeons (family Acipenseridae) comprise an ancient group of fishes that are found in the northern hemisphere. Some species are anadromous, and others are freshwater; all spawn in freshwater rivers and streams. The tail fin is heterocercal (having unequal lobes), and the vertebrae are cartilaginous. All species have five prominent rows of bony scutes on the body that run from behind the head to the tail. Four barbels are on the ventral surface of the snout in front of the protrusible mouth.

Three species of sturgeon have been collected from the Platte River. The most common and widespread is the shovelnose sturgeon (Scaphirhynchus platorynchus). It is found in large rivers throughout the Mississippi River and Missouri River drainages. It is now confined in most years to the lower Platte and its tributaries upstream to the downstream-most dam or obstruction. However, before construction of mainstream dams, it was collected as far west as Casper, Wyoming, in the North Platte River (Baxter and Stone 1995). The lake sturgeon (Acipenser fulvescens) has a wide distribution from the Mississippi River and St. Lawrence River drainages north into Canada but has rarely been collected from the lower



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Endangered and Threatened Species of the Platte River 7 PALLID STURGEON This chapter addresses the question, Do current habitat conditions in the lower Platte River affect the likelihood of survival and limit (adversely affect) the recovery of the pallid sturgeon? The segment of the lower Platte River of interest in this question extends downstream from the confluence of the Elkhorn River with the lower Platte to the mouth of the lower Platte at its confluence with the Missouri River. Sturgeons (family Acipenseridae) comprise an ancient group of fishes that are found in the northern hemisphere. Some species are anadromous, and others are freshwater; all spawn in freshwater rivers and streams. The tail fin is heterocercal (having unequal lobes), and the vertebrae are cartilaginous. All species have five prominent rows of bony scutes on the body that run from behind the head to the tail. Four barbels are on the ventral surface of the snout in front of the protrusible mouth. Three species of sturgeon have been collected from the Platte River. The most common and widespread is the shovelnose sturgeon (Scaphirhynchus platorynchus). It is found in large rivers throughout the Mississippi River and Missouri River drainages. It is now confined in most years to the lower Platte and its tributaries upstream to the downstream-most dam or obstruction. However, before construction of mainstream dams, it was collected as far west as Casper, Wyoming, in the North Platte River (Baxter and Stone 1995). The lake sturgeon (Acipenser fulvescens) has a wide distribution from the Mississippi River and St. Lawrence River drainages north into Canada but has rarely been collected from the lower

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Endangered and Threatened Species of the Platte River Platte River and is uncommon in the Missouri River bordering Nebraska. In July 2000, a specimen was captured near Schuyler, Nebraska; that constitutes the furthest upstream documented locality in the Platte River. The pallid sturgeon was described from specimens captured in the Mississippi River near Grafton, Illinois, by Forbes and Richardson (1905). Specimens lack the spiracle found in the genus Acipenser, are light in color, and have a caudal peduncle that is fully armored and laterally compressed (Figure 1-5). The breast or belly of the pallid sturgeon is naked; this distinguishes it from the shovelnose sturgeon, whose belly is covered with scale-like plates. Keenlyne et al. (1994) measured shovelnose and pallid sturgeon from the upper Missouri River drainage and found overlap in morphometric characters between the two species. Sheehan et al. (1999) developed an index with meristic and morphometric characters to distinguish pallid and shovelnose sturgeon. Differences between species of the genus Scaphirhynchus have been debated over the last several years (Phelps and Allendorf 1983), but recent genetic studies are providing valuable insight into the evolutionary relationships of these species and have confirmed the genetic distinction between pallid and shovelnose sturgeon (Campton et al. 2000; McQuown et al. 2000; Simons et al. 2001; Birstein et al. 2002). There is no way to distinguish male and female pallid sturgeon visually. A common denominator in the distribution of sturgeon species appears to be access to flowing freshwater, at least for spawning and initial development. That is especially true for members of the genus Scaphirhynchus (also known as river sturgeons), which spend virtually their whole lives in riverine environments. Sturgeons are long-distance travelers. Tagging studies have documented movements of well over 100 km. Sturgeon eggs are demersal and adhesive. Shortly after hatching, the larvae become buoyant and drift with the current. SPECIES DISTRIBUTION Pallid sturgeon were found in the Mississippi River from near Keokuk, Iowa, downstream to the Gulf of Mexico, and in the Achafalaya River in Louisiana. The main part of their range is the Missouri River, from its confluence with the Mississippi River upstream to Fort Benton, Montana (Figures 7-1 and 7-2). In addition, pallid sturgeon have been documented from downstream reaches of several major tributaries of the Missouri River. In Kansas, they are known only from the lower 65 km of the Kansas River, mostly during the time of floods. In Nebraska, they have been captured up to 46 km upstream of the mouth of the Platte River. In Montana, their distribution extends 113 km up the Yellowstone River. Damming, water diversions, flood control, and channelization have modified rivers throughout much of its range (Keenlyne 1989). Today, the distribution of pallid

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Endangered and Threatened Species of the Platte River FIGURE 7-1 Map showing where pallid sturgeon had been caught in Missouri and Mississippi Rivers up to 1980. Each dot represents one location. Source: Lee 1980. Reprinted with permission; copyright 1980, North Carolina State Museum of Natural Sciences. sturgeon in the Missouri River drainage is disjunct, with a northern subpopulation in Montana that is associated with the Yellowstone River confluence and another subpopulation downstream of the Gavins Point Dam near Yankton, South Dakota. In this lower section of the Missouri River drainage, the Platte River is the only tributary from which pallid sturgeon have been captured regularly in the last 20 years. Pallid sturgeon were probably more abundant in the past, but no accurate records have been tabulated. At the time when pallid sturgeon were described, Forbes and Richardson (1905) stated that the ratio of pallid to shovelnose sturgeon in the catch at Grafton, Illinois, was 1:500, but at the mouth of the Missouri River the ratio was 1:5. Carlson et al. (1985) collected 4,355 sturgeon, of which 11 were identified as pallid sturgeon, for a ratio of 1:396. Watson and Stewart (1991) found one pallid sturgeon among

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Endangered and Threatened Species of the Platte River FIGURE 7-2 Map showing where pallid sturgeon had been caught in Platte River and its tributaries in 1979-2003. Each dot represents one location. Source: D. Feit, Nebraska Game and Parks Commission, unpublished data, 1979-2003. 350 sturgeon captured in the Yellowstone River. In comparison, continuing studies in the Platte River during 2000-2003 have captured four pallid and 929 shovelnose sturgeon, for a ratio of 1:232. Factors contributing to the decline in abundance of pallid sturgeon are diverse and in some cases incompletely documented. The illegal harvest of adult sturgeon for their eggs is probably a serious drain on the breeding stock. In the Platte River, catches of pallid sturgeon are uncommon but occur regularly. In 1979-2002, anglers reported catching pallid sturgeon in the Platte and Elkhorn Rivers. Most of the catches have been in the reach of the Platte River from the mouth upstream to about the mouth of the Elkhorn River (Table 7-1, Figure 7-2). As of 2003, no wild pallid sturgeon have been

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Endangered and Threatened Species of the Platte River TABLE 7-1 Angler Reports of Pallid Sturgeon Catches from Platte and Elkhorn Rivers, Nebraska Date Location Length, Weight May 10, 1979 I-80 bridge 37 in, 6.25 lb May 25, 1993 1 mi below Elkhorn River 35 in, 6.5 lb April 15, 1995 NE Hwy 50 bridge 36 in, 8-10 lb May 10, 1997 Elkhorn River mouth 41 in, 6.25 lb May 25, 1997 0.5 mi below Elkhorn River —, 6.5 lb June 9, 1997 US Hwy 6 bridge 36 in,— May 25, 1998 Elkhorn River mouth 18 in,— May 22, 1999 1 mi E. Ak-Sar-Ben Aquarium 42.5 in,— May 30, 1999 Louisville, NE —,— September 5, 1999 Elkhorn River 3 mi N. NE Hwy 36 30 in,— May 23, 2002 Elkhorn River 3 mi N. NE Hwy 91 36 in,—   Source: D. Feit, Nebraska Game and Park Commission, unpublished data, 1979-2003. documented in the Platte River upstream of the mouth of the Elkhorn River confluence. In addition to those fish, three wild and one hatchery-reared pallid sturgeon have been captured by researchers on the Platte River. The wild fish were caught in the vicinity of the Nebraska Highway 50 bridge at river kilometer (RK) 26 on May 3, 2001, and May 23, 2002, and near the US Highway 75 bridge at RK 10 on April 3, 2003. They were tagged with passive integrated transponder tags, implanted with radiotransmitters, and released back into the Platte River. They were later tracked. The hatchery-reared fish was captured on May 22, 2003, at RK 25; it had been stocked in the Missouri River at Bellevue, Nebraska, in 2002. REPRODUCTION AND POPULATION TRENDS There have been no direct observations of reproduction by pallid sturgeon in the wild, but movement patterns and collection of ripe females suggest that they spawn during June or July in South Dakota and as early as March in Louisiana. Spawning in most sturgeon is not an annual event, and current evidence suggests that pallid sturgeon males spawn on a 2- to 3-year cycle and pallid sturgeon females on a 3- to 5-year cycle (Keenlyne and Jenkins 1993). Successful spawning by pallid sturgeon in the lower Platte River has not been recorded. No spawning areas have been documented, and no trend data exist for pallid sturgeon, but Scaphirhynchus sp. larvae have been collected from the lower Platte River. Until recently, collections of the larvae had been rare; until 1999, no larvae had ever been collected in the wild. In August 1999, several pallid sturgeon larvae were collected at the

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Endangered and Threatened Species of the Platte River Big Muddy Wildlife Refuge in Missouri; where they were spawned has not been determined. In 1996, 1998, 1999, 2000, 2001, and 2002, sturgeon larvae less than a day after hatching (protolarvae) were collected from the Platte River near Ashland, Nebraska, during late May and early June. They were in all likelihood shovelnose sturgeon larvae, but the fact that sturgeon are hatching in the Platte River provides clues to the potential sites of pallid sturgeon spawning. Although there is no direct evidence of pallid sturgeon spawning in the lower Platte River, a female carrying a large quantity of eggs was captured on May 3, 2001, and a radiotransmitter was implanted in her. She remained in the lower Platte River for more than a month before moving back to the Missouri River in early June 2001. Another pallid sturgeon tagged on May 23, 2002, showed no evidence of reproductive products; it moved consistently downstream and left the lower Platte River within 6 days. During both years, the downstream movements occurred within a week of the time when sturgeon larvae were collected in the lower Platte River. These larvae were too small for their species to be accurately identified. In spring 2003, netting on a gravel bar in the Mississippi River captured 44 adult sturgeon, including one pallid sturgeon. After that collection of adult fish, sturgeon larvae were collected at the same site (Robert Hrabik, Missouri Department of Conservation, pers. comm., March 18, 2004). The lower Platte River seems to be the tributary of the Missouri River most likely used for spawning; no recent records of pallid sturgeon in other major tributary streams, such as the Kansas River, exist. That seems to be borne out by the observation that most collection takes place during the spring and early summer, when spawning is expected. However, the rarity of these animals, the turbid water, and the sparseness of our knowledge of their biology in the wild make the likelihood of observing their reproduction small. In addition, all wild-caught radio-tagged pallid sturgeon have been caught in April or May; they have moved out of the Platte River by early June. High water temperatures and loss of connectivity during years of low discharge may be important limiting factors. Systematic sampling for pallid sturgeon commenced in May 2000. Before that, fish sampling was sporadic and concentrated on techniques that were more suited to collection of catfish and other species. Therefore, there are few data on which to base an assessment of population trends. HABITAT COMPONENTS REQUIRED FOR SURVIVAL Pallid sturgeon are described as fish of large turbid rivers (Cross and Collins 1995; Harlan and Speaker 1951; Bailey and Allum 1962; Lee 1980). The areas where they are most regularly found include areas with many islands (Bramblett and White 2001), the mouths of tributaries, and the

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Endangered and Threatened Species of the Platte River downstream ends of islands and sand bars where currents converge (Hurley 1999; Snook et al. 2002). Pflieger (1997) describes their habitat as areas that exhibit strong current and firm substrate along sand bars and behind wing dikes. Studies in the upper Missouri River by Bramblett and White (2001), the Mississippi River by Hurley (1999), and the lower Platte River by Snook et al. (2002) describe the diversity of general conditions under which pallid sturgeon can live but indicate several common threads of habitat requirements. Depth Depths used by pallid sturgeon vary with river system and range from less than 1 m in the Platte River to 12 m in the Mississippi. Table 7-2 summarizes depth use for pallid sturgeon habitat by river studied. Snook (2001) and Snook et al. (2002) found that the hatchery-reared pallid sturgeon in the Platte River used depths of 0.15-2.75 m; the average was 0.84 m. Swigle (2003) found that wild pallid sturgeon caught in the Platte River used water depths that averaged 1.29 m. Bramblett (1996) and Bramblett and White (2001) found that pallid sturgeon in the upper Missouri and Yellowstone Rivers used depths that averaged 3.3 m (range, 1-7 m), and Hurley (1999) found that pallid sturgeon in the Mississippi River used depths of 6-12 m. Erickson (1992) studied habitats used by pallid sturgeon in Lake Sharpe, South Dakota, and stated that they were found in water with an average depth of over 4 m in this lake environment. Catches of pallid sturgeon from the tail race of the Missouri River at Fort Peck Dam, Montana, came from depths of 1.2-3.7 m (Clancey 1990). Watson and Stewart (1991) captured them at 0.6-14.5 m in the upper Missouri and Yellowstone Rivers, Montana, and Constant et al. (1997) captured them at an average of 15.2 m in a constructed channel of the Atchafalaya River in Louisiana. Wild pallid sturgeon caught in the Platte TABLE 7-2 Depth Use of Pallid Sturgeon Documented with Telemetry Study Location Average Depth (Range), m Reference Missouri and Yellowstone Rivers (Montana) 3.3 (0.6-14.5) Bramblett (1996), Bramblett and White (2001) Mississippi River (Illinois) —(6-12) Hurley (1999) Missouri River at Lake Sharpe (South Dakota) (<5kg) 4.62 (1.52-6.71) (>5kg) 4.66 (1.22-10.4) Erickson (1992) Platte River (Nebraska) 0.84 (0.15-2.75) Snook (2001),a Snook et al. (2002)a Platte River (Nebraska) 1.29 (0.58-2.71) Swigle 2003 aHatchery-reared fish.

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Endangered and Threatened Species of the Platte River River, Nebraska, during 2001 and 2002 were captured in water that was 0.7 and 1.5 m deep, respectively (Ed Peters, University of Nebraska, unpublished material, May 2, 2001, and May 23, 2002). Water Velocity General descriptions of current velocity often use terms like strong (Lee 1980) and swift (Carlson et al. 1985; Pflieger 1997), but these refer to the appearance of the river from the surface. In a laboratory study of swimming endurance, juvenile pallid sturgeon were able to sustain swimming at 0.25 m/s and attain burst speeds of up to 0.7 m/s (Adams et al. 1999). It seems likely that pallid sturgeon are using low-velocity microhabitats near the substrate of higher-velocity river reaches. To describe the water velocities experienced by pallid sturgeon quantitatively, both column velocities and bottom velocities have been used to characterize pallid sturgeon habitats. Column velocities tend to be more variable, as would be expected over the wide range of water depths where pallid sturgeon occur. Bottom velocities, which more accurately describe conditions of the sturgeon near the substrate, generally are less variable. Table 7-3 summarizes column velocities and bottom velocities from several studies across the range of the pallid sturgeon. Water velocity measured during collection of pallid sturgeon was 0.46-0.96 m/s (Clancey 1990) in the Missouri River tail race of Fort Peck Reservoir in Montana. In the Platte River, Nebraska, wild pallid sturgeon were captured in habitats with a mean column velocity of 0.82 m/s in 2001 and a range of 0.38-0.65 m/s in 2002 (Ed Peters, University of Nebraska, unpublished material, May 2, 2001, and May 23, 2002 ). Bottom velocity at those sites was 0.31 m/s in 2001 and ranged from 0.12 to 0.36 m/s in 2002. TABLE 7-3 Mean Column Velocities and Bottom Velocities at Pallid Sturgeon Sites in Telemetry Studies Study Location Column Velocity (Average), m/sa Bottom Velocity (Average), m/s Reference Missouri and Yellowstone Rivers (Montana) (0.90) 0.0-0.70 0.0-1.55 Bramblett (1996), Bramblett and White (2001) Lake Sharpe (South Dakota) 0-0.73 (0.40) 0-0.55 (0.18) Erickson (1992) Platte River (Nebraska) 0.03-1.26 (0.69) 0.17-0.97 (0.38) Snook (2001),b Snook et al. (2002)b Platte River (Nebraska) 0.43-1.28 (0.86) 0.28-0.84 (0.58) Swigle (2003) aAverage velocities measured in water column at point 0.6 × water depth for water less than 1 m deep or by averaging velocity measurements at 0.2 × and 0.8 × water depth for water depths greater than 1 m. bHatchery-reared fish.

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Endangered and Threatened Species of the Platte River Other Factors Sand appears to be the substrate of choice for pallid sturgeon, in contrast with the preference of shovelnose sturgeon for gravel substrates (Bramblett and White 2001). The most common feature of the substrate that characterizes pallid sturgeon habitat is an irregular bottom contour (Bramblett and White 2001; Hurley 1999; Snook et al. 2002), which occurs at the downstream ends of sunken sand bars and open channels with dunes (Swigle 2003). Pallid sturgeon seem to tolerate a wide range of temperatures, 0-33°C in the Platte River, Nebraska (temperatures averaged 24.8°C during summer 1998 and 20.6°C during summer 1999). That is consistent with the range of temperatures noted by Dryer and Sandvol (1993). Little is known of the dissolved-oxygen requirements of pallid sturgeon, but they are generally found in areas where dissolved oxygen is considered good for most species (>5 mg/L). Pallid sturgeon seem to prefer “excessively turbid waters” (Lee 1980) and avoid areas that lack turbidity (Bailey and Cross 1954; Erickson 1992). In the lower Platte River, suspended-solids concentrations where hatchery-reared pallid sturgeon have been found ranged from 56 to 1,000 mg/L (Snook 2001). Suspended-solids concentrations at locations where wild pallid sturgeon were captured in the lower Platte River range from 313 to 359 mg/L (Swigle 2003). Turbidity may act as a component of cover for pallid and shovelnose sturgeon. Studies in the Platte River (Snook 2001; Swigle 2003) found sturgeon away from objects, such as submerged logs, that afford cover to many species of riverine fishes (Peters et al. 1989), whereas studies in low-turbidity sections of the Missouri River have noted use of such areas by telemetry-tagged pallid sturgeon. Rivers where pallid sturgeon are found are often high in dissolved solids and high in conductivity, but little is known of their specific requirements for these conditions. Similarly, virtually nothing is known about their tolerance of pesticides and other pollutants. Ruelle and Keenlyne (1993) examined tissues from three pallid sturgeon and found a variety of organic pesticides but documented no specific effects. Current studies by the U.S. Fish and Wildlife Service and the U.S. Geological Survey are assessing the health of shovelnose sturgeon in the Platte River. Biologic Interactions Pallid sturgeon occur in rivers that support a wide variety of other fish species, but the habitat conditions that they seem to prefer probably limit their direct associations to species that inhabit the same microhabitats. It is generally accepted that pallid sturgeon are piscivorous as adults (Coker 1930; Herb Bollig, U.S. Fish and Wildlife Service, pers. comm., 1998), but there have been no supporting quantitative field studies. Therefore, we

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Endangered and Threatened Species of the Platte River surmise that smaller fishes that are found in habitats with pallid sturgeon are important food resources. In the Platte River, fishes that use the habitats where pallid sturgeon have been found include speckled chubs (Macrhybopsis aestivalis), sturgeon chubs (M. gelida), and a host of other minnow and sucker species. Methods for Determining Habitat Components The methods used to determine habitat use by fishes in general have centered on measuring a variety of habitat characteristics in areas where the species of interest has been collected or observed. It is particularly challenging for species, like pallid sturgeon, that seem to prefer turbid water, in which direct observation of behaviors, such as feeding and spawning, is unlikely or nearly impossible. Therefore, biologists need to depend on focused collection techniques and telemetry. Recent advances in sonar technology are affording additional methods for imaging fish in turbid river habitats. CURRENT PLATTE RIVER CONDITIONS The lower Platte River still maintains the braided channel pattern with abundant areas of converging currents that pallid sturgeon seem to select. The shifting sand has been noted as a preferred substrate by pallid sturgeon. Patches of gravel substrate may afford habitats for spawning during the spring warmup of the Platte River and allow spawning earlier in the season than in the Missouri River. On the basis of surveys, the lower Platte appears to afford pallid sturgeon usable habitat up to the vicinity of the Loup Power Canal confluence near Columbus, Nebraska. Flows upstream of this point are too unpredictable to provide reliable habitat, and diversion structures impose barriers to access. Even though conditions in the lower Platte may be appropriate for pallid sturgeon use, these areas of acceptable or even preferred habitat may be isolated by sections of inhospitable habitat during periods of low flow, such as those of the drought in 2002 and 2003. The section of the lower Platte downstream of the mouth of the Elkhorn River appears to retain most of the appropriate habitat conditions and the connectivity that reliably allows use by pallid sturgeon. Historically, those conditions may not have posed as much of a problem for the pallid sturgeon and other riverine fishes; today, channelization and damming of the Missouri River have depleted pallid sturgeon habitats throughout its former range. Pallid sturgeon have been captured in the Elkhorn River, but they must traverse the lower Platte to reach it. There are no other known habitats for pallid sturgeon in tributaries of the lower Platte River system or in nearby Missouri River tributaries. For those reasons, the lower Platte River may be even more important for the recovery of pallid sturgeon.

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Endangered and Threatened Species of the Platte River RECOVERY OF PALLID STURGEON The pallid sturgeon recovery plan (Dryer and Sandvol 1993) has as its goal the downlisting of the species from endangered to threatened, or to delisting by 2040. Because of the large and fragmented range of the pallid sturgeon, the recovery plan identifies six recovery priority management areas (Figure 7-3). The mouth of the Platte River is included in recovery priority management area 4. Although no specific population target densities have FIGURE 7-3 Locations of priority management areas 1-6 for recovery of pallid sturgeon. Source: Dryer and Sandvol 1993.

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Endangered and Threatened Species of the Platte River been identified, a population structure that includes at least 10% sexually mature females is part of the recovery goal. It is proposed that this goal be accomplished by a combined effort that uses protection from harvest, protection of habitat, and supplemental stocking of hatchery-reared fish as major features. The purpose of the planned efforts to stock hatchery-reared pallid sturgeon is to make up for the apparent lack of reproduction of the native populations. The parentage of the released offspring is wild fish that were captured from and released back into their native areas after 1 or 2 years of spawning. The offspring are reared for the shortest time possible to retain their natural, wild behavioral characteristics and because larger fish are expensive to feed. An additional consideration in how long they are reared is whether they can accept tags that allow them to be identified as hatchery-reared fish, which would distinguish them from non-hatchery-reared stocks. Hatchery-reared pallid sturgeon have not been released into the Platte River since the late 1990s; 75 were released in 1998 (of which 10 were radio-tagged) and 25 in 1999 (of which 15 were radio-tagged). All releases since then have been in the Missouri River. Evidently, some fish stocked in the Missouri River are finding their way into the lower Platte River. The committee views the use of hatcheries in restoration programs as a last resort for an extremely depleted population, as did the recent National Research Council report on Atlantic salmon in Maine (NRC 2004c). That report reviewed recent literature on the genetic and other hazards that can be imposed by hatcheries used to rehabilitate depressed salmonid populations; it also described protocols that can reduce some of the adverse effects, although some genetic hazards cannot be avoided by any hatchery program. Much has been learned about salmonid hatchery programs, especially in the Pacific Northwest, Canada, the United Kingdom, and Scandinavia, and research continues there. The life history of sturgeon differs from that of salmon in important ways, but much of the experience with salmon hatcheries is applicable to any diploid, heterosexual organism. Any hatchery program for sturgeon should be thoroughly informed by that experience. The current range of conditions in the Platte River seems to attract pallid sturgeon to this river in numbers at least as great as those in the rest of its range in recovery-priority management area 4. Pallid sturgeon have been documented as moving long distances during their lives, so it seems likely (though not proved) that they are attracted to the Platte River. Because of low population densities and rather recent initiation of regular sturgeon sampling, it has not been determined what role the Platte plays in the ultimate recovery of pallid sturgeon in the middle and lower Missouri River population. Today, the lower Platte receives water from the central Platte and three main tributaries—the Loup system, the Elkhorn system,

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Endangered and Threatened Species of the Platte River and the Salt Creek system. Discharge from the Loup system is modified by irrigation withdrawals and peaking power generation at Columbus, Nebraska. Those fluctuations and runoff events from the Elkhorn River, Salt Creek, and smaller tributaries result in irregular rises in discharge in the lower Platte that may be important for its use by pallid sturgeon; many riverine fishes require rising flows to initiate spawning activities. The tributaries also contribute fine sediments to the river. IMPORTANCE OF THE LOWER PLATTE RIVER TO PALLID STURGEON Critical habitat has not been designated for pallid sturgeon. However, recovery-priority management areas have included the mouth of the lower Platte River and adjacent portions of the Missouri River on the basis of the preponderance of collections of pallid sturgeon in the region. Research has documented use of the lower Platte by pallid sturgeon during April, May, and June in the last 3 years. The question of the importance of Platte River flows for the existence of the pallid sturgeon is still problematic. On the one hand, the lower Platte River accounts for only a small fraction of the distribution of the remaining area available to the pallid sturgeon of the lower Missouri population. On the other hand, the lower Platte is the only tributary in this reach where pallid sturgeon are regularly captured, and the captures coincide with conditions and times that correspond to the presumed reproductive requirements of the pallid sturgeon. Although there is no recorded documentation of pallid sturgeon reproduction in the lower Platte River, it seems to be the tributary of the lower Missouri River most likely to have the combination of discharge, substrate, and channel morphometry necessary for spawning to occur. There is recent evidence that sturgeon are spawning on sand and gravel bars in the lower Missouri River and the Mississippi River. As for habitat conditions, the lower Platte seems to afford pallid sturgeon the turbid, shifting sand substrate that was apparently typical of most of the Missouri River before the imposition of the Pick-Sloan impoundments that stabilized river flows and reduced sediment loads. The major habitat differences between the lower Platte and the Missouri may be in water depth and volume of flow. Those differences may limit the usefulness of the lower Platte River as overwintering habitat for pallid sturgeon. During periods of low flow and high temperatures, the Platte may exceed their thermal tolerance. However, it is important to recognize that the pallid sturgeon using the lower Platte River are part of a much larger population that may include all the Missouri River from Gavins Point Dam downstream to the Mississippi River and possibly beyond. Efforts that are under way to mitigate the impacts of the channelization that occurred in the

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Endangered and Threatened Species of the Platte River Missouri River over the last 60 years may eventually lessen the importance of the lower Platte River for the recovery of pallid sturgeon in this part of its range. Today, however, the lower Platte habitat is important for the recovery of the pallid sturgeon by virtue of their relative abundance in the vicinity of the mouth of the Platte River in the Missouri River. It is clear that the lower Platte River by itself can be only a part of the overall recovery of the lower Missouri River population of the pallid sturgeon. Continued efforts to expand appropriate habitats for spawning, overwintering, development, and maturation of pallid sturgeon that will increase the overall population are urgently needed. Furthermore, recovery of pallid sturgeon and down-listing or delisting of the species will depend on population responses to the introduction of hatchery-reared fish (dependence on hatcheries to sustain populations that reproduce inadequately essentially ensures continued listing status), on control of contaminants in key habitats, and on assurance that accidental overharvest does not affect the known populations that apparently are naturally sparse. SUMMARY AND CONCLUSIONS The endangered population of pallid sturgeon is exceptionally small and apparently declining. Much of its original habitat in the Missouri River has been altered by dams and their reservoirs. The lower Platte River remains the habitat with a flow regime most similar to the original, unaltered habitat of pallid sturgeon. The braided channel of warm, sediment-rich waters with shifting sandbars and islands and a sandy substrate offers suitable habitat that serves as a refuge for pallid sturgeon and a possible starting point for expanding the population. For those reasons, the committee concluded that current habitat conditions in the lower Platte do not adversely affect the likelihood of survival or recovery of the pallid sturgeon. The loss of lower Platte River habitat would probably result in a catastrophic reduction in the pallid sturgeon population. Any recovery effort for the pallid sturgeon will of necessity include the lower Platte River. Questions about the biology of the pallid sturgeon and the role of the lower Platte River in its recovery are as follows. What is the connection between pallid sturgeon that use the lower Platte River and those found throughout the middle and lower Missouri River? Knowledge of the connection could help to answer the question of the importance of the Platte for the recovery of the pallid sturgeon in this section of its range. How does the year-to-year variability of the discharge in the Platte River affect the reproduction of the pallid sturgeon? Concerted studies on the pallid sturgeon in the lower Platte River started in 2000, and the Platte has since experienced some of the lowest discharge volumes on record.

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Endangered and Threatened Species of the Platte River Continuing the current studies for another 5-10 years may allow an evaluation of how pallid sturgeon respond to higher flows. In addition, evaluation of the recovery of any animal with a maturation time of 10-plus years that reproduces on a 3- to 5-year cycle requires longer-term studies. How do proposed modifications along the Platte River affect flow patterns and instream habitat in the river? The habitat in the lower Platte River needs to be evaluated with models that more accurately predict channel responses to altered river management. Such prediction requires intensive studies of the hydrological relations in shifting sand-bed streams.