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Paddlefishes and Sturgeons of the Yangtze and Mississippi Rivers: Status, Biology, and Management.

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  • Yangtze river fisheries research institute , Chinese Academy of Fishery Sciences
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... In addition to the previously mentioned fish species, the Chinese sturgeon and other endemic fish species were previously commercially harvested for food in the Yangtze River . For instance, the annual commercial harvested number of Chinese sturgeon from the Yangtze River was around 400 to 600 during 1972-1980, and then reached 1163 in 1981 when large numbers of Chinese sturgeon were not able to pass the Gezhouba Dam and gathered below the dam ( Fig. 2a; Phelps et al. 2016;Huang et al. 2017). Also, the pre-spawn population of Chinese sturgeon in the mainstem Yangtze River below Gezhouba Dam was about 2176 individuals in 1980, but was reduced to \ 1000 in 1989, and then to only around 20 in 2017 (Fig. 2a;Zhu et al. 2009;Huang 2013;CMEE 2017;CMOA 2019). ...
... Likewise, the population of Chinese paddlefish in the mainstem Yangtze River below Gezhouba Dam dramatically decreased from about 32 individuals in 1985 to functional extinction. That is, the total existing population became too low to support natural reproduction around 1995 ( Fig. 2b; Wei et al. 1997;Phelps et al. 2016;CMOA 2019). ...
... Data sources:Zhu et al. (2009) and CMEE(2017)Ó Royal Swedish Academy of Sciences 2019 www.kva.Total harvested number and pre-spawn of Chinese sturgeon Acipenser sinensis (the vertical line indicates the initial operation of the Gezhouba Dam in 1981) in the Yangtze River and b total estimated number of adult Chinese paddlefish Psephurus gladius below the Gezhouba Dam in the mainstem of the Yangtze River. Data sources:Wei et al. (1997),Zhu et al. (2009), Huang (2013,Phelps et al. (2016),Huang et al. (2017), CMEE (2017, and CMOA (2019) Total estimated number of Baiji or Yangtze river dolphin Lipotes vexillifer and Yangtze finless porpoise Neophocaena asiaeorientalis in the mainstem of the Yangtze River. Data sources:Chen and Hua (1985),Chen et al. (1993),Zhang et al. (1993Zhang et al. ( , 2003, Turvey et al. (2007), Mei et al. (2014), Chen et al. (2016), CMEE (2017), and CAS (2018) Royal Swedish Academy of Sciences 2019 www.kva.se/en ...
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These relatively complete fossils are extremely rare and belong to the species †Paleopsephurus wilsoni MacAlpin, 1947 from the Upper Cretaceous Hell Creek Formation of Montana (redescribed here); †Polyodon sp. nov. from the lower Paleocene Tullock Formation of Montana (described here); and †Crossopholis magnicaudatus Cope, 1883 from lower Eocene deposits of the Green River Formation in Wyoming (described in detail here for the first time based on much new material). Among the fossils, †Crossopholis is of particular interest, because it is now known by several nearly complete skeletons (reported and described here for the first time). †Paleopsephurus and †Polyodon sp. nov. are known from a single partial skull each, and some fragmentary postcranial material, but are well enough represented to include in our phylogenetic analysis. Other, more fragmentary, Upper Cretaceous and Paleocene material (some newly reported here) is placed in Polyodontidae incertae sedis.There are two extant species in the family, frequently referred to in the literature as “living fossils.” These two species are Polyodon spathula from the Mississippi River drainage of North America, and Psephurus gladius primarily from the upper Yangtze River drainage in China. The osteology of Psephurus gladius is described here in detail for the first time.Previous studies of Polyodontidae have been almost exclusively based on Polyodon spathula, because specimens of the other species are extremely rare and very poorly known. After accumulating and preparing material for several years, we were able to describe more fully all polyodontid species and examine the family phylogenetically. Our study reveals that Polyodon spathula is a highly derived member of the family, and generally not a good representative of the family when the family is used as an outgroup for higher-level phylogenetic studies of actinopterygians. The use of Polyodon spathula as the primitive polyodontid has previously led to several serious errors in the literature. We therefore also summarize certain primitive characters of Polyodontidae in this paper.Contrary to previous studies, we show that †Paleopsephurus has the stellate rostral bones unique to polyodontids (in addition to the long median rostral bones and extremely elongate snout also unique to the group). Also contrary to previous studies, Polyodontidae (at least primitively) does have ossifications of the quadratojugal, interclavicle and postcleithrum. 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Normal-sized, reproductively active adult paddlefishes lack particular ossifications of the scapulocoracoid, vertebral column and neurocranium present in other osteichthyans; but as we show here, these elements eventually ossify in very large, older individuals. In this sense, paddlefishes appear to be the axolotls of the fish world, offering one of the best documented cases of paedomorphosis (or neoteny) known to date.
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A novel electrosensory function is presented for the large, plankton-feeding, freshwater paddlefish, Polyodon spathula, along with a hypothesis which accounts for the distinctive, elongated rostrum of this unusual fish. Behavioural experiments conducted in the 'dark' (under infrared illumination), to eliminate vision, show that paddlefish efficiently capture planktonic prey to distances up to 80-90 mm. They make feeding strikes at dipole electrodes in response to weak low-frequency electrical currents. Fish also avoid metal obstacles placed in the water, again in the dark. Electrophysiological experiments confirm that the Lorenzinian ampullae of paddlefish are sensitive to weak, low-frequency electrical signals, and demonstrate unequivocally that they respond to the very small electrical signals generated by their natural zooplankton prey (Daphnia sp.). We propose that the rostrum constitutes the biological equivalent of an electrical antenna, enabling the fish to accurately detect and capture its planktonic food in turbid river environments where vision is severely limited. The electrical sensitivity of paddlefish to metallic substrates may interfere with their migrations through locks and dams.
Article
Chinese sturgeon, Acipenser sinensis Gray, an anadromous fish, mainly distributes in the mainstream of Yangtze River, the continental shelf of East China Sea and Yellow Sea. Historical spawning grounds of A. sinensis were in the upper Yangtze and the lower Jingsha River, covering a stretch of at least 600 km of river length. After the damming of Yangtze River by the Gezhouba Dam in January 1981, the spawning grounds for A. sinensis were no longer accessible and fish alternatively spawned within a 30 km section below the Gezhouba Dam. Moreover, only one regular spawning ground has been found in the downstream area of the Dam. It is in a narrow portion within a 7 km length, just below the dam. A minor spawning ground was also found at Huyatan, around 15 km below the dam. The present attempts to estimate stock size of A. sinensis suggest that the major source of recruitment from a complementary colony is still obtained through natural reproduction. A. sinensis requires a certain water temperature, moderate bottom velocity, and special riverbed topography to spawn successfully. Due to the restriction of experimental instruments and methods, the research on the topography of spawning grounds is a first attempt to tentatively characterize the area in terms of fine-scale bottom profile and structure. Based on measurements taken during five surveys between 1999 and 2006, we analyzed the river bed morphology by assessing the elevation, slopes, and the bottom "roughness" of the spawning grounds by ArcGIS 9.0. The terrain of the riverbed was simulated by Inverse Distance Weighted scheme with an 8.0 m of grid size. The results showed that the roughness in regularly used alternative spawning site directly at the dam and the minor spawning ground further downstream are 1.74 × 10-3 and 0.36 × 10-3, respectively. Moreover, the slope of area between Gezhouba Dam and Moji Hill is much bigger than that between the Linjiang Stream and Huyatan. The average and standard deviation of length of elevations at the upstream and downstream spawning sites are 32.35 ±2.18 m and 30.14 ±3.85 m, respectively. Taking the average water level for spawning between 1983 and 2002 as a standard, the average water depth on the upstream and downstream spawning sites were 11.66 m and 13.87 m, respectively. These depths may be the optimal water depth for A. sinensis to spawn under the given hydrodynamic regime in the Yangtze River. Four views are considered in this article. (1) The topographical characteristics of the spawning ground which locates in the reaches between the Gezhouba Dam and the Shilihong are very similar to those on the historical spawning grounds. The natural reproduction of A. sinensis rarely occurs in the reach between the Linjiang Stream and Huyatan, and it is postulated that the major reason is to be found in the unsuitable terrain, which makes the hydrological regime and hydraulic conditions not suitable to trigger spawning activity. (2) The topography of the spawning ground below the Gezhouba Dam has been changed drastically during recent decades because of the River Regime Regulating Work. The increased complexity of the topography may yield some benefit for A. sinensis to attract fish to inhabit before the natural reproduction stage is reached, but the destruction of slopes on the spawning grounds perhaps goes against the dispersion of fertilized eggs and embryonic development. (3) The results provide further support for the hypothesis of "Dividing Function of the Spawning Ground of A. sinensis" in the area below the Dam at the present time. The turning of riverway and adverse slope of riverbed may be the absolutely necessary requirement for a suitable spawning ground of A. sinensis. Although the data are limted for such a working hypothesis, further studies should combine the accurate terrain data from historical spawning grounds of A. sinensis with thos from the new grounds to verify this potentially valid hypothesis. (4) The substrate, hydrological and hydraulis conditions are affected by the topography of the riverbed to a certan extent. It is necessary to make further investigation on the influence of riverbed terrain on the natural reproduction of A. sinensis, especially when considering the changes that may be anticipated with the full functioning of the Three Gorges Dam which must have also an effect on the hydrodynamics downstream even beyond the Gezhouba Dam.
Article
The shovelnose sturgeon, Scaphirhynchus platorynchus, is a freshwater sturgeon of the Mississippi and Missouri rivers and their tributaries. It is one of the smaller North American sturgeons, seldom weighing more than 2.5 kg over most of its range except in the upper Missouri River, where individuals of over 7 kg have been found. Spawning occurs in spring at temperatures between 17 and 21 °C over rock or gravel substrate downstream from dams, near rock structures, or in tributaries, most males reach sexual maturity at 5 years, most females at 7 years. Adults do not spawn every year. Shovelnose sturgeon are found in large, turbid rivers and frequently concentrate in areas downstream from dams or at the mouths of tributaries. Population densities range up to 2500 fish per km. They are commonly found in areas of current over sandy bottoms or near rocky points or bars, where they feed primarily on aquatic invertebrates. The shovelnose sturgeon is classified as a sport species in 12 of 24 states where it occurs. Commercial harvest is allowed in seven states, where fresh shovelnose sturgeon sell for 55 to 88 cents per kg, smoked shovelnose for about $5.75 per kg, and roe from 33 to 110 dollars per kg. About 25 tons of shovelnose sturgeon are harvested commercially each year. Shovelnose sturgeon are considered extirpated in three states, fully protected in four states, and rare, threatened, or of special concern in eight states. Populations are considered stable throughout most of the upper Mississippi, lower Missouri, Red, and Atchafalaya rivers. Three states, Wyoming, West Virginia, and New Mexico, have developed plans to reintroduce the species into rivers where it has been extirpated.
Chapter
The sturgeons belong to the family Acipenseridae and to the order Acipenseriformes. The Polyodontidae or paddle fishes are also included in this order.
Chapter
The status of paddlefi sh Polyodon spathula in the United States was first described in two surveys published in 1986 and 1997; in this paper, we report the results of a similar survey of state and federal agency personnel that we conducted in 2006. From the 1970s through the 1990s, the status of paddlefi sh stocks was on a downward trend throughout much of the species’ range. The 2006 survey results suggest that the status of paddlefish stocks has improved since the fi rst survey was conducted; 17 of 26 states in 2006 reported that their paddlefi sh populations were stable or increasing, compared to only 14 states in 1983 and 1994. The number of states with closed fisheries (i.e., no commercial or sport harvest) increased to 12 in 2006 from 8 in 1983. The number of states reporting declining or stable/declining paddlefi sh populations dropped from seven states in 1983 to only three states in 2006. The two principal reasons cited for reported declines have remained the same for more than three decades: habitat loss and overfishing. Two states where paddlefi sh were listed as extirpated (New York and Pennsylvania) have begun restoration efforts that may one day allow the status of paddlefi sh in those states to be changed. As long as the demand for caviar remains strong, pressure on paddlefi sh stocks will undoubtedly remain high in the seven states where they are commercially exploited. however, earlier fears of a basin-wide collapse in paddlefi sh stocks should continue to diminish if resource managers are successful in combating overfi shing and continued habitat destruction, which will always threaten the long-term viability of paddlefi sh stocks throughout the Mississippi River basin.
Article
Metabolic rate, branchial morphology, and modes of gill ventilation were studied in young (2-10 g) North American paddlefish, Polyodon spathula, with anatomical, behavioral, and physiological methods. Polyodon lacks the oral and opercular valves that are typical for fishes that rely on a buccal pump system to ventilate the gills, and the jaw opening system of Polyodon is poorly suited for regular pumping movements. Unrestrained, undisturbed juvenile paddlefishes swim constantly at a mean speed of 1.1-1.5 body lengths · $s^{-1}$ (bls). The maximum speed sustainable for > 10 min is 1.6-1.8 bls. When forced to swim at slow speeds in flow tanks or water tunnels, ventilation of the gills by buccal pumping occurs at a frequency of 50-80 · $min^{-1}$ . As swimming speed increases, buccal ventilation becomes intermittent and continuous ram ventilation occurs above 0.6-0.8 bls, which means that Polyodon is essentially an obligate ram ventilator under normal conditions. Oxygen consumption ( $\dot{M}O_{2}$ ), carbon dioxide production ( $\dot{M}O_{2}$ ), and the gas exchange ratio (R) were determined as a function of inspired Po₂ during undisturbed swimming in still water at 25° C Oxygen consumption, buccal pressure, and swimming performance were also measured at set swimming speeds in a flow tank and small water tunnel. Oxygen consumption at the preferred swimming speed of 1.25 bls was 6-7 μmol O₂, · $g^{-1}$ · $h^{-1}$ . Carbon dioxide production was 3-4 μmol CO₂ · $g^{-1}$ · $h^{-1}$ , yielding an R of 0.5-1.0. Paddlefishes are O₂ regulators in mild hypoxia (150 down to 90 mmHg) but die quickly at Po₂ < 90 mmHg. During steady swimming in normoxia, paddlefishes normally maintain 70%-80% of the maximum sustainable speed. This results in a normal minimum metabolic rate that is about twice that of the minimum (resting) rate of other acipensiform fishes. From a phylogenetic standpoint, other acipenseriforms also use ram ventilation, leading to the hypothesis that the evolutionary origin of a reliance on ram ventilation in Polyodon probably predates the origin of the filter feeding habit. Constant swimming may be metabolically expensive, but it would appear to allow some energy to be conserved by ram ventilation. This may be particularly advantageous for species such as P. spathula that combine filter feeding and ram ventilation.
Article
North American paddlefish Polyodon spathula historically occurred in the Mississippi, Saint Lawrence, and Alabama rivers, as well as several other Gulf of Mexico coastal drainages. Recent population declines in some locations and the local extirpations of others purportedly have been due to habitat loss, overharvest, and other biotic and abiotic changes to riverine ecosystems. Mitigation for these declines has emphasized hatchery production and supplemental stocking programs, which use a number of geographical sources for parental stock. To investigate phylogeographic relationships within and among several watersheds, we surveyed allozyme and mtDNA variation among 189 individuals collected from six regions of the Mississippi River drainage plus the Mobile Bay and Pearl River drainages. Paddlefish exhibited lower levels of allelic and haplotypic diversity than other freshwater species, a characteristic shared with other “primitive” fishes. Allozyme variation at 64 presumptive loci was segregated into two major groups: a Mobile Bay drainage group and a group from the Mississippi River and Pearl River drainages. More subtle population divergences were observed among major regional tributaries within the Mississippi River, although a clear and unambiguous level of geographical clustering was not delineated. Furthermore, several of the localities harbored “private,” but rare, polymorphisms. These results demonstrate that the populations are sufficiently divergent to warrant differential management considerations for any rehabilitation, restoration, or protection measures.
Article
Dabry's sturgeon, Acipenser dabryanus, is a relatively small (130 cm, 16 kg) and now rare sturgeon restricted to the Yangtze River Basin. It behaves as a resident freshwater fish, does not undertake long distance migrations (except for spawning), and lives in a variety of habitats. It historically spawned in the upper Yangtze River, but the spawning sites are unknown. Acipenser dabryanus reaches maturity earlier than do other Chinese sturgeons, which gives the species aquaculture potential, and artificial spawning has been carried out. However, the native population in the Yangtze has sharply declined in the last two decades due to overfishing, pollution and habitat alteration and destruction, especially since the construction of the Gezhouba Dam, which was built in 1981 across the Yangtze River at Yichang, Hubei Province. Since 1981, Dabry's sturgeon rarely occurs below the Gezhouba Dam because downstream movements are blocked. Clearly, conservation of Dabry's sturgeon must be emphasized. Conservation methods may include protecting habitats, controlling capture and stock replenishment.
Article
Paddlefish Polyodon spathula (n = 576) were collected from Kentucky Lake, Kentucky–Tennessee, with experimental gill nets in 2003–2004 to assess population characteristics and the potential for commercial overfishing. Additional data were collected from 1,039 paddlefish caught by commercial gillnetters in this impoundment. Since the most recent study in 1991, size and age structure have been reduced and annual mortality has tripled. In the 1991 study, 37% of the fish collected were older than the maximum age we observed (age 11), and in 2003 annual mortality for paddlefish age 7 and older was high (A = 68%). Natural mortality is presumably low (
Article
Age at sexual maturity has not been described previously for the pallid sturgeon Scaphirhynchus albus, an endangered species. Age and reproductive development data were obtained for five male and nine female pallid sturgeons collected from 1983 to 1991. Spawning bands were observed in pectoral fin ray sections of age-25 and age-41 females. Males reached sexual maturity at ages 5–7. Females began egg development at ages 9–12 and first spawned at age 15. Eight of the specimens we examined were collected from the headwaters of the Atchafalaya River, where pallid sturgeons had not been previously reported.
Article
Paddlefish fisheries in some states have become increasingly important, but in other states populations have declined severely and the species range has been reduced. Commercial paddlefish harvests today come mainly from the impounded Tennessee, Mississippi, and Arkansas Rivers. Sport fisheries have replaced commercial fisheries in some areas and even though they are extremely localized, they account for sizable harvests. Declining paddlefish populations reported in several states are a result of loss of spawning habitat. Recent studies in these and other states have increased our knowledge about the paddlefish and its management. Identification and protection of spawning areas is critical to the maintenance of the fisheries.
Article
Growth, mortality, fecundity, egg diameter, and age at maturity were determined from samples of 270 paddlefish Polyodon spathula collected from the Atchafalaya River basin and Lake Pontchartrain, Louisiana, during a commercial harvest moratorium from 1987 to 1989. Early growth of paddlefish determined from back-calculation of annular measurements on dentary cross sections was high relative to paddlefish populations in the upper Mississippi River drainage; lengths of age-1 fish ranged from 411 to 455 mm in eye-fork length. Natural mortality of Louisiana paddlefish was high (26–48%), and data suggested a reduction in age at maturity (100% females mature at age 10), lower fecundity (average fecundity, 9,500 eggs/kg body weight), and larger eggs (average diameter, 2.67 mm) relative to other paddlefish populations. Changes in reproductive life history characteristics may reflect rapid growth and high natural mortality rates. As the postmoratorium commercial fishery for Louisiana paddlefish develops, conservative harvest regulations should be promulgated until the effects of harvest on paddlefish stock dynamics can be determined.
Article
This study quantified the effects of temperature and fish mass on routine metabolism of the American paddlefish Polyodon spathula. Thermal sensitivity, as measured by Q10 value, was low in P. spathula. Mean Q10 was 1·78 while poikilotherms are generally expected to have Q10 values in the 2·00-2·50 range. Mass-specific metabolism did not decrease with increased fish size to the extent that this phenomenon is observed in teleosts, as evidenced by a mass exponent (β) value of 0·92 for P. spathula compared with 0·79 in a review of teleost species. Other Acipenseriformes have exhibited relatively high β values for mass-specific respiration. Overall P. spathula metabolism appears to be more dependent on body mass and less dependent on temperature than for many other fishes. An equation utilizing temperature and fish mass to estimate gross respiration for P. spathula was derived and this equation was applied to respiratory data from other Acipenseriformes to assess inter-species variation. Polyodon spathula respiration rates across water temperature and fish mass appear most similar to those of Atlantic sturgeon Acipenser naccarii and white sturgeon Acipenser transmontanus.
Article
Gut contents of shovelnose and pallid sturgeon from the lower and middle Mississippi River were obtained by colonic flushing, a safe and easily implemented alternative to gastric lavage. Diets of both species were dominated numerically by immature Trichoptera, Ephemeroptera, and Diptera. Primary prey, based on volume, for shovelnose sturgeon were Trichoptera, and for pallid sturgeon were various fishes. Geographic and seasonal nuances in diet were observed for both species, but the general dichotomy of shovelnose sturgeon as browser on invertebrates and pallid sturgeon as predator on fishes did not change. Data indicate that both species require hard substrates for feeding. Data demonstrate that colonic flushing is an effective technique for describing diet and inferring ecological and behavioral information about sturgeon.
Article
Microsatellite variation from 13 disomic loci is reported for a total of 208 individuals of the genus Scaphirhynchus. This includes 105 individuals of the pallid sturgeon (Scaphirhynchus albus) from the lower Mississippi River, 11 pallid sturgeon from the Upper Mississippi and Missouri rivers, 65 shovelnose sturgeon (S. platorynchus) from the lower Mississippi River, six Alabama sturgeon (S. suttkusi), and 21 individuals of sturgeon identified as intermediate between S. albus and S. platorynchus. Results indicate that all five of the above population/species units are significantly differentiated from one another based on pairwise FST estimates. Locus Spl-7 was diagnostic for the Alabama sturgeon and serves to further differentiate this allopatric species from other Scaphirhynchus. Classification of genotypes with and without a priori designations failed to clearly delineate the species and intermediates in the latter case but was successful for the species but not in the intermediates in the former case. The presence of six unique alleles in five of the 21 morphologically ‘intermediate’ sturgeon examined requires additional evaluation but suggests that these individuals are possibly not the result of hybridization. We hope that raising these important issues will bring all stakeholders to the table to establish a concerted effort needed for both morphological and molecular analyses to adequately address the question of hybridization and the origin of the morphological variation in these fishes.
Article
1.Dabry's sturgeon, a large, long-lived migratory fish is endemic to the Yangtze River. Over-fishing and habitat destruction have caused large-scale declines in natural stocks in the last two decades.2.Examining patterns of genetic diversity has become an integral component of many management plans for endangered species. DNA fingerprinting was applied to detect genetic diversity in Dabry's sturgeon collected in 1958–1959, 1980–1981 and 1998–1999.3.Studies on direct genetic parameters (genetic variability, hypervariable loci and heterozygosity) and indirect parameters (band-sharing coefficient and allelic frequency) showed that the continuous decline in wild populations has caused the loss of genetic diversity in present-day sturgeon.4.The present-day populations have the lowest genetic variability; thus, effective management is needed to preserve genetic diversity.5.A conservation strategy is urgently required, comprising artificial rearing facilities coupled with breeding management plans. Copyright © 2003 John Wiley & Sons, Ltd.
Article
SummaryA multiyear study of pallid sturgeon distribution and relative abundance was conducted in the lower and middle Mississippi river (LMR and MMR, respectively). The LMR and MMR comprise the free-flowing Mississippi River extending 1857 river kilometers (rkm) from its mouth at the Gulf of Mexico upstream to the mouth of the Missouri River. A total of 219 pallid sturgeon and 6018 shovelnose sturgeon was collected during the periods 1996–1997 and 2000–2006. Trotlines baited with worms were the primary collecting gear. The smallest pallid sturgeon captured on trotlines was 405 mm FL and the largest was 995 mm FL. Mean size of pallid sturgeon was statistically smaller in the Mississippi River below the Atchafalaya River near Baton Rouge, LA (621 mm FL). Mean abundance (catch per trotline night) of pallid sturgeon was highest at water temperatures around 10°C. There was a latitudinal trend in mean abundance of pallid and shovelnose sturgeon, but the pattern differed between species. Pallid sturgeon abundance was statistically (P < 0.05) higher (0.3 fish per trotline night) in the lower reach between the Atchafalaya River and New Orleans (rkm 154–507), and at the Chain of Rocks (COR), a low water dam near the mouth of the Missouri River. Pallid sturgeon abundance between these two locations was statistically the same (0.12–0.23). Shovelnose sturgeon abundance increased going upstream, but was disproportionally higher at the COR (22 fish per line compared with <6 fish per line in other reaches). Overall, the ratio between pallid and shovelnose sturgeon varied from a high of 1 : 6 at the lower reach, and gradually decreased upstream to a low of 1 : 77 at the COR. Based on differences in sturgeon abundance, size and habitat characteristics, the free-flowing Mississippi River can be divided into two reaches in the MMR (i.e. COR is a separate location), and four reaches (i.e., including the Atchafalaya River) in the LMR where management goals may differ.
Article
Trotlines were used to capture pallid sturgeon in the free-flowing Mississippi River, which extends from the Gulf of Mexico to the mouth of the Missouri River. Trotlines were baited with worms, and set overnight usually along the channel border. The pectoral fin rays of 165 pallid sturgeon caught in the Mississippi River were aged; 118 were from the lower Mississippi River (LMR) between the Gulf and mouth of the Ohio River, and 47 were from the middle Mississippi River (MMR) between the mouths of the Ohio and Missouri rivers. Initial agreement within ±1 year between two readers ranged from 53% for the LMR specimens, which were read first, to 84% for the MMR. Final age was agreed upon by both readers. For LMR pallid sturgeon, final age estimates ranged from 3 to 21 years with a mean (±SD) of 11.0 ± 4.7. For MMR pallid sturgeon, final age estimates ranged from 5 to 14 years with a mean of 9.5 ± 2.1. Seven pallid sturgeon marked with coded wire tags (CWT), indicating hatchery origin, were collected in the MMR. Age estimates for CWT fish were 7–8 years representing 1997 stocked fish, and 11–12 years representing 1992 progeny stocked in 1994. Von Bertalanffy growth equations for length indicated that pallid sturgeon in the MMR had higher growth rates for a given age than pallid sturgeon in the LMR. However, there were no significant differences (anova, P > 0.5) in the length–weight relationships between reaches. In the LMR, pallid sturgeon fully recruited to trotlines at age 11 and instantaneous total mortality (Z; slope of catch curve) was estimated at −0.12 (n = 10 year classes, r2 = 0.55, P = 0.01). Of the 118 sectioned rays from the LMR, 28 could not be reliably aged (only one section from the MMR could not be aged). Therefore, age was predicted from length using the von Bertalanffy equation. The catch curve was re-calculated using the predicted ages of the 28 pallid sturgeon in the LMR resulting in Z = −0.07. In the MMR, pallid sturgeon fully recruited to trotlines at age 9 and Z was estimated at −0.36 (n = 6 year classes, r2 = 0.67, P = 0.04), which was significantly higher (anova, P = 0.04) than the LMR estimate. Higher mortality in the MMR may be due to habitat limitations compared to a larger, more diverse channel in the LMR, and incidental take of larger, older individuals during commercial harvesting of shovelnose sturgeon. Commercial take of shovelnose does not occur in the LMR except in the northern portion of the reach. Considering the presence of pallid sturgeon with CWT, recruitment of older individuals in the MMR may have been influenced by stocking a decade earlier. Management strategies for this endangered species should consider the differences in mortality rates among reaches, the impacts of commercial fishing on recovery of pallid sturgeon in the MMR, and the long-term effects of hatchery fish now recruiting into the free-flowing Mississippi River.