Pallid sturgeon

Species of fish

The pallid sturgeon (Scaphirhynchus albus) is an endangered species of ray finned fish, endemic to the waters of the Missouri and lower Mississippi river basins of the United States. It may have even reached the St. Croix River before colonization. Named for its pale coloration, it is closely related to the relatively common shovelnose sturgeon (Scaphirhynchus platorynchus), but is much larger, averaging between in length and in weight at maturity. This species takes 15 years to mature and spawns infrequently, but can live up to a century. A member of the sturgeon family, Acipenseridae, which originated during the Cretaceous period 70 million years ago, the pallid sturgeon has changed little since then. In 1990, the U. S. Fish and Wildlife Service placed the pallid sturgeon on its endangered species list because few young individuals had been observed in the preceding decade and sightings had greatly diminished; the species is now rarely seen in the wild. It was the first fish species in the Missouri River drainage area to be listed as endangered, and a loss of its habitat is thought to be responsible for its decline. The vast majority of the Missouri River drainage system has been channeled and dammed, reducing the gravel deposits and slow moving side channels that are its favored spawning areas. Until the middle of the 20th century, pallid sturgeon were common and anglers found catching such a large fish in fresh water a rewarding experience. The species is considered to be good tasting, and its eggs have been used as caviar, although less commonly than those of many other sturgeon. Efforts to prevent the species from becoming extinct have had modest success. Pallid sturgeon are actively being raised in a dozen hatcheries and the offspring are being released back to the wild every year. To better understand pallid sturgeon behavior, researchers have implanted radio transmitters to track their movements and help identify possible spawning areas. Federal and state agencies are working together to improve habitat by restoring spawning areas since restoration of these areas is required if the species is to survive in the wild.

Taxonomists S. A. Forbes and R. E. Richardson classified the pallid sturgeon in 1905, grouping it in the genus Parascaphirhynchus and the family Acipenseridae, which includes all sturgeon worldwide. Its closest relatives are the shovelnose sturgeon (Scaphirhynchus platorynchus), which is still relatively common, and the critically endangered Alabama sturgeon (Scaphirhynchus suttkusi), which may soon become extinct. These three species belong to the subfamily Scaphirhynchinae, which has only one other genus, Pseudoscaphirhynchus, represented by three species found in west central Asia. The word pallid means "deficient in color", and compared to other species of sturgeon, the pallid is noticeably paler. The scientific name for the fish is derived from Scaphirhynchus, a Greek word meaning "spade snout" and albus which is Latin for "white".

To better protect the pallid sturgeon from extinction, research on its DNA and that of other closely related species was conducted to assess the differences within various populations of pallid sturgeon, and the differences between pallid and shovelnose sturgeon. Early DNA research indicated that pallid sturgeon and shovelnose sturgeon were a single species. However, a 2000 study comparing DNA sequences in the three members of the genus Scaphirhynchus(pallid, shovelnose, and Alabama sturgeon) showed that the three are distinct species. Between 2001 and 2006, several studies examined two populations of pallid sturgeon located in the upper Great Plains section of the Missouri River and compared them to a southern population located in the Atchafalaya River in Louisiana. These DNA studies concluded that the northern populations of pallid sturgeon are reproductively isolated and are genetically distinct from the Atchafalaya population. However, the genetic variability among pallid sturgeon was found to be far less than that between them and the shovelnose sturgeon. Another reason for DNA testing was to determine the rates of hybridization between pallid and shovelnose sturgeon. The southern populations have more hybrids than are found in the middle sections of the Missouri River basin, while the northernmost populations have had few reports of hybrids. Like the shovelnose sturgeon, their tails are heterocercal, with the top tail fin being longer than the bottom fin, though this is more pronounced in pallid sturgeon. As with other sturgeon, pallid sturgeon lack the scales or bones found in more "modern" species of fish. Instead, they have cartilaginous skeletons with five rows of thick cartilage plates that extend along their sides, undersides, and backs, as well as over most of the head. These thick cartilage plates are covered by the skin and serve as a protective armor. They lack bones and scales, which makes it more difficult to establish their age and determine exactly how long they live. As is true for many long lived species, pallid sturgeon reach reproductive maturity relatively late. Males reach sexual maturity between the ages of 5 and 7 years, while females are believed to become capable of reproduction when they are at least 15 years old. One study of nine females indicated that they begin egg development between the ages of 9 and 12 years, but do not reach reproductive maturity until they are 15 years old. Reproduction does not take place every year; the average interval between spawnings is three years, although other studies suggest an interval as long as 10 years. Spawning usually takes place May to July. One female pallid sturgeon caught in the upper Missouri River was estimated to be carrying 170,000 eggs, representing over 11 percent of its total body weight. After fertilization, pallid sturgeon eggs hatch in 5 to 8 days,

The pallid sturgeon's historical range spanned the entire Missouri River and into the Mississippi River. Historically, the species was rare to nonexistent in the upper Mississippi, probably due to a lack of proper habitat. Currently, the species is considered imperiled throughout its entire range. As of 2008, pallid sturgeon can still be found throughout their original range, but their population numbers have severely declined from the mid 20th century. The Missouri and Mississippi rivers from Montana to Louisiana, as well as the Atchafalaya River in Louisiana, continue to harbor an aging population of pallid sturgeon. Between 1985 and 2000, the ratio of pallid sturgeon to all sturgeon netted declined from one in about 400 to one in nearly 650. A 1996 study concluded that between 6,000 and 21,000 pallid sturgeon remained in their natural habitat at that time. Pallid sturgeon are believed to have preferred the muddy and generally warmer waters that existed prior to Missouri River dam construction. Fish is a more important dietary staple for pallid sturgeon than it is for shovelnose sturgeon. In one study comparing dietary tendencies between adult shovelnose sturgeon and immature pallid sturgeon, the pallid sturgeon was found to consume far greater numbers of small fish such as cyprinids (minnows).

Though never believed to be common, pallid sturgeon populations rapidly declined during the late 20th century and the species was listed as endangered on September 6, 1990. Pallid sturgeon were previously considered a prized trophy game fish species, until their numbers declined and they were placed on the endangered species list. All captured pallid sturgeon must now be released back to the wild. The species was known for being very palatable and the roe from females was used as caviar. These alterations of the river have had a detrimental impact on a number of native fish species. In the 13 U. S. states where the pallid sturgeon is found, only a few other fish species are listed as critically endangered. Although substantial efforts are being implemented to ensure the survival of this species, the rarity of self sustaining populations of pallid sturgeon ensures that it will remain federally protected for many decades.

Two populations of pallid sturgeon in the Missouri and Yellowstone rivers of Montana are both at risk of extinction, and current projections are that wild pallid sturgeon populations in Montana will be extinct by 2018. In Nebraska, a small number of pallid sturgeon have been captured along the lower reaches of the Platte River. Unlike most rivers in the Mississippi Missouri River System, the Platte River has only a few dams and they are well upstream from its confluence with the Missouri River. The lower Platte River is shallow with numerous sand bars and small islands. Though pallid sturgeon prefer more turbulent and deeper rivers than the Platte, between 1979 and 2003, over a dozen pallid sturgeon, including some from hatcheries, have been captured from the Platte River. A number of these pallid sturgeon have been fitted with radio transmitters which track their return to the Platte River when water levels and turbidity conditions are favorable. Coinciding with the majority of the pallid sturgeon that have been captured, the period that is generally most favorable is during the spring and early summer. By midsummer, a reduction in water levels and turbidity on the Platte River encourages pallid sturgeon to return to the Missouri River. The lower reaches of the Platte River, a more than 30 mile (48 km) stretch from the Elkhorn River to its confluence with the Missouri River, has suitable spawning habitat for pallid sturgeon, although no conclusive evidence has been found that spawning is occurring in this region. The side channel was apparently being used by the larva pallid sturgeon for protection from the swifter currents of the Missouri River. In 2007, the USFWS concluded that hatchery based reproduction efforts should be continued, along with monitoring of any population changes, to determine the effectiveness of human intervention. The 2007 findings also emphasized the need to determine the most likely areas of spawning, to identify any parasite or disease that may be impacting the reproductive capabilities of pallid sturgeon, and to examine engineering possibilities that may permit recreation of suitable habitats without reducing the USFWS's ability to protect people from harmful and destructive flooding, and to maintain its ability to provide adequate water impoundment for irrigation and recreation purposes.