American paddlefish

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American Paddlefish
Paddlefish USFWS.jpg
Conservation status
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Acipenseriformes
Family: Polyodontidae
Genus: Polyodon
Lacépède, 1797
Species: P. spathula
Binomial name
Polyodon spathula
(Walbaum in Artedi, 1792)

American paddlefish (Polyodon spathula) are basal chondrostean ray-finned fish closely related to sturgeons in the order Acipenseriformes. They are freshwater fish, commonly called "paddlefish", but are also referred to as "Mississippi paddlefish", "spoon-billed cats", or "spoonbill". American paddlefish are one of only two extant taxa in the paddlefish family, Polyodontidae. The other is the critically endangered Chinese paddlefish, Psephurus gladius, endemic to the Yangtze River Basin in China where there is growing concern the species may now be extinct. American paddlefish are highly derived with adaptations specifically for filter feeding. Their rostrum and cranium are covered with tens of thousands of sensory receptors for locating swarms of zooplankton which is their primary food source.

American paddlefish are endemic to the Mississippi River Basin, and once moved freely under the relatively natural, unaltered conditions that existed prior to the early 1900s. They commonly inhabited large, free-flowing rivers, braided channels, backwaters, and oxbow lakes throughout the Mississippi River drainage basin, adjacent Gulf drainages, the Great Lakes and rivers in Ontario, Canada. Naturally occurring populations have been extirpated from much of their peripheral range in the Great Lakes and Canada, New York, Maryland, Virginia, and Pennsylvania. The current range of American paddlefish has been reduced to the Mississippi and Missouri river tributaries, and Mobile Bay drainage basin. They are currently found in only twenty-two states in the U.S., and those populations are protected under state and federal laws. There are only thirteen states that allow commercial or sport fishing.

American paddlefish populations have declined dramatically primarily because of overfishing, habitat destruction, and pollution. Poaching has also been a contributing factor to their decline, and will continue to be as long as the demand for caviar remains strong. The roe of American paddlefish can be processed into caviar that is similar in taste and texture to the highly prized Sevruga sturgeon caviar from the Caspian Sea. Therefore, in an effort to protect vulnerable populations of American paddlefish in the wild, various state, federal, and international regulations were enacted to restrict harvest, and prohibit the trafficking of American paddlefish roe. Violations can result in substantial monetary fines, and imprisonment.

Taxonomy, etymology and evolution[edit]

An American paddlefish in a large aquarium tank

American paddlefish are basal chondrostean ray-finned fish closely related to sturgeons in the order Acipenseriformes.[1] Lacepéde, 1797, established the genus Polyodon which today includes a single extant species, Polyodon spathula. A second, extinct species, P. tuberculata, fossils of which are found in the Lower Paleoscene Tullock Formation in Montana, approximately 60 million years ago.[2] When establishing the genus, Lacepéde dismissed speculation by some contemporary taxonomists who suggested paddlefish may be a unique genus of sharks because of some morphological similarities such as a heterocercal tail, and cartilaginous skeleton.[3]

The critically endangered, possibly extinct Chinese paddlefish, Psephurus gladius, is the closest extant relative of American paddlefish. It is endemic to the Yangtze River Basin in China, and unlike the planktivorous American paddlefish, they are strong swimmers, grow larger, and are opportunistic piscivores that feed on small fishes and crustaceans. Some distinct morphological differences of Psephurus gladius include a narrower, sword-like rostrum, and a protrusible mouth. The Chinese paddlefish also has fewer, thicker gill rakers unlike those of Polyodon spathula which are composed of extensive comb-like filaments, believed to have inspired the etymology of the genus name Polyodon, a Greek compound word meaning "many toothed." Adult American paddlefish are toothless, although numerous small teeth less than 1 mm (0.039 in) were found in a 630 mm (25 in) paddlefish; spathula references the elongated, paddle shaped snout, or rostrum.[4][5]

American paddlefish are the only living species in the genus Polyodon. They are often referred to as primitive fish because of morphological characters they retained from their earliest ancestors whose fossil record dates from the Late Cretaceous, seventy to seventy-five million years ago.[6][7] Some of their primitive characteristics include a skeleton composed primarily of cartilage, and a deeply forked heterocercal caudal fin similar to that of sharks, although they are not of the same taxon.[8] Fossil paddlefish with recognizable rostrums date from the Upper Cretaceous and Paleocene periods, 65 million years ago.[6] An elongated rostrum is a morphological character of the Polyodontidae, but only the genus Polyodon (P. spathula and the extinct P. tuberculata) have characteristics adapted specifically for filter feeding including the jaw, gill arches, and cranium. Compared to the Chinese paddlefish, and fossil genera, the American paddlefish (and by extension, its fossil relative P. tuberculata) is a highly derived animal that is adapted exclusively for planktivory.[9]


Paddlefish ram suspension-feeding zooplankton in aquarium

American paddlefish are among the largest and longest lived freshwater fishes in North America.[10] They have a shark-like body, average 1.5 metres (4.9 ft) in length, weigh 27 kilograms (60 lb), and can live in excess of thirty years.[11] However, the median age for most populations is five to eight years, and maximum age is fourteen to eighteen years.[10]

American paddlefish are smooth skinned, and almost entirely cartilaginous. Their eyes are small and directed laterally. They have a large, tapering operculum flap, a large mouth, and a flat, paddle-shaped rostrum that measures approximately one-third its body length. During the initial stages of development from embryo to fry, American paddlefish have no rostrum. It begins to form shortly after hatching.[12][13] The rostrum is an extension of the cranium, not of the upper and lower jaws, or olfactory system as with the long snouts of other fishes.[9][10] Other distinguishing characteristics include a deeply forked heterocercal caudal fin, and dull coloration, often with mottling, and ranging from bluish-gray to black dorsally grading to white ventrally.[5]

Feeding ecology and physiology[edit]

The debate regarding the function of the American paddlefish's rostrum began when the species was described in the late 1700s.[6] Scientists once believed it was used to excavate bottom substrate, or functioned as a balancing mechanism and navigational aid.[12] However, laboratory experiments dating from 1993 which utilized advanced technology in the field of electron microscopy have established conclusively that the rostrum of American paddlefish is covered with tens of thousands of sensory receptors, morphologically similar to the ampullae of Lorenzini of sharks and rays, and that they are indeed passive ampullary-type electroreceptors used by American paddlefish to detect plankton.[6] Clusters of electroreceptors also cover the head and operculum flaps. The electroreceptors can detect weak electrical fields which not only signal the presence of zooplankton, the primary diet of American paddlefish, but they can also detect the individual feeding and swimming movements of zooplankton appendages.[12] When a swarm of zooplankton is detected, American paddlefish will swim forward continuously with their mouths wide open forcing water over their gill rakers to filter out prey. Such feeding behavior is considered ram suspension-feeding. Research further indicates their electroreceptors may also serve as a navigational aid to mediate obstacle avoidance.[6][12]

American paddlefish have small undeveloped eyes that are directed laterally. Unlike most fishes, American paddlefish hardly respond to overhead shadows, or changes in illumination. Electroreception appears to have largely replaced vision as a primary sensory modality, which indicates a reliance on electroreceptors for detecting prey.[6][12] However, the rostrum is not their only means of food detection. Some reports suggest a damaged rostrum would render American paddlefish less capable of foraging efficiently to maintain good health, but laboratory experiments, and field research indicate otherwise.[9][12] In addition to electroreceptors on the rostrum, American paddlefish also have sensory pores covering nearly half of the skin surface extending from the rostrum to the top of the head down to the tips of the operculum flaps. Studies have indicated that American paddlefish with damaged, or abbreviated rostrums are still able to forage and maintain good health.[9][12]

Reproduction and life cycle[edit]

Development of paddlefish from embryo to larval stage
Stages of rostrum development

American paddlefish are long-lived, sexually late maturing, pelagic fish. Females do not begin spawning until they are seven to ten years old, some as late as sixteen to eighteen years old. Females do not spawn every year, rather they spawn every second or third year. Males spawn more frequently, usually every year or every other year beginning around age seven, some as late as nine or ten years of age.[12][14]

American paddlefish begin their upstream spawning migration sometime during early spring; some begin in late fall.[14] They spawn on silt-free gravel bars that would otherwise be exposed to air, or covered by very shallow water were it not for the rises in the river from snow melt and annual spring rains that cause flooding.[15] Although availability of preferred spawning habitat is essential, there are three precise environmental events that must occur before American paddlefish will spawn.[12][16] The water temperature must be from 55 to 60 °F (13 to 16 °C); the lengthened photoperiod which occurs in spring triggers biological and behavioral processes that are dependent on increasing day length; and there must be a proper rise and flow in the river before a successful spawn can occur. Historically, American paddlefish did not spawn every year because the precise environmental events only occurred once every 4 or 5 years.[16]

American paddlefish are broadcast spawners, also referred to as mass spawners or synchronous spawners. Gravid females release their eggs into the water over bare rocks or gravel at the same time males release their sperm. Fertilization occurs externally. The eggs become sticky after they are released into the water, and will attach to the bottom substrate. Incubation varies depending on water temperature, but in 60 °F (16 °C) water, the eggs will hatch into larval fish in about seven days.[14] After hatching, the larval fish drift downstream into areas of low flow velocity where they forage on zooplankton.[17]

Young American paddlefish are poor swimmers, and susceptible to predation, therefore rapid first-year growth is important to their survival.[14] Fry can grow about 1 in (2.5 cm) per week,[18] and by late July the fingerlings are around 5–6 in (13–15 cm) long.[14] Their rate of growth is variable, and highly dependent on food abundance. Higher growth rates occur in areas where food is not limited. The feeding behavior of fingerlings is quite different from that of older juveniles and adults. They capture individual plankton one by one which requires detection and location of individual Daphnia on approach, followed by an intercept maneuver to capture the selected prey.[6] By late September, fingerlings have developed into juveniles, and are around 10–12 in (25–30 cm) long. After the 1st year, their growth rate slows, and is highly variable. Studies indicate that by age 5, their growth rate averages around 2 in (5.1 cm) per year depending on the abundance of food, and other environmental influences.[19]

American paddlefish can live to be 60 years or older. Age is best determined by dentary studies, most of which occur on fish harvested during snagging season. The dentary is removed from the lower jawbone, cleaned of any remaining soft tissue, and cross-sectioned to expose the annual rings. The dentary rings are counted in much the same way a tree is aged. Females typically live longer and grow larger than males.[20]

Habitat and distribution[edit]

Map of the United States showing distribution of paddlefish

American paddlefish are highly mobile, and well adapted to living in rivers.[8] They inhabit many types of riverine habitats throughout much of the Mississippi Valley and adjacent Gulf slope drainages. They occur most frequently in deeper, low current areas such as side channels, oxbows, backwater lakes, bayous, and tailwaters below dams. They have been observed to move more than 3,218.68 km (2,000.00 mi) in a river system.[8]

American paddlefish are endemic to the Mississippi River basin, historically occurring from the Missouri and Yellowstone rivers in the northwest to the Ohio and Allegheny rivers of the Northeast; the headwaters of the Mississippi River south to its mouth, from the San Jacinto River in the southwest to the Tombigbee and Alabamarivers of the Southeast.[10] They were extirpated from much of their peripheral range in the Great Lakes region and Canada, New York, Maryland, and Pennsylvania.[16] However, in 1991 Pennsylvania implemented a reintroduction program utilizing hatchery-reared American paddlefish in an effort to establish self-sustaining populations in the upper Ohio and lower Allegheny rivers. In 1998, New York initiated a stocking program upstream in the Allegheny Reservoir above Kinzua Dam, and a second stocking in 2006 in Conewango Creek, a relatively unaltered section of their historic range. Reports of free ranging adults captured by gill nets have since been documented in Pennsylvania and New York, but there is no evidence of natural reproduction.[21][22] They are currently found in only twenty-two states in the U.S., and are protected under state and federal laws. There are only thirteen states that allow commercial or sport fishing for American paddlefish.[12]

Human interaction[edit]

Propagation and culture[edit]

C-section surgery to extract roe, fertilize, and incubate. Blind Pony Hatchery in Missouri, 1995

The growing importance of American paddlefish for its meat and roe became the catalyst for further development of culture techniques for aquaculture in the U.S. rather than restoration.[23] Artificial propagation requires broodstock which, because of the late sexual maturation of American paddlefish, are initially obtained from the wild, and brought into a hatchery environment.[24] The fish are injected with LH-RHhormone to stimulate spawning. The number of eggs a female may produce depends on the size of the fish and can range anywhere from 70,000-300,000 eggs. Unlike most teleosts, the oviduct branches of American paddlefish and sturgeons are not directly attached to the ovaries, rather they open dorsally into the body cavity. Ova staging is performed to determine status of progression toward maturation, and requires a minor procedure that involves a small abdominal incision from which to extract a few sample oocytes. The oocytes are boiled for a few minutes until the yolk is hardened, and then cut in half to expose the nucleus for microscopic examination to determine stage of maturity.[23]

Once maturation is confirmed, one of three procedures is used to extract the eggs from a female paddlefish. The three procedures are (1) the traditional hand-stripping method, considered to be time consuming and laborious; (2) Caesarean section, a relatively quick surgical method of extracting eggs through a 4 in (10 cm) abdominal incision; considered faster than hand stripping, however suturing can be time consuming, and the incision may result in muscular stress and poor suture retention which lowers survival rate; and (3) MIST, (minimally invasive surgical technique) which is the fastest of the three procedures because it requires less handling of the fish, and eliminates the need for suturing. A small internal incision is made in the dorsal area of the oviduct which allows direct stripping of eggs from the body cavity through the gonopore, bypassing the oviductal funnels.[25][26]

Spermiating males can produce large volumes of milt over the course of three to four days. Milt is collected by inserting a short plastic tube with syringe attached into the urogenital opening of the male, and applying light suction with the syringe to draw the milt. The collected milt is diluted in water just prior to adding it to the eggs, and the combination is gently stirred for about a minute to achieve fertilization. Fertilized eggs are adhesive and demersal, therefore if incubation is to take place in a flow-through hatching jar, the eggs must be treated to prevent clumping. Incubation usually takes anywhere from five to twelve days.[23]

Global commercial market[edit]

Tin of freshly processed paddlefish caviar

Advancements in biotechnology have created a global commercial market for the polyculture of American paddlefish. In 1970, American paddlefish were stocked in several rivers in Europe and Asia. Introduction began when five-thousand hatched larvae from Missouri hatcheries in the United States were exported to the former USSR for aquacultural utilization.[27] Reproduction was successful in 1988 and 1989, and resulted in the exportation of juveniles to Romania and Hungary. American paddlefish are now being raised in Germany, Austria, the Czech Republic, and the Plovdiv and Vidin regions in Bulgaria. In May 2006, specimens of different sizes and weights were caught by professional fisherman near Prahovo in the Serbian part of the Danube River.[28]

In 1988, fertilized American paddlefish eggs and larvae from Missouri hatcheries were first introduced into China.[28] Since that time, China imports approximately 4.5 million fertilized eggs and larvae every year from hatcheries in Russia, and the United States. Some American paddlefish are polycultured in carp ponds, and sold to restaurants while others are cultured for brood stock and caviar production. China has also exported American paddlefish to Cuba where they are farmed for caviar production.[29]

Sport fishing[edit]

American paddlefish are a popular sport fish where their populations are sufficient to allow such activity. Areas where there are no self-sustaining populations rely on state and federal restocking programs to maintain a viable fishery. A 2009 report includes the following states as allowing American paddlefish sport fishing per their respective state and federal regulations: Arkansas, Illinois, Indiana, Iowa, Kansas, Kentucky, Mississippi, Missouri, Montana, Nebraska, North Dakota, Nebraska, Oklahoma, Montana, South Dakota, and Tennessee.[21] Since American paddlefish are filter-feeders, they will not take bait, or lures, and must be caught by snagging. The official state record in Kansas is a 144 lb (65 kg) American paddlefish snagged in 2004; Montana, a 142.5 lb (64.6 kg) American paddlefish snagged in 1973; and in North Dakota, a 130 lb (59 kg) American paddlefish snagged in 2010.[14] The largest American paddlefish on record was captured in Lake Okoboji, Iowa in 1916 by a spear fisherman, and measured 85 in (2.2 m) long, and weighed an estimated 198 lb (90 kg).[14]

Population declines[edit]

Angler landing a large paddlefish

Overfishing and habitat destruction[edit]

American paddlefish populations have declined dramatically primarily as a result of overfishing and habitat destruction. In 2004 they were listed as Vulnerable (VU A3de ver 3.1) on the IUCN Red List of Threatened Species, and are now proposed for listing as VU 3de throughout their range as the result of a U.S. Fish & Wildlife Service assessment that suggests "an overall population size reduction of at least 30% may occur within the next 10 years or three generations due to actual or potential levels of exploitation and the effects of introduced taxa, pollutants, competitors or parasites."[30] American paddlefish are filter feeding pelagic fish that require large, free-flowing rivers with braided channels, backwater areas, oxbow lakes that are rich in zooplankton, and gravel bars for spawning.[15] Series of dams on rivers such as those constructed on the Missouri River have impounded large populations of American paddlefish, and blocked their upstream migration to spawning shoals.[15] Channelization and groynes, or wing dykes have caused the narrowing of rivers and altered flow, destroying crucial spawning and nursery habitat.[10][16][21] As a result, most impounded populations are not self-sustaining, and must be stocked in order to maintain a viable sport fishery.[15]

Federal and state resource agencies utilize artificial propagation techniques to mitigate areas where self-sustaining populations no longer exist, including impoundments created by dams that block upstream migration to spawning shoals, and where environmental conditions fail to meet their precise spawning requirements. The artificial propagation of American paddlefish began with the efforts of the Missouri Department of Conservation during the early 1960s, and was oriented primarily on the maintenance of the sport fishery.[23]

Zebra mussels[edit]

Zebra mussel infestations in the Mississippi River, Great Lakes, and other Midwest rivers are also negatively impacting American paddlefish populations. Zebra mussels are an invasive species that are well adapted for explosive population growth. As filter feeders, zebra mussels rely on plankton, and can filter significant amounts of phytoplankton and zooplankton in the water, altering the availability of an important food source for paddlefish, and native unionidae.[16][31]


Poaching is also a contributing factor to declining populations of American paddlefish in the states where they are commercially exploited, particularly while the demand for caviar remains strong.[21] Since the 1980s, a trade embargo on Iran restricted imports of the highly sought after, and most expensive Beluga caviar from the Caspian Sea, limiting U.S. sources of caviar. The most sought after caviar is produced by sturgeons in the Northern Caspian Sea, but overfishing and poaching have exhausted the supply. American sturgeon and paddlefish populations were targeted as likely substitutes.[12]

The roe of American paddlefish can be processed into caviar similar in taste and texture to Sevruga sturgeon caviar.[16] It has been described by caviar gourmets as a good substitute for Beluga, "with its clear, glossy beads, buttery flavor, and steel gray to light or even golden-gray roe."[32] Several cases of mislabeled American paddlefish roe sold as Caspian Sea caviar have been prosecuted by the U.S. Fish and Wildlife Service (USFWS).[12]

State and federal regulations restricting the harvest of American paddlefish populations in the wild, and the illegal trafficking of their roe are strictly enforced. Related violations such as the illegal transport of American paddlefish roe have resulted in convictions with substantial fines, and prison sentences.[33][34] Paddlefish are also protected under CITES, (Convention on the International Trade in Endangered Species of Fauna and Flora).[35]


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