American Paddlefish

It is speculated that the large size and extended life span of P. spathula is an adaptation to avoid predation. Producing high concentrations of paddlefish offspring in a single season may be a predator satiating mechanism used to maximize the chances of survival for a smaller, but significant proportion of P. spathula. When American paddlefish are in their larval stage they are easy prey for many different birds and fishes, but at maturity their only real predators are humans (Wills, 1993).

Known Predators:

• humans (Homo sapiens)
• fish (Actinopterygii)
• birds (Aves)

American paddlefish are clearly distinguishable from other North American fish by the presence of an extended snout, or rostrum. This rostrum is covered with electroreceptors to locate zooplankton and facilitate migratory behavior. American paddlefish are large, reaching maximum lengths of up to 2.5 meters and weighing from 18 to 70 kg. Males are generally larger than females (Wilkens, 2002). Large gill rakers are present in these fishes for zooplankton consumption from turbid waters (Russell, 2002). They also have a cartilaginous skeleton, heterocercal tail, and lack scales (Wills, 1993).

Range mass: 70 (high) kg.

Average mass: 18-45 kg.

Range length: 2.5 (high) m.

Other Physical Features: ectothermic ; heterothermic ; bilateral symmetry

Sexual Dimorphism: male larger

American paddlefish are relatively long-lived, they may live up to 55 years. The average lifespan as estimated in dentary studies seems to be about 20 to 30 years (Wills, 1993).

Range lifespan
Status: wild: 55 (high) years.

Typical lifespan
Status: wild: 20 to 30 years.

American paddlefish are mainly freshwater fish but can survive in brackish water (Billard, 2001). They generally reside in large rivers with deep water (greater than 6 meters) and slow moving currents (less than 5 cm/s) (Zigler, 2003; Southall, 1984). Turbid (muddy) water is preferred by P. spathula. During migratory breeding events, P. spathula needs access to areas with sand or gravel bars (Wills, 1993).

Habitat Regions: temperate ; freshwater

Aquatic Biomes: lakes and ponds; rivers and streams; brackish water

Polyodon spathula was first described by J. Walbaum in 1792 (Walbaum, 1792).

The large rostrum (paddle) of P. spathula is covered with electroreceptors (Russell, 2002). This paddle is essentially a highly sensitive antenna used to gather information about the surrounding environment through changing electrical fields. This electric sense is used by American paddlefish to locate prey and successfully migrate during spawning seasons (Wilkins, 2002). This system is so sensitive that juveniles can locate single zooplankton from up to 9 cm away by sensing the miniscule electrical pulses given off by the animal’s muscle contractions (Wilkins, 2002; Wills, 1993). It is unclear how P. spathula individuals communicate during spawning, but they may use a combination of visual and tactile cues.

Communication Channels: visual ; tactile

Perception Channels: visual ; tactile ; chemical ; electric

Polyodon spathula populations are most threatened by dams throughout the Mississippi River basin. These cause a separation of American paddlefish populations which limits gene flow and thus genetic variability. Dams also prevent the natural migratory spawning behavior of American paddlefish (Wills, 1993). Due to their highly valuable meat and roe, P. spathula has been over-harvested in the past. This has lead to more regulations on paddlefish harvesting, and many states now actively stock areas of river with American paddlefish (Graham, 1997). This species is presumed to be extirpated from the state of Michigan.

US Federal List: no special status

CITES: appendix ii

State of Michigan List: probably extirpated

IUCN Red List of Threatened Species: vulnerable

After fertilization, developing American paddlefish may be seen through their transparent roe, or eggs. In 24 hours the notochord develops, and a heartbeat is apparent by day four. By the fifth day, young P. spathula hatch and begin their larval stage of life. They are then carried somewhere downstream by slow moving river currents (Wills, 1993). Once gill rakers fully form, juvenile paddlefish can effectively filter feed (Mims, 2001). Female American paddlefish are not fully mature until they reach 12 to 14 years of age and males are mature at ages of 6 to 7 years (Wills, 1993).

Development - Life Cycle: metamorphosis ; indeterminate growth

There are no known adverse affects of P. spathula on humans.

Perhaps to the benefit of humans and the detriment of American paddlefish, these fish are highly valuable in many ways. Due to their large size and succulent flesh, P. spathula has long been utilized as a source of meat. Also, being closely related to sturgeons (family Acipenseridae), American paddlefish produce highly coveted roe, or caviar. In the 1980’s a trade embargo of Iranian imports severely limited the amount of caviar that was imported to the United States from the Caspian Sea. As the demand for caviar increased, American paddlefish suffered heavy population losses as the illegal acquisition of their roe became a highly profitable endeavor (Wills, 1993).

Beyond P. spathula being a source of caviar, their skin is thick, scaleless, strong, and tans well, making it a marketable product. There is also a growing interest in the immune system of American paddlefish. Despite their extremely simple immune system, they rarely, if ever, get cancer due to their cartilaginous skeleton. Cartilage prohibits blood flow to cancerous cells that is necessary for their growth, thereby stopping the spread of cancer. Therefore, American paddlefish have been a useful and promising test subject in the field of aquatic pharmacology (Wills, 1993).

Since American paddlefish are extremely beneficial to humans in many ways, they are currently being farm raised in the U.S. (Mims, 1999), and as far away as Russia, Romania, and Moldavia (Vedrasco, 2001). Most of the fish at these farms are produced for their valuable meat and caviar (Vedrasco, 2001).

Positive Impacts: food ; body parts are source of valuable material; research and education

American paddlefish are predators of zooplankton and prey to other fishes, birds, and humans. Also, silver lampreys have been found to use American paddlefish as hosts (Cochran, 2004). Beyond these relationships, relatively little is known of the role of American paddlefish in their ecosystem.

Polyodon spathula is a faunivore specialized for filter feeding. As described above, American paddlefish use electroreceptors to locate zooplankton in turbid water (Wilkins, 2002). Examples of animals in the P. spathula diet are copepods, cladocerans such Daphnia pulex, and ephemeropteran nymphs (Hoxmeier, 1997). Polyodon spathula possesses huge gill rakers along with jaws that are anatomically independent from the neurocranium (Carroll, 2003). These features allow for a larger surface area to be filtered by American paddlefish.

Animal Foods: insects; aquatic crustaceans; zooplankton

Foraging Behavior: filter-feeding

Primary Diet: planktivore

Polyodon spathula (American paddlefish) is currently found in 22 states that are part of the Mississippi River drainage (Mims, 2001). American paddlefish distribution is now restricted to this system of large, slow-moving rivers. In the past, P. spathula was located in 4 more states in this drainage, the Great Lakes, and Canada (Graham, 1997). The many dams that have been added throughout the Mississippi River drainage have limited the ranges of many populations of these fish (Wills, 1993). Due to these unnatural blockages, migratory breeding behavior has been disrupted, and many areas previously sustaining paddlefish must be annually stocked by local conservation agencies (Graham, 1997).

American paddlefish have also been introduced into the lower Danube River, in the Balkans region of Europe, as sportfish.

Biogeographic Regions: nearctic (Native ); palearctic (Introduced )

Paddlefish are broadcast spawners whereby multiple males swim near and release milt onto the eggs liberated by a female. The eggs are very sticky and thus adhere to a substrate such as gravel or sand (Wills, 1993). A spawning migration occurs within the Mississippi River system making paddlefish a potamodromous species. During the peak of the breeding season in the spring, large shoals of male and female fish form in specific breeding areas to spawn (Billard, 2001).

Mating System: polygynandrous (promiscuous)

Female American paddlefish are very particular about when they will release their eggs for reproduction. It has been noted that spawning generally occurs only every 2 to 3 years based upon certain environmental stimuli. At a specific spring photoperiod, there must be a rise in the water level of the river accompanied by a water temperature of approximately 55 to 60 degrees Fahrenheit (Wills, 1993).

Breeding interval: American paddlefish breed every 2 to 3 years.

Breeding season: American paddlefish spawn in the late winter and spring.

Average number of offspring: many thousands.

Range gestation period: 5 (high) days.

Range age at sexual or reproductive maturity (female): 10 to 12 years.

Range age at sexual or reproductive maturity (male): 6 to 7 years.

Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; fertilization (External ); broadcast (group) spawning; oviparous

Beyond the production of roe and milt, female and male American paddlefish provide nothing in the form of parental investment. The yolk sack of larval padlefish is consumed after hatching which may be considered a form of pre-fertilization provisioning by the female (Wills, 1993). These fishes are an excellent example of a species with a life history strategy designed to maximize success by producing many more eggs than will survive, rather than providing parental care.

Parental Investment: no parental involvement; pre-fertilization (Provisioning)

The paddlefish is one of the few freshwater fish to feed by straining plankton from the water, which it does by sweeping through the water with its lower jaw dropped and the sides of the head inflated (4), allowing it to filter feed and ventilate its gills simultaneously (7). The large snout is covered with electroreceptors used to gather information about the surrounding environment, including locating prey (7). The peak breeding season occurs in spring, during which large shoals of paddlefish migrate upstream and congregate in specific breeding areas to spawn. Spawning appears to require very specific environmental requirements and therefore generally only occurs every two to three years based on environmental stimuli (7). Single females can lay a huge number of eggs, from 300,000 to 600,000 (8), after which no parental care is invested and many will die (7). Males attain sexual maturity in around seven years, females in nine to ten (9). These relatively long-lived fish may live up to 55 years, although the average lifespan seems to be around 20 to 30 years (7).

The historical decline of paddlefish has led to greater regulations on paddlefish harvesting (7). The Mississippi Interstate Cooperative Resource Association (MICRA) was set up in 1991 to address the fishery management issues in the Mississippi River Basin, with several states actively participating in the five year MICRA paddlefish project by stocking rivers with this species (1). Indeed, over a million hatchery-reared juveniles have been tagged and released since 1994, helping to bolster current numbers and to provide valuable data (9). The results of tag monitoring and harvesting reports indicate that the total population of paddlefish exceeds 10,000 individuals and can sustain current harvesting levels (1).

The paddlefish has a highly distinctive appearance, so named for its peculiar, elongate, paddle-shaped snout, which may act as a sensory organ or help to channel plankton into the mouth (2) (4). The paddlefish's genus name, Polydon, derives from a Greek word meaning 'many tooth', referring to their hundreds of gill rakers, specially adapted to their method of filter-feeding plankton (5). This large fish is grey to blue-black, paler underneath, lacks scales, and has a deeply forked, shark-like tail (5) (6). Males are generally larger than females (7).

A freshwater fish (although capable of surviving in brackish water) that generally inhabits slow-flowing water of large rivers, usually at depths greater than 1.3 m (2) (7). Access to areas with sand or gravel bars is required during migratory breeding events (7).

Currently found in 22 U.S. states that are part of the Mississippi River basin (7), including the Missouri River into Montana, the Ohio River, and their major tributaries (2).

Classified as Vulnerable (VU A3de) on the IUCN Red List 2004 (1) and listed on Appendix II of CITES (3).

The paddlefish has suffered heavily from legal and illegal harvesting in the past, due to its valuable meat and eggs, which are sold as caviar. However, the species is now threatened more by habitat destruction and river modification as a result of dams throughout the Mississippi River basin (7). Dams have helped eliminate traditional spawning sites, interrupt natural spawning migrations (9), and separate paddlefish populations, which limits gene flow and therefore genetic variability (7). Additionally, agricultural development along much of the Missouri, Ohio, and Mississippi rivers and their tributaries has increased soil erosion, and the fertiliser and pesticides used run off into these rivers. These problems are combined with severe industrial pollution and municipal waste in a number of areas across the fish's range. Although much of the caviar and meat produced now comes from farmed paddlefish, commercial harvest was still permitted in six states as of 1997, and sport harvest in 14 states (9).

Snout prolonged, expanded into a thin flat blade, somewhat flexible. Spiracles present. Opercular skin produced behind into a long acute flap, nearly reaching to ventrals. No tongue. No barbels. Jaws and palatines with many fine deciduous teeth. Gill rakers long and in the hundreds. Gill rakers become fully functional when fish reach 225-250 mm eye-to-fork length (Rosen & Hales, 1981). Isthmus papillose in the young. Gill membranes considerably connected to one another, free from the isthmus. Ventrals near the middle of the body. Dorsal well behind them. Anal mostly behind the dorsal and somewhat larger. Caudal fin heterocercal, the lower caudal lobe is well developed, so that the fin is nearly homocercal. D: 50-60; A:50-65; V:45; caudal fulcra:13-20. Skin smooth, or nearly so, except rhombic plates on the sides of the bent portion of the tail. Colour pale olivaceous (mostly from Jordan & Evermann, 1896)

Maximum size: 221 cm TL; maximum weight: 91 kg; maximum reported age: 30 years.

The Mississippi paddlefish inhabits slow-flowing water of large rivers, usually in water deeper than 1.3 m, and with abundant zooplancton. Adult specimens typically swim through the water both day and night.Adult specimens typically swim with the nonprotrusible mouth open, strining zooplancton and aquatic insect larvae indiscriminately through the gill rakers. Adult paddle fish are indiscriminate filter feeder ingesting all material strained from the water colum by the gill rakers. Its stomachs contain the same organisms as the plankton but smaller forms (small cyclopoid, copepodits, nauplii, rotifers, etc. ) were almost entirely absent. Daphnia pulex, calanoids copepodits, Diaptomus siciloides was the most important food item by volume (Rosen & Hales, 1981). Juveniles, in which neither gill rakers nor the paddle are completely developed, pick individual zooplankters present, mainly insects (Rosen & Hales, 1981). The age at first maturity is 7 to 9 years for males and 10 to 12 years for females. The spawn only at 2- to 5-year intervals. Appropiate spawning areas are degradated by damming, which decreases water flow and leads to siltation.

Arkansas and Tennessee report the largest commercial fisheries: 136.000-181.000 Kg and Tennessee 60.328 kg, respectively, in 1992. All states bordering the Missouri River now prohibit commercial fishing for paddlefish because harvest were small and size of fish was decreasing (Graham, 1997).

Snout long, canoe-paddle-shaped; gill cover with large, fleshy, pointed flap (Ref. 86798).

Potamodromous. Migrating within streams, migratory in rivers, e.g. Saliminus, Moxostoma, Labeo. Migrations should be cyclical and predictable and cover more than 100 km.

Inhabits slow-flowing water of large rivers, usually in water deeper than 1.3 m. Feeds on plankton. The extended upper jaw (= paddle) may help to channel plankton into the mouth. Fish larger than 22.5 cm SL are indiscriminate tow-net filter feeders on plankton, while young paddlefish are selective plankton feeders (Ref. 46977).

Usually inhabits water deeper than 1.2 m in slow-flowing large rivers, lakes, and impoundments (Ref. 86798). Feeds on plankton. The extended upper jaw (= paddle) may help to channel plankton into the mouth. Threatened due to over harvesting, habitat loss and pollution (Ref. 58490).

fisheries: commercial; aquarium: public aquariums