Paddlefish

Paddlefish is the common name for the ray-finned fish comprising the family Polyodontidae of the Order Acipenseriformes, characterized by an elongated, paddle-like snout with minute barbels, large mouth with minute teeth, and long gill rakers. The other extant family in the Acipenseriformes order is that of the sturgeons, Acipenseridae, but the sturgeons lack teeth in adults, have fewer than 50 gill rakers, and have a flattened rostra, among other differences. In some areas in the United States, paddlefish are referred to as "spoonbills," "spoonbill catfish," or "spoonies."

There are only two modern species of these fish: the plankton-feeding American paddlefish (Polyodon spathula), found in the Mississippi River drainage system, and the piscivorous Chinese paddlefish (Psephurus gladius), found in the Yangtze River; however, there are concerns that the Chinese species may now be extinct.

Paddlefish provide important economic, ecological, and aesthetic values. During the last century, paddlefish and sturgeon have been commercially exploited for their eggs (roe), called caviar. Paddlefish and sturgeon are two of the most important fish for freshwater caviar. The large number of eggs produced by females also mean they can be important in food chains, with the young providing a food resource for predators. Their unique shape and large size also adds to the joy of nature for humans, including sports fishing. However, exploitation and habitat changes, among other factors, have significantly reduced the populations of paddlefish, with the Chinese paddlefish not having a confirmed sighting since 2007 and the American paddlefish no longer found in the Great Lakes region.

Overview and description

American Paddlefish, Polyodon spathula

The Order Acipenseriformes includes two extant families: the paddlefishes (family Polyodontidae) and the sturgeons (family Acipenseridae). Members of this order are characterized by a largely cartilaginous structure, an elongated body, an intestine with spiral valve, a heterocercal caudal fin, the absence of gulars, lack of vertebral central, and fin rays more numerous than their basals (Nelson 2006).

The paddlefish family, Polyodontidae, is characterized by its members having an elongated, spatula-like snout, called a rostrum, which is longer than the rest of the head. It takes it common name, and the American paddlefish (Polyodon spathula) takes its scientific name from this distinctive paddle-shaped snout. Other characteristics include minute barbels on the snout; long gill rakers, which can reach the hundreds in Polyodon; minute teeth; and a body lacking large scutes, but having small "scales" in some regions (Nelson 2006). The piscivorous Chinese paddlefish, Psephurus gladius, is characterized by a protrusible mouth, while the plankton-feeding American paddlefish (Polyodon spathula) has a nonprotrusible mouth. Members of the sturgeon family, Acipenseridae, differ from the paddlefish in that the acipenserids lack teeth in adults, have five rows of large bony scutes or plates on the body, have fewer than 50 gill rakers; and have a flattened rostra (Nelson 2006).

Paddlefish are not closely related to sharks, which are in a different taxonomic class, but they do have some body parts that resemble those of sharks such as their skeletons, primarily composed of cartilage, and their deeply forked heterocercal tail fins. As in many of the distantly related shark class, the paddlefish's rostrum contains electroreceptors that can detect weak electrical fields.

Classification

There are two currently or recently extant genera in this family and four (if not five) extinct genera: Polyodontidae

Subfamily †Paleopsephurinae
- Genus †Paleopsephurus MacAlpin, 1947
  - Species †Paleopsephurus wilsoni MacAlpin, 1947
    
    Protopsephurus liui fossils
Subfamily Polyodontinae
- Genus †Crossopholis Cope, 1883
  - Species †Crossopholis magnicaudatus Cope, 1883
- Genus Polyodon Lacépède, 1797
  - Polyodon spathula Walbaum, 1792 American paddlefish
  - †Polyodon tuberculata Grande & Bemis, 1991
- Genus Psephurus Günther, 1873
  - Psephurus gladius E. von Martens, 1862 Chinese paddlefish (Not recently verified extant, and perhaps now extinct)
Subfamily †Protopsephurinae Grande & Bemis, 1996
- Genus †Protopsephurus Lu, 1994
  - Species †Protopsephurus liui Lu, 1994

Fossils of other kinds of paddlefish have been found. One such species is Crossopholis magnicaudatus, from the Eocene-age Green River Shale deposit in Wyoming.

American paddlefish

American paddlefish, Polyodon spathula

The American paddlefish, Polyodon spathula, also known as the Mississippi paddlefish, lives in slow-flowing waters of the Mississippi River drainage system. This includes slow-flowing waters of the Mississippi River itself, as well as various tributaries, including the Missouri River, Ohio River, Yellowstone River, Wisconsin River, Des Moines River, and Arkansas River systems. These fish were also found historically in Lake Erie, in the Great Lakes, but appears to have become extinct in that area. In May 2000, the Canadian Species at Risk Act listed the paddlefish as being extirpated in Canada.

The American paddlefish is one of the largest freshwater fish in North America. They commonly reach 5 feet (1.5 meters) or more in length and can weigh more than 60 pounds (27 kilograms). The largest American paddlefish on record, weighing 144 pounds (65 kg), was caught by Clinton Boldridge in the Atchison Watershed in Kansas.

Paddlefish take many years before they are able to spawn. A female may take eight to twelve years, and males spawn when about seven years old (range from about 5-9 years old) and they are about 40 inches inches long when they start to spawn. The female releases adhesive eggs randomly over the water bottom and abandons them. They are capable of producing over a half million eggs a year, but they may not spawn every year.

The American paddlefish is believed to use sensitive electroreceptors on its paddle to detect prey, as well as to navigate while migrating to spawning sites. The American paddlefish feeds primarily on zooplankton but also feeds on crustaceans and bivalves.

Population decline

Polyodon spathula

Paddlefish were at one time very abundant in most central U.S. river systems, but populations have declined greatly due to overharvesting, sedimentation, river modification, and other factors.

Paddlefish need free-flowing rivers that have shallow pools with sandy, rocky bottoms for their spawning, and modification of rivers by the construction of dams and dredging, as well as water removal for agricultural use, has reduced paddlefish spawning grounds. Dams and other barriers block paddlefish migration routes that are very important to the fish for spawning and can prevent the fish from recolonizing places where they once occurred. Notably, fish ladders that could allow paddlefish to navigate around dams are avoided because of the metal rebar used in construction, which disrupts their electro-magnetic sense organs.

Free-flowing lakes with reservoirs can also provide paddlefish breeding habitat. One such area is the Missouri River-Lake Sakakawea system in North Dakota. This area is capable of producing good paddlefish numbers because it is a free-flowing system with many good areas for paddlefish to spawn.

Paddlefish are protected by law over a large part of their range. In some regions, sports fishing is allowed, However, even in protected areas, paddlefish are sometimes argeted by poachers for their valuable eggs. Paddlefish are a protected species in Wisconsin, where they occur in the Wisconsin River south of the Prairie du Sac hydroelectric dam and in the lower Saint Croix River in Pierce County.

There are efforts to reintroduce paddlefish. Pennsylvania Fish and Boat Commissioners are reintroducing the species to historical habitats in the Ohio and Allegheny rivers in an effort to establish a secure breeding population once again. Reintroduction efforts may take many years, since paddlefish mature slowly, lengthening the time required to establish a breeding population.

Until about 1900, the species was also found in Lake Erie and in associated river systems in the U.S. and Canada. Invasive species such as zebra mussels have reduced the number of zooplankton in the Great Lakes to such low levels that any hypothetical reintroduction program would seem likely to fail.

Recently, American paddlefish were spotted in the Danube river. It has not been determined whether these fished escaped from Romanian or Bulgarian fish farms during the 2006 European floods, or whether they were let into the Danube earlier and matured in the river.

Sport fishing

The American paddlefish remains a popular sport fish in those parts of its range where populations are sufficient to allow harvesting. Because paddlefish are filter feeders, they do not take conventional lures. Taking paddlefish is done with a bow and arrow, a spear, or by snagging (deliberately foul-hooking the fish in the fins or tail). Snagging is the usual method.

In most of its range the paddlefish is a protected species, and fishing for paddlefish is illegal in many areas. Any paddlefish caught accidentally should be released unharmed as quickly as possible. However, a few states still allow sport fishing for paddlefish. Several states, including Missouri, have enacted stocking programs for these fish in reservoirs where the resident populations were low or nonexistent, or in areas where historical populations are no longer naturally sustainable. Oklahoma has drastically reduced sportfish harvest of paddlefish to one per person per day to help sustain populations.

Poachers also use these methods to target paddlefish in areas where paddlefish fishing is not legal. Suspect paddlefish fishing activity can be reported to fish and wildlife officers who will verify legality. Some jurisdictions pay a financial reward to citizens whose report leads to prosecution of a poacher. One example is the Iowa DNR, and their Turn In Poachers (TIP) Program which was started in August 1985.

Chinese paddlefish

The Chinese paddlefish, Psephurus gladius (simplified Chinese: 白鲟; traditional Chinese: 白鱘), also known as Chinese swordfish and "elephant fish" (because its snout resembles an elephant trunk), is listed as one of two extant paddlefish species, although there are concerns it may be extinct. In modern days, they were found only in the Yangtze River Basin in China, mostly in the middle or lower part of the Yangtze (Chang Jiang), occasionally in large lakes.

The Chinese paddlefish has a white belly and its back and head is gray. They feed on other fish, as well as small amounts of crabs and crayfish. It is suspected of being anadromous, spending time in marine waters but spawning in the Yangtze River; however, it is so rare that little is actually known about its habits (Bourton 2009).

The Chinese paddlefish is a very large fish. Three-meter (9-feet) specimens weighing 300 kilograms (660 pounds) have been recorded. It is said that the zoologist Bǐng Zhì (秉志) recorded around the 1950s that some fishermen caught a paddlefish of 7 meters (23 feet), although the authenticity of the story is unconfirmed. It is said that the Chinese paddlefish can grow to a weight of 500 kilograms (1,100 pounds), but little research on a maximum size can be conducted today due to the species' scarcity. Paddlefish are sexually mature at age seven or eight, when they have reached a body length of about 2 meters (6 feet) and a weight of about 25 kilograms (55 pounds).

Overfishing and habitat change are key factors in the decline of paddlefish populations. In 1983, the People's Republic of China official recognized the paddlefish as critical endangered in order to prevent fishing of paddlefish young or adults. Paddlefish are also threatened by dams (such as the Three Gorges Dam), which divide the population into isolated groups.

The fish are rarely seen, recently raising concerns that the species might already be extinct. In 2009, Revkin reported that the Chinese paddlefish "has not been seen alive for six years, despite thorough surveys." During a three-year search conducted from 2006–2008, a research team from the Chinese Academy of Fisheries Science in Jingzhou failed to find even one specimen (Bourton 2009). There was a confirmed sighting of the fish alive on January 24, 2003 on the Yangtze (Bourton 2009). On January 8, 2007, a 3.6-meter, 250 kilogram specimen was captured by illegal fishing in Jiayu County (Gao et al. 2009). Local villagers contacted officials, who rushed to the site. Zeb Hogan of Monster Fish on National Geographic Channel and other conservationists transferred the fish to a holding pen in hope that it would survive. But shortly afterwards it died due to unrecoverable injuries sustained while thrashing in the net. This was the last known observation.

Notes

↑ R. Froese and D. Pauly, eds., Family Polyodontidae: Paddlefishes , Fishbase (2009). Retrieved August 23, 2012.

References
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Barry, P. M., R. F. Carline, D. G. Argent, and W. G. Kimmel. 2007. Movement and habitat use of stocked juvenile paddlefish in the Ohio River System, Pennsylvania. North American Journal of Fisheries Management 27: 1316-1325.

Bettoli, P. W., G. D. Scholten, and W. C. Reeves. 2007. Protecting paddlefish from overfishing: A case history of the research and regulatory process. Fisheries, American Fisheries Society 32: 390-397.

Boone, E. A., and T. J. Timmons. 1994. Density and natural mortality of paddlefish,Polyodon spathula, in an unfished Cumberland River subimpoundment, South Cross Reservoir, TN. Journal of Freshwater Ecology 10: 421-431.

Bourton, J. 2009. Giant fish 'verges on extinction.' BBC News. September 29, 2009. Retrieved August 29, 2012.

Cheblukov, V. P., and V. Ye. Kharin. 2009. The first finding of the American Paddlefish, Polyodon spathula (Polyodontidae) in waters of the Russian Far East. Russian Journal of Marine Biology 35: 611-613.

Gao X., Wang J.W. & Brosse S. (2009), Threatened fishes of the world: Psephurus gladius (Martens, 1862) (Acipenseriformes: Polyodontidae). Environmental Biology of Fishes. 84 : 421-422.

Miller, S. E., and D. L. Scarnecchia. 2008. Adult paddlefish migrations in relation to spring river conditions of the Yellowstone and Missouri rivers, Montana and North Dakota, USA. Journal of Applied Icthyology 24: 221-228.

Nelson, J. S. 2006. Fishes of the World, 4th edition. Hoboken, NJ: John Wiley & Sons. ISBN 0471250317.

Onders, R. J., S. D. Mims, C. D. Webster, and A. L. Gannam. 2009. Apparent digestibility coefficients of protein, lipid and carbohydrate in practical diets fed to paddlefish, Polyodon spathula (Walbaum). Wiley Interscience 40: 1785-1788.

Pitman, V. M., and J. O. Parks. 1994. Habitat use and movement of young paddlefish, Polyodon spathula. Journal of Freshwater Ecology 33: 181-190.

Revkin, A. C. 2009. For Chinese paddlefish, a long goodbye. New York Times September 30, 2009. Retrieved August 29, 2012.

Rosen, R. A., and D. C. Hales. 1981. Feeding of paddlefish,Polyodon spathula. Copeia 2: 441-455.

Russell, D. F., L. A. Wilkens, and F. Moss. 1999. Use of behavioral stochastic resonance by paddlefish for feeding. Nature 402: 291-294.

Scholten, G. D., and P. W. Betoli. 2005. Population characteristics and assessment of overfishing for an exploited paddlefish population in the lower Tennessee River. Transactions of the American Fisheries Society 134: 1285-1298.

Zigler, S. J., M. R. Dewey, B. C. Knights, A. L. Runstrom, and M. T. Steingraeber. 2004. Hydrologic and hydraulic factors affecting passage of paddlefish through dams in the upper Missouri River. Transactions of the American Fisheries Society 133: 160-172.

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[FB-1] R. Froese and D. Pauly, eds., Family Polyodontidae: Paddlefishes , Fishbase (2009). Retrieved August 23, 2012.

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