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Paddlefish (Polyodon spathula)

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JADE McCARLEY


Department of Wildlife and Fisheries, Mississippi State University, Mississippi State, Mississippi, 39762, USA



Abstract

The Paddlefish Polyodon spathula is a ram suspension feeding planktivore that resides in fresh water habitats. While there are numerous hypotheses about the external morphology of the Paddlefish, none truly support a definite finding.



  • Fig. 1. Example of an American Paddlefish

Context & Content[+] Expand

The Paddlefish Polyodon spathula is classified as the following: Kingdom: Animalia, Phylum: Chordata, Class: Actinopterygii, Order: Acipenseriformes, Family: Polyodontidae, Genus: Polyodon, Species: spathula (Ross and Brenneman 2001, Starnes 2016). The order Acipenseriformes includes the sturgeons (family Acipenseridae) and the paddlefishes (family Polyodontidae; Ross and Brenneman 2001). The Chinese Paddlefish, Psephurus gladius, is the only other species in the family Polyodontidae (Wilkens and Hofmann 2007) and resides exclusively in China's Yangtze River (Ross and Brenneman 2001). There are a variety of common names associated with the species such as Paddlefish, Spadefish, Spoonbill Catfish, or Duck-Billed Catfish (Ross and Brenneman 2001). Walbaum portrayed the Paddlefish as a shark in 1792; but was corrected by Laceped? in 1797, who classified Paddlefish in the genus Polyodon (McKinley 1984).

General Characteristics[+] Expand

Paddlefish are an ancient species and exhibit less derived traits. They are typically characterized by the presence of heterocercal caudal fins and the absence of teeth in adults (Ross and Brenneman 2001). Polyodon means 'many teeth', while Adult Paddlefish do not have teeth, but do have many gill rakers which they use to filter zooplankton from the water column (Ross and Brenneman 2001) and spathula means 'flat piece', referring to the long snout, or rostrum, which makes up around one third of the total body length of adults (Wilkens and Hofmann 2007). For the most part Paddlefish have a smooth shark-like exterior appearance and possess a small amount of scales located on the operculum and the bottom lobe of the tail. The Paddlefish skeleton consists mostly of cartilage with the jaws being the only exception (Ross and Brenneman 2001).

Distribution[+] Expand

Ross and Brenneman (2001) documented Paddlefish in the: Noxubee River system, Pearl drainages of the Gulf of Mexico Basin, and areas of the Mississippi River Basin. Paddlefish have also been documented throughout the Licking, Tennessee, Mississippi, Ohio, Kentucky, and Cumberland rivers and as far north as the Great Lakes (Burr 1980).

Form & Function[+] Expand

The main theory pertaining to the rostrum of Paddlefish is that it is used as an electrosensory apparatus for locating plankton in aquatic habitats (Wilkens and Hofmann 2007) While earlier writers suggested that the Paddlefish used the elongate rostrum to dig into the benthic layer to locate items for nourishment, later scientific research confirmed that they feed primarily on plankton (Ross and Brenneman 2001).

Paddlefish total length is often measured using eye-fork length, taken from the eye to the fork of the tail (Ross and Brenneman 2001). The rostrum of fingerlings and smaller fish account for most of the body length which may increase error if measuring total length from the paddle to the fork of the tail. Eye-fork length (EFL) is the best method for determining an accurate total body length (Ross and Brenneman 2001).

Ontogeny & Reproduction[+] Expand

Paddlefish journey upstream each spring to spawn. Upon locating areas having an average water temperature of 12?C, Paddlefish spawn over coarse gravel (Ross and Brenneman 2001). The timing for spawning varies with location as more northern populations mature later and have differing thermal regimes. There appears to be a positive relationship between seasonally warming waters and adult Paddlefish abundance (Miller et al 2011), suggesting spawning depends on water temperature.

Initially, eggs float, but once the fertilization process is completed the eggs sink, sticking to hard surfaces for up to one week. Eggs hatch when the water temperature ranges from 18-21?C (Jennings and Zigler 2000). Maturity rates seem to vary by location, with males tending to mature at age six (Ross and Brenneman 2001). In most cases females mature later in life around age nine, although a study concluded that half matured by age nine and half matured at age ten (Reed et al 1992).

Ecology[+] Expand

Paddlefish roe provides a domestic alternative to purchasing expensive imported sturgeon caviar (Clarke-Kolaks et al 2009). However, increased poaching to harvest eggs, driven by the compensation fishers receive for roe, has led to rising concerns Paddlefish conservation. In 2006, fishers could earn as much as $22.50 per pound of roe harvested (Bettoli et al 2009). Although this may be an ongoing problem, management plans have established guidelines for legal harvest regulations, such as regulating areas where harvest is still permitted to prevent over-exploitation. As different habitats can support varying numbers of fish, limits must also vary according to current population status. A commercial harvest of Paddlefish was permitted during the summer of 2006 in Mississippi. Possessing a Paddlefish that was over thirty inches in length was prohibited and the catch limit was two fish per day (Bettoli et al 2009).

Paddlefish were plentiful in the Mississippi River drainage before 1900 (Jennings and Zigler 2000 However, Paddlefish are listed as vulnerable by the International Union for Conservation of Nature (IUCN) (Grady 2004), largely due to habitat alteration. Paddlefish movement is negatively impacted by the construction of dams as they increase the difficulty of moving upstream to spawn. Increasing river modification, could significantly reduce spawning success and ultimate result in population decline.

Coker (1928) participated in a study to determine whether Paddlefish were injured while passing through the Keokuk Dam located on the upper Mississippi River. The results found that Paddlefish were in fact harmed more often than other species of fish, but Paddlefish were able to move through the system at times unharmed (Coker 1928). River modification can result in loss and degradation of spawning habitats due to the destruction of passageways. Jennings and Zigler (2000) cite river modification (along with pollution and over-exploitation) as they discussed environmental issues relating to declines in Paddlefish abundance. Following modification, Paddlefish populations may be confined to an area, resulting in negative impacts on population size, increased competition for resources, and increased rates of disease transmission and mortality. Piers and other channel construction, while not obstructive, may result changes in Paddlefish movement. Wilkens and Hofmann (2007) noted the Paddlefish avoided structures containing steel, perhaps due to abnormal electric signals. Fish in laboratory trials also avoided metal structures. (Wilkens and Hofmann 2007).

The Paddlefishes along with sturgeons, are protected by the Convention on International Trade in Endangered Species of Wild Fauna and Flora. This organization was established in order to prevent over-exploitation of any species due to international trade. As extensive concern for Paddlefish populations began to grow people began to get involved with the conservation of the species, as of March 1992 Paddlefish were listed within the Appendix II section titled "Class Actinopterygii" (Jennings and Zigler 2000; CITES 2015). Currently in the state of Mississippi, the Paddlefish is not considered an endangered species, but is listed by Mississippi Natural Heritage Program as a species of special concern (MNHP 2015; Ross and Brenneman 2001).

Behavior[+] Expand

Paddlefish feed on plankton using a method known as ram suspension feeding in which they open their mouths and continuously swim forward, forcing water to flow through their gill rakers, which filter out plankton (Ross and Brenneman 2001). These fish happen to be the only filter feeders in the family Polyodontidae and have adapted specifically for this method of feeding, as their gill arch and jaw configuration produces a wider gape that allows for successful filter feeding (Wilkens and Hofmann 2007).

Genetics[+] Expand

There is no information on the subject at this time

Conservation[+] Expand

There is no information on the subject at this time

Acknowledgements[+] Expand

Upon completion of this paper I would like to acknowledge Jason Bies for being a wonderful mentor and teacher throughout the species review. Also, Dr. Mike Colvin who assisted in providing me with a substantial amount of resources and information that led to the successful completion of literary research to make this paper possible.

Remarks[+] Expand

There is no information on the subject at this time

Literature Cited[+] Expand

Bettoli, P. W., J.A. Kerns, G. D. Scholten. 2009. "Status of paddlefish in the United States." American Fisheries Society Symposium, Volume 66.


Burr, B.M. 1980. "A distributional checklist of the fishes of Kentucky." Brimleyana, Number 3. 53–84.


Clarke, S. J., J. R. Jackson, and S. E. Lochmann. 2009. "Adult and juvenile paddlefish in the floodplain lakes along the lower white river, Arkansas." Wetlands, volume 29, number 2. 488–496.


Coker, R. E. 1928. "Keokuk Dam and the Fisheries of the Upper Mississippi River." Bureau of Fisheries.

Order[+] Expand

Acipenseriformes are an order of primitive ray–finned fishes that includes the sturgeons and paddlefishes, as well as some extinct families.

Family[+] Expand

The Polyodontidae family live primarily in freshwater and are distributed only in China and the United States. In Greek, the word "poly" means "a lot of" and "odous" means "tooth;" these planktivorous fish possess numerous (in the hundreds) of long gill rakers and a paddle-like snout. The body is smooth, scaleless except for small scales on the caudal peduncle and caudal fin.




About This Project.

This website is an ongoing project by Ichthyology students of the Department of Wildlife, Fisheries, and Aquaculture, within the College of Forest Resources to provide information on the biology and ecology of fishes that occur in Mississippi. These accounts were written by undergraduate students as a course assignment, generally follow the format of Mammalian Species, and nomenclature follows Nelson 1994.

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