Paddlefish (Polyodon spathula)
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Description: This ancient species, also called a Spoonbill or Shovelnose Cat, has been in existance for an estimated 300,000 years. They reache up to 7 feet in length and 100 lbs during their 30 year lifespan, making them one of the largest freshwater fish. They have a greatly elongated paddle-shaped snout and a large mouth on the extreme underside of its head. The paddle-shaped rostrum, as the snout is properly called, is approximately one-third the fish's total length.The tail fin is deeply forked with a long upper lobe. The skin is smooth and nearly scaleless except for a patch near the tail fin. Paddlefish vary in color from black to bluish-gray, which fades to white on the sides and belly. They have small eyes near the base of the rostrum (snout), providing only poor vision. The large tail, with the top portion bigger than the bottom portion, gives them a shark-like appearance. The large, toothless mouth combined with the closely set gill rakers makes an effective seine to filter the zooplankton on which the fish feeds. The elongated opercula (gill covers) allow for expansion of the gill openings and mouth while filter-feeding.
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Habitat and Habits
Paddlefish are found only in North America. They prefer backwaters, ox bow lakes, and areas where currents are reduced. Big islands, bridge pilings,
shoals, and sand bars provide such sheltered areas. Examples of good paddlefish habitat in the Three Rivers area are the confluence of the
Beaver and Ohio rivers, around Neville Island in the Ohio, and the islands and bridges at the upper end of Pool 2 on the Allegheny. Although lakes and reservoirs can provide good feeding areas for paddlefish, the dams that create them have inundated suitable spawning habitat and are barricades to paddlefish during spawning migrations. Paddlefish are native to the Missouri River and have been introduced into many lakes and quarries across the midwest. They normally like quiet, slow-flowing waters, and swim continuously near the surface or in shallow waters. They feed on zooplankton and insect larvae filterd from the water through their elaborate gill rakers.
Males mature at about 7 years, females at 9-10 years. Paddlefish spawn in early spring on gravel bars of large, free-flowing rivers. Spawning is not successful without flooding at critical times. Eggs sink to the bottom, adhere to the first object contacted and hatch approximately 9 days later.
This species was formerly the most commercially important fish in the
Misssissippi Valley, prized for its meat and caviar.
Distribution
The paddlefish is one of two living species of this family in the world. The other species in the family Polyodontidae is found in China's Yangtze Valley. Paddlefish are restricted to the Mississippi River and its tributaries and adjacent Gulf drainages in portions of Texas and Alabama. Under former natural conditions, the numerous oxbows and backwaters of the large, free-flowing rivers of the Mississippi Valley provided feeding grounds as well as extensive spawning habitat (gravel bars). In South Dakota, a stable
paddlefish population occurs in the Missouri River below Gavins Point Dam, with remnant populations in other Missouri River reservoirs in the state.
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Paddlefish biology
Paddlefish have a skeleton made of cartilage, even though they are considered bony fishes. Although paddlefish fry are not born with the bill for which they are named, the rostrum starts growing shortly after birth. Biologists are not sure about the rostrum's function. For years, many people believed wrongly that it was used to dig in the mud. The rostrum has many sensory and electro receptors that may aid in detecting concentrations of
zooplankton. The rostrum may help balance the fish while swimming with its large mouth open. However, paddlefish that have lost part or all of the rostrum seem to suffer no ill effect and so cast doubt on its true use.
Compared to most fish, paddlefish mature late in life. Males reach sexual maturity at seven to nine years of age, or after about one-fourth of their expected life span. Females mature at 10 to 12 years of age, or after about one-third of their expected life span. Although males may spawn every year, females require two or more years to produce up to 10 to 12 pounds of eggs. The eggs are valuable as a source of high-quality caviar. A recent estimate of the value of a pound of processed paddlefish eggs was $360.
Until 1960, little was known about paddlefish spawning behavior or habitat requirements. Environmental conditions have to be just right for spawning to occur. Paddlefish need clean gravel bars with good current flows. The water temperature must be near 60 degrees, the water flows must be high and rising, and all of this needs to happen in the spring. A sudden water level rise of six feet has been documented to trigger spawning activity. These environmental factors don't coincide every year. Thus, female paddlefish may not spawn every year.
When spawning does occur, it is a simple act. A large female, accompanied by a couple of usually smaller males, swims over the selected gravel bar. Both sexes release their respective eggs and milt. Fertilization occurs in the water and the eggs become sticky. The sticky eggs adhere to the gravel substrate so that they don't wash away. The high water flow keeps the eggs well-oxygenated and keeps debris and silt from covering them. In about a week, the eggs hatch and the larval paddlefish are swept downstream to quieter nursery waters. In a few days, the yolk sac is absorbed and the young fish feed first on small insects. Then they begin their lifelong pursuit of zooplankton.
Young paddlefish are not good swimmers. Their only defense from being eaten is to grow large fast. It is amazing that the paddlefish can attain its great size feeding only on creatures the size of the period at the end of this sentence. This means that volume of food is the key to paddlefish survival and growth.
In some states, paddlefish populations are healthy enough to support active sport fisheries. Because paddlefish feed on microscopic organisms, traditional fishing techniques are not successful. Most anglers catch paddlefish by snagging. In recent years, a few paddlefish have been inadvertently "foul-hooked" and released by Pennsylvania anglers in the Ohio and Allegheny rivers. The paddlefish season is permanently closed in Pennsylvania. The flesh is compared to swordfish in flavor and the roe (eggs) is sought for caviar.
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