July 02, 2012
What American fish can grow to over 100 pounds, has no teeth, eats microscopic plankton, and resembles a shark with a big nose? If you solved this riddle, you're familiar with the paddlefish, one of our largest, oldest, and most fascinating fish.
Early European explorers encountered this strange beast in North America's large rivers. Records of the paddlefish date back to DeSoto's Expedition through the Southeast from 1539 to 1542. A writer known only as "A Gentleman of Elvas" wrote, "There was another fish called pexe pella (translated 'peel fish'). Its snout was a cubit in length and the tip of its upper lip was shaped like a shovel."
Archeologists have uncovered fossils of paddlefish similar to our Polyodon spathula in rock formations around the Green River in Wyoming that date back over 50 million years. Yet major aspects of its biology remain mysterious. And its common name, spoonbill cat, suggests that some anglers remain unsure of its place in the animal kingdom.
In early America, paddlefish inhabited the large free-flowing rivers of the Mississippi Valley, extending north into Ontario and west to the Missouri River in Montana. Commercial fishing and dam construction later eliminated paddlefish from several river drainages and four states. They thrive in some impoundments, however, and today are found in 22 states.
The paddlefish skeleton is cartilage, except for a small amount of bone in the jaw. To feed, adult paddlefish swim through clouds of zooplankton, engulfing whatever can't swim out of their way. Their long thin gill rakers then strain the water, filtering food organisms from it. Young paddlefish feed by sight on zooplankton until their gill rakers fully develop at a length of about 16 inches.
Paddlefish do not spawn until they are relatively old--about 8 for males and 10 for females. Harvest of immature fish (females may weigh 30 pounds and be immature) has hurt populations in several areas. Spawning takes place over shallow gravel bars with current, but only when appropriate water temperature, day length, and flow rate coincide. Throughout its range, spawning may take place from late March to late June, when water temperature is in the mid-50F range.
Dams have flooded many natural spawning sites and even where spawning sites persist, sudden fluctuations in water level caused by flood control and hydroelectric operations strand fertilized eggs or hatchlings.
Young paddlefish grow quickly, reaching up to 20 inches at age-1. After age-5, growth rate slows to about 2 inches per year. Maximum age is around 40 years.
Commercial harvest has been severely limited throughout the United States but anglers have sought paddlefish for food and the thrill of battling a big fish. Since these filter feeders rarely take bait or lures, weighted snagging hooks are cast on heavy tackle and blindly jerked to stick fish. This method works when paddlefish migrate upstream in spring, congregating below dams.
Where paddlefish populations are stable, fishing may be legal, though the season and bag limit are restricted. In some regions, stocking has buoyed paddlefish numbers. On Missouri's Osage River, the paddlefish population was doomed by construction of Truman Dam, which blocked their migration from Lake of the Ozarks. An innovative hatchery program has maintained a strong fishery, with the state record set at 130 pounds. Kansas and Arkansas also have obtained fingerlings from this hatchery to strengthen populations in their rivers. And artificial spawning has been successful at the Garrison Dam National Fish Hatchery in North Dakota.
Because of the grave problem of poachers slaughtering paddlefish for their roe, the sale or barter of paddlefish or their eggs is illegal. The value of the roe has spurred the development of commercial aquaculture of paddlefish.
Where natural spawning areas for paddlefish remain, it's critical that channelization or damming be prohibited and that pollution be minimized. Harvest must be carefully monitored to ensure that populations aren't reduced to a point that spawning is limited. Where populations are sustained by stocking, harvest must be limited so these ancient fish can maintain the place in our rivers they've held for millions of years.