Captive Care Notes: Paddlefish (Family Polyodontidae)
Like sturgeons, Paddlefish are poor subjects for home aquaria. They grow large, need lots of food, and damage their sensitive rostra easily. Occasionally, hatchery reared juveniles are for sale in pet shops, but you should resist the urge to purchase these undeniably cute fish. The best place to observe and admire Paddlefish is in public aquaria. Even here, most specimens have damaged rostra and are conspicuously skinnier than their brethren in the wild. Although the Paddle may be useful in locating microscopic prey, it does a poor job of telling the Paddlefish when it's about to bump into the aquarium glass and various submerged structures. Should you insist in keeping juvenile Paddlefish, elliptical tanks or round pools are preferred, the larger the better.
Even though they're primarily plankton feeders in the wild, captive Paddlefish readily adapt to commercially prepared foods. At the Tennessee Aquarium, Paddlefish are fond of pellets and will flip upside down to take them from the surface. They're also fond of live brine shrimp, which are dripped into the tank through an air hose leading from a bucket placed above the water's surface. The Paddlefish immediately sense when the brine shrimp begin hitting the water and commence their open-mouthed, filter-feeding behavior. Aquarists at the Mississippi Museum of Natural Science allow Paddlefish to filter feed by turning off the tank's filtration system and dropping a "plankton cocktail" into the water a couple of times every day. Aquarists at Cleveland Metroparks Zoo feed used to sink frozen blocks of brine shrimp, flake food, chopped smelt, and shrimp into their 60,000-gallon Wolf Wilderness exhibit, but the Paddlefish could not compete with more aggressive feeders and began to perish. Eventually aquarists hit upon the idea of training their Paddlefish to accept smelt dangled from a pole in much the same way sharks are fed in ocean exhibits. They're pole-fed about three times a week and hand-fed by divers when cleaning the exhibit.
Hatcheries spawn Paddlefish with the use of hormones. Eggs are kept in plastic jars, where they hatch 5-7 days later when kept at 18C (65F). The larvae are placed into earthen rearing ponds as soon as their yolk sacs are absorbed. These ponds are prepared 10-14 days in advance. First, they are drained, cleaned and refilled to eliminate predacious aquatic insect populations and aquatic vegetation. (Larval Paddlefish are poor swimmers and easily get entangled in filamentous algae.) Organic fertilizers such as hay, alfalfa meal, rice and wheat bran, brewer's yeast, and cow manure are used to culture zooplankton. Stocking usually takes place on cloudy days or after sundown to prevent "sunlight shock."
Paddlefish can also be reared indoors in environmentally controlled tanks or troughs supplied initially with pond-caught daphnia, then gradually replaced by a commercially prepared diet. Since the Paddlefish are kept in tanks that are usually smaller than outdoor ponds, overcrowding can be a problem. Overcrowded juvenile Paddlefish will bite each other's tails, or exhibit a behavior called billing, in which they swim with their paddles out of the water. The function of the biting is unknown, but billing appears to be the Paddlefish's effort to deflect oxygen-rich water from the surface into its mouth. Billing Paddlefish have stopped feeding and will soon die. When Paddlefish are moved to more spacious quarters, billing and tailbiting cease.