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INTRODUCTION: Paddlefish are an ancient creature large freshwater fish, Polyodon spathula is also called spoonbill or duckbill and named for its flattened, paddle-shaped snout. The largest specimens weigh well over 150 lb (67.5 kg) and reach 6 ft (183 cm) in length. The snout may be a third of the length of the body; it is equipped with sense organs that assist the fish in finding its prey of small crustaceans, which it strains out with gill rakers. Paddlefish have a large, toothless mouth on the underside of their head. Paddlefishes are primitive; unlike most modern fishes, they have skins with reduced scales, almost wholly cartilaginous skeletons, and upturned tail fins. They are uniform leaden gray in color. The tail is deeply forked with the upper lobe longer than the lower. Valued as food fish, their greenish black eggs, like the more highly valued ones of the distantly related sturgeon , are used to make caviar . A Chinese species found in the Chang (Yangtze) River is said to grow to 20 ft (610 cm). Paddlefishes are classified in the phylum Chordata , subphylum Vertebrata, class Osteichthyes, order Acipenseriformes, family Polyodontidae. DISTRIBUTION: Paddlefish are native to the Mississippi River basin. In South Dakota, they are found primarily in the Missouri River (and reservoirs), sometimes being observed short distances up into Missouri River tributaries such as the James or White rivers. Historically, paddlefish were free to move great distances and were recorded traveling 1200 miles. Now in South Dakota and elsewhere, many paddlefish populations have been confined and isolated between dams. Paddlefish usually are found in quiet, slow-flowing waters. They typically swim continuously near the surface. NATURAL HISTORY AND HABITAT: Paddlefish are a unique species, representing a link from modern times to our historic past. In North America, fish in the sturgeon family represent the only close relative to the paddlefish. Their closest relative is another species of paddlefish found only in a large freshwater river in China. Paddlefish are long-lived, with some fish known to exceed 30 years of age. Male paddlefish generally mature at about 5 to 8 years and females at 8 to 12 years old. Large females can produce over a half-million eggs. Paddlefish spawn in early spring when water temperatures approach the mid-50 F range. Females deposit their eggs over gravel bars in large free-flowing rivers. Immediately after being fertilized, the eggs sink to the bottom, sticking to the first object contacted, and hatch approximately seven to nine days later. Newly hatched young, which look very different from the adults, are carried by currents into areas where food is available. At about one month old, the young paddlefish look very similar to the adults. Paddlefish, young and adults, feed almost exclusively on zooplankton, although there are isolated instances where fish and insects have been found in their diet. They feed differently than many other fish, using their long thin gill rakers to efficiently filter food from the water. SIGNIFICANCE: At one time, paddlefish were one of the most commercially important fish. Paddlefish, like sturgeon, are highly valued for their greyishblack roe (eggs), which is processed into caviar, and for their boneless, firm, white meat. Most paddlefish products are obtained from wild populations. However, overexploitation and contamination by organochlorine pollutants such as polychlorinated biphenyls (PCBs) and chlordane have made it necessary for many state agencies to close down this valuable fishery. Paddlefish are also prized as a sport fish in many parts of their range. In South Dakota, paddlefish are considered a sport fish. At one time, sport fisheries existed below each of the dams on the Missouri River. However, most of those populations rapidly declined due to lack of suitable spawning habitat. AQUACULTURE POTENTIAL: Paddlefish aquaculture will provide alternative fish sources for the marketplace in the era of strict federal and state regulations on wild populations. Aquaculture of paddlefish is in a research and developmental phase. Most broodstock are obtained from wild sources, though some mature fish have been developed in captivity. Artificial propagation techniques are resulting in > 80 % egg fertility. Larval paddlefish are initially raised in organically fertilized, zooplankton-rich (i.e. Daphnia sp.) ponds, and then trained on extruded trout/salmon diets until the juvenile fish are > 30 cm in total length. Tank culture is also an alternative for raising juvenile fish. Juvenile paddlefish, a filter feeder that requires zooplankton as its primary food, are being grown in reservoirs and in polyculture with channel catfish (Ictalurus punctatus) in Kentucky, Indiana, Missouri and Alabama. Production yields are 200-400 kg·ha-1 in polyculture and in reservoirs ranching resulted in 55-175 kg·ha-1. Reservoir ranching is ideal for caviar production; whereas, paddlefish (1.5 to 4.0 kg) cultured with catfish is for meat production. A system to produce all-female progeny through artificial propagation with sex-inverted, gynogenetic broodstock and attempting to develop optimal cryopreservation techniques for the milt of these unique broodstock is currently being tested. Paddlefish aquaculture is unique to Kentucky. Paddlefish armlings (8 to 10 inch long fish) are stocked in conjunction with catfish in catfish ponds at 50-70/ac. Paddlefish are zooplanktivores and do not require additional feeding. Paddlefish are harvested at 18-month age and at an average weight of 5-6 lb/head. Cost of production is approximately $0.68/lb, based on live weight. PROS AND CONS OF PADDLEFISH PRODUCTION: Paddlefish require only zooplankton as a food source for most of their life, and grow rapidly (up to 10 pounds or 4.5 kg per year). They can be harvested easily with gill nets or seines. Paddlefish can be propagated artificially and fingerlings raised intensively up to 12 inches or 30 cm (total length) in ponds, then grown for meat and/or roe extensively in reservoirs or intensively in ponds with catfish. Mature female paddlefish can produce about 15 percent of their body weight in roe (3 to 10 pounds or 1.5 to 4.5 kg per fish). There are also some disadvantages to paddlefish production. They have poor tolerance for low dissolved oxygen (< 2 mg/L), and show handling stress when water temperatures are higher than 70 F (21 C). Artificial propagation and fingerling production is complex, and fingerlings are vulnerable to bird predation. Also, there is a waiting period of at least 7 years before females produce eggs, and fish must be sacrificed to harvest caviar. Although the paddlefish is still considered an experimental food fish, ongoing research and demonstrations have shown commercial potential. This publication provides the basic, current information about propagation, production and marketing of paddlefish. PROPAGATION: Collection and Transport of Broodstock: Broodstock are generally obtained from wild sources because of their long maturation period (7 to 9 years in the southern U.S.). Broodstock are captured in 6-inch (15- cm) or larger bar-mesh gill nets that are set in rivers and lakes in the winter or early spring when water temperature is less than 60 F (16 C). Males usually weigh about one-half to two-thirds as much as females, and have minute tubercles on their heads and opercular flaps. Mature females have few, if any, tubercles, and swollen abdomens. The gonopore areas may be distended and reddish in color during the prespawning period. Broodstock can be transported to the hatchery in conventional hauling tanks holding 200 to 300 gallons (750 to 1100 L) of water supplied with agitation and oxygen and mixed with 0.25 to 0.50 percent sodium chloride salt. Approximately 2 pounds of broodstock per gallon of water (0.25 kg/L) is a safe quantity for transport. Broodstock can be held in ponds until water temperatures are suitable for propagation. Hormone Injections: Paddlefish can be propagated when water temperature is 55 to 65 F (13 to 18 C). Broodstock must be held in circular tanks (diameter 8 feet or 2.4 m) in the hatchery so the fish can swim continuously and aerate their gills (known as ram ventilation). Water temperatures of 62 to 65 F (16 to 18 C), flow rate exchange of 25 percent of the total tank volume per hour, and water saturated with oxygen (100 percent; about 10 mg/L at 62 F) are optimal conditions for broodstock in the hatchery tanks. Broodstock should be injected intraperitoneally with LHRH analogue of des-Gly10(D-Ala6) LHRH ethylamide (currently not approved by FDA; Investigational New Animal Drug permit is required). Females should receive a total dosage of 100 µg/kg body weight (BW) administered in a priming injection (10 µg/kg BW) and a resolving injection (90 µg/ kg BW) 12 hours apart. At 63 F (17 C), females can be expected to ovulate within 12 to 24 hours after the resolving dose. Males should receive a single dose of 50 µg per kg BW when the females are given the priming injection; they will spermiate within 24 hours and continue for 3 to 4 days. Milt and Egg Collection: For milt (sperm) collection, the fish is blotted dry around the gonopore area. Tygon tubing (diameter of 3/16 inch or 0.5 cm; length 2 inches or 5 cm) attached to a 10-cc plastic syringe is inserted into the urogenital pore. With gentle suction, milt is collected from the fish. A large volume (five to ten syringes) of milt can be obtained each time from one male for 3 to 4 days. However, milt from two to three males should be used to fertilize the eggs in order to increase genetic diversity. Milt is checked microscopically and 75 to 100 percent of the sperm should be motile. If sperm motility is less than 75 percent, the milt should be discarded and milt from other males should be considered. Milt can be collected several hours before needed and stored without aeration in sealed containers placed in a refrigerator (39 F or 4 C) or on wet ice (ice sprinkled with water). Hand-stripping was the traditional method of collecting eggs when culturists believed that paddlefish could not ovulate all their eggs at one time. The method was labor intensive and often required three or more individuals to collect and fertilize a small portion of the eggs (4 to 5 ounces or 120 to 150 ml) every 30 to 60 minutes over a 12-hour period before the majority of eggs were removed. We now know that all eggs do ovulate within 1 hour; however, because of a unique oviductal system not all the eggs can flow freely out of the fish. Therefore, two other methods have been developed to expedite egg collection—Caesarean section and the Minimally Invasive Surgical Technique (MIST). Caesarean section is a relatively quick surgical method (30 minutes) that removes eggs through a 4-inch (10-cm) external abdominal incision; however, suturing is time consuming and muscular stress on the incision usually results in poor suture retention and low survival of broodstock. The MIST makes removal of ovulated eggs even faster (about 10 minutes) and requires much less handling than the other methods and no suturing. With the MIST method a small internal incision (0.5 inches or 1.3 cm) is made in the posterior- dorsal area of the oviduct. This permits direct stripping of eggs from the body cavity through the gonopore, but bypasses the oviductal funnels. Regardless of the method used, eggs must be collected free of water. A female paddlefish weighing 20 to 80 pounds (9 to 36 kg) can release 70,000 to 300,000 eggs. Fertilization and Hatching: The eggs should be fertilized using the “wet method.” Milt is added to water at a 1:100 200 ratio (milt to water) and then immediately poured onto the eggs (0.5 ounces of undiluted milt per quart of eggs or 13 ml/L). There are approximately 50,000 eggs per quart. The fertilized eggs are stirred for 1 minute and then coated with a Fuller’s earth suspension (2/3 cup of Fuller’s earth per gallon of water or 40 ml/L) for 20 minutes. The eggs are then rinsed free of Fuller’s earth, volumetrically measured, and loaded into McDonald incubation jars at about 75,000 eggs per 2.5-gallon (10-L) jar. Fungus can be controlled by frequently siphoning dead eggs that float on the top layer and maintaining water temperature near 64 F (18 C). Incubating eggs at this temperature will allow fry to hatch in approximately 6 days. Fry require another 5 to 6 days before they will consume external (exogenous) food. During this interim, the mouth and fore-gut develop and the residual yolk is absorbed. Dark pigmented material that can be seen in the spiral valve of the hindgut will be excreted at about the time exogenous feeding begins. Cannibalism is not uncommon during this period, especially if fry are different sizes. NURSERY PHASE:
Pond Culture: Pond preparation should begin about 2 weeks before propagation. Ponds should be drained completely and dried if possible. After fish have been propagated, the pond should be flooded with well water or with water from a reservoir that is filtered through a saran sock. Rice bran (11.4 percent crude protein) has been recommended as the best organic fertilizer for paddlefish nursery ponds in Kentucky. Other organic fertilizers such as cottonseed, soybean and alfalfa meals can be used, but quantities should be adjusted for a total nitrogen amount of 40 pounds per acre or 45 kg/ha. During the initial fertilization period (week 0), large zooplankton such as Daphnia spp. should be inoculated into the pond at eight Daphnia per gallon (2/L). Other zooplankton such as copepods, rotifers and ostracods will not substitute for daphnia because they are fast swimming and/or too small for fry to capture. Fry can be stocked at a rate of 25,000 fish per acre (62,000 fish per ha) in fertilized earthen ponds when water temperature is higher than 65 F (18 C). Fish can grow up to 1/3 pound (150 g) and 14 inches (35 cm) in total length in about 6 months. During this time, survival rate can range from 30 to 80 percent. Bird predation can be controlled by covering ponds with netting. Raceway Culture:: Raceways or flow-through systems have been successful for producing fingerlings (less than 10 inches or 25 cm). Ground water or surface water can be used. Ground water should be aerated and heated to more than 72 F (22 C). Surface water should be filtered and, if possible, aerated and heated. Regardless of the source, incoming water should be tested for contaminants and other poor water quality parameters. However, with proper flow through of about 50 percent of the tank volume per hour, water quality doesn’t need to be monitored regularly except for temperature and oxygen. Outdoor raceways should be covered with 95 percent shade cloth to minimize sunlight and ultraviolet light exposure, which can cause sunburn and even mortality. Shade cloth also may provide some protection against predation by birds. Fry are initially stocked at eight fish per gallon (two fish per L). After 14 days, fry should reach about 2 inches (5 cm) in length. To prevent crowding, fish should be reduced to 2.5 fish per gallon (0.7 fish per L). After another 14 days, the fish should reach about 4 inches (10 cm) and the density should be further reduced to three-fourths fish per gallon (two tenths fish per L). Culturists should watch for fish swimming at the surface with their paddles above the water—a behavior known as “billing.” Billing indicates stress caused by high density; reducing fish density will usually stop this behavior. Because of the large space requirements, fish in raceways are usually harvested when less than 10 inches (25 cm) long rather than 12 inches (30 cm) as in pond culture. In raceways, survival rates can range from about 50 to 80 percent. FEEDING: In pond culture, fry initially feed on relatively large, slow-swimming zooplankton such as Daphnia spp. (water fleas) and insect larvae. These food items are preferred during the first 3 to 4 weeks. Fry cannot effectively filter- feed until their gill rakers are developed, at about 5 inches (12 cm) long or 5 to 6 weeks old. During this initial feeding period, some culturists supplement the fish diet with trout/salmon crumbles (#00-03; 50 percent protein) at a rate of 15 pounds per acre (17 kg/ha). Once the fish are about 3 inches (7.5 cm) long they can be trained to accept extruded pellets (1/16-inch or 1.5-mm; 45 percent protein). If fish are trained on a prepared diet, larger extruded pellets will be accepted by larger fish. Feed conversion ranges from 1.5 to 2 pounds of feed per pound of fish. In raceway culture, fry can be trained on a sinking diet of trout/salmon #00-03 crumbles (more than 50 percent protein). Some culturists use a 1/16-inch (1.5-mm) extruded pellet once the fish reach about 3 inches (7.5 cm) in 3 to 4 weeks. Fry (3/4 to 1.5 inch or 2 to 4 cm) are fed by automatic feeders every 15 to 20 minutes for the first 7 to 10 days. Thereafter, a combination of automatic feeders and hand feeding is used to feed the fish about every 2 hours until they are stocked into ponds or reservoirs. At this time, feed conversion ranges from 2 to 4 pounds of feed per pound of fish. WATER QUALITY: Dissolved oxygen is particularly important; a daily oxygen monitoring program is necessary to achieve maximum yields. Because paddlefish have a rostrum (paddle), they are unable to obtain oxygen from the surface film of water as catfish do when dissolved oxygen is low (less than 2 ppm or mg/L). It is recommended that the dissolved oxygen be kept above 30 percent of saturation at any given water temperature to prevent stress or loss of paddlefish. (For example, 100 percent of DO saturation at a water temperature of 85 F would be about 7.6 mg/L, so at 30 percent of saturation the DO would be about 2.3 mg/L). Paddlefish can survive a wide range of water temperatures from just above 32 F (0 C) to about 100o F. Water temperatures for best fish growth are 70 to 80 F (18 to 27 C). However, paddlefish handle best at stocking and harvesting when water temperature is less than 70 F (less than 21 C). Other water quality parameters are similar to those required by catfish: pH 6-9; un-ionized ammonia less than 0.2 ppm as N; and nitrite level dependent upon chloride level. DISEASES: Diseases have not appeared to be a problem in production of paddlefish in ponds or reservoirs, probably because of low stocking densities. However, a few diseases have been reported and studied for intensively cultured paddlefish in raceways. Rostrum (paddle) degenerative disease causes deformity of the rostrum, including a narrowing and/or downward curvature. Both Aeromonas bacteria and columnaris disease bacteria have been isolated from the rostrum and are believed to cause this disease. Chloramine-T at a rate of 20 mg per L for 1 hour has experimentally stopped the progress of this disease in raceways. Paddlefish have also been known to have “Ich” infections (Ichthyophthirius multifilis). Both treating with salt at 3 ppt and raising water temperature to 86 C for several days have been successful in eliminating this parasite in raceways. PRODUCTION STRATEGIES: Reservoir ranching and pond culture with catfish are two practical systems of raising paddlefish for caviar and/or meat. Both systems rely on the presence of natural zooplankton so that paddlefish grow-out requires little cost and management. Diet-trained paddlefish stocked into these systems (at low stocking rates) will switch to filter feeding and will not compete with the other fish for prepared diets. Reservoir ranching is an extensive aquacultural production system in which young fish (more than 12 inches or 30 cm) are stocked into a reservoir, permitted to forage on the natural food supply, and harvested after 2 or more years. This system is a very economical one for paddlefish caviar production. It uses existing reservoirs that were primarily developed for the storage of water, flood control and hydroelectric purposes. It has been demonstrated that paddlefish stocked at low densities of four to eight fish per acre (ten to twenty fish per ha) can reach 10 pounds (4.5 kg) in about 18 months (Alabama and Kentucky). They then can be sold for their meat or permitted to grow until mature, and then harvested for their roe. Fish are harvested with gill nets with nearly 90 percent efficiency. It is estimated that 50 to 150 pounds per acre (55 to 170 kg/ha) could be harvested yearly depending on the fertility, food supply and temperature of the water. There is ongoing research on reservoir ranching. Pond culture of paddlefish with catfish is a more intensive system than reservoir ranching. Paddlefish stocked at 30 per acre (75/ha), with catfish at 5,000 per acre (12,500/ha) have been reported to reach up to 7 pounds (3.2 kg) in about 12 months (Kentucky). Producing paddlefish with catfish depends on the pond system’s production of zooplankton and the water quality. If catfish feeding rates are increased, more zooplankton could be produced and higher yields of paddlefish could be expected. However, higher feeding rates for catfish will cause greater oxygen demand and possibly lower water quality. Fish can be harvested by seining with nearly 100 percent efficiency. Paddlefish are easily sorted by hand from the catfish and can be held in holding nets (at a water temperature less than 60 F) until loaded for transport to a processing plant. Because of the typical harvest cycle for catfish (every 6 to 12 months), this system is best for paddlefish meat production, even though the fish can be returned to the pond for further grow-out to maturity. More information is needed on stocking paddlefish with catfish in commercial operations in different parts of the South. MARKETING: Marketing efforts for paddlefish indicated an unlimited potential. Recently, researchers have identified a few white tablecloth restaurants in Kentucky that are willing to accept 100lb of fresh fillets per week at $6.50/lb. While this is very encouraging, the lack of a commercial paddlefish hatchery is thwarting production efforts. The full potential of paddlefish as an aquaculture crop will be realized with the advent of a large-scale hatchery. Value-added products such as hot and cold smoked paddlefish are the major effort being developed for the marketplace. Consumer acceptability of value-added products from paddlefish has been better than channel catfish. CAVIAR: Paddlefish caviar, which is often compared to sevruga (Acipenser stellatus), the most popular Caspian Sea caviar, is an alternative product. Paddlefish and sevruga have similar egg size (known as small grains) and a greyish color with a mild, less salty taste. A 40-pound (20-kg) female paddlefish can yield 4 to 6 pounds (about 2 to 3 kg) of roe. Current wholesale prices range from $30 to 65 per pound ($65 to 143 per kg), with retail prices of more than $150 per pound ($330 per kg). Caviar is marketed through buyers who sell to exclusive restaurants, gourmet shops and mail-order retail outlets. Caviar from paddlefish raised in ponds and reservoirs could alleviate pressure on wild fish stocks. The contaminants sometimes found in caviar from wild fish have not been found in cultured paddlefish caviar. Further, caviar from cultured paddlefish has been described as having a more buttery taste (more fat in egg) than caviar from wild paddlefish. In a blind taste test comparing cultured paddlefish caviar with the top three types of Caspian Sea sturgeon caviar (beluga, osetra and sevruga), the paddlefish caviar tied for second with osetra (A. gueldenstaedti) and was second only to beluga (Huso huso). MEAT: Paddlefish meat is firm, white, boneless, and very similar to sturgeon in taste and texture. Paddlefish meat has been well accepted by consumers. Even those who do not eat fish regularly liked paddlefish meat products in taste tests. Several smoked paddlefish meat products have been developed and are well received in exclusive restaurants and gourmet shops. Smoked paddlefish wholesales for $9 to 11 per pound ($20 to 24 per kg) and retails for more than $18 per pound ($40 per kg). Paddle-fish meat also has been tested for surimi (imitation crab meat) production with promising results. REFERENCES:
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