Trout and Freshwater Prawn Culture in Georgia Ponds
Evaluation of freshwater prawn production in ponds in south Georgia reveals the adaptability of Machrobrachium rosenbergii to climates and culture systems, provided management effort is intensified. Preparation of ponds for production of zooplankton as food for larval and juvenile prawns is important. Control of pH can be achieved by combining aeration and application of gypsum. Partial harvest of prawns to remove large individuals at 4 m allows staggered sales, reducing inventory volume during September and October. Stocking rainbow trout behind freshwater prawns achieved a second crop in the same ponds. Total revenue was $20,395/ha ($8,498/A) for the double cropping system.
Freshwater prawn, Machrobrachium rosenbergii, are currently popular with small scale aquaculturists in the southeastern United States. Research at Kentucky State University has reported larger yields (Tidwell et al. 1999) than other states in the region (D'Abramo et al. 1998), reportedly due to the cool summer climate and reduced competition between reproductively active individuals. Several other aspects of that research can lead to improved production over other systems attempted in the Southeast. Utilization of water from a reservoir rather than a well can allow zooplankton to be seeded in the prawn ponds prior to initial fertilization and larval prawn stocking. The pH of a surface water reservoir can be lower than well water from calcium carbonate containing aquifers due to watershed geology and lack of alkalinity in rainwater that fills the reservoir. The effective use of structure to supplement pond bottom surface area allows prawns access to their natural foods and refuge from aggressive individuals, especially during molting.
Prawn culture methods outlined by those promoting the growth of these animals assumes that natural food organisms are present when ponds are stocked with post-larvae or small juveniles. Fertilization is recommended to produce blooms of algae and zooplankton, but many prawn producers fail to achieve the necessary zooplankton numbers to provide adequate food for the prawns they stock. Often, plastic tanks and plastic lined ponds fail to produce prawns because these artificial systems lack natural food organisms. Earthen ponds will eventually establish zooplankton blooms if fertilization is applied, but the timing of prawn stocking must be delayed until a suitable bloom is present.
The approximate pH maximum for freshwater prawn survival is 9.0. Alkaline waters, obtained from aquifers in the southeast, have a pH of 8.5. There is little margin for pH increases that are likely to occur due to changes in the amount of carbon dioxide or dissolved oxygen in pond water.
Without structure, most prawn yields are approximately 500 kg/ha. Work with different amounts of structure at Kentucky State University has indicated that prawns benefit from additional surface area. Past work at Tifton indicated that vertical substrate encouraged the growth of filamentous algae and wasted potential surface area material by allowing the material to remain above the water line. An ideal material for providing additional surface area would be one that is water stable, allows feed to fall through it to reach the pond bottom, and is able to be totally submerged in the pond. Natural food organisms grow on the substrate, providing additional nutrients to the prawn population. Immature males and molting individuals may find refuge in the substrate as mature males guard their harems of females on the pond bottom.
Rainbow trout have been proposed as a second crop for channel catfish raised in ponds or raceways in the southeastern United States. Water temperatures drop low enough to support trout growth during the month of November and last until early March each year. Warm water fish grow slowly during this 5 m cool season. Prawns must be removed from ponds before water temperature drops below 16°C (60°F). Early stocking of prawn post-larvae is recommended in Georgia due to suitable temperatures in March and April. A 6 m prawn season and 6 m trout season is probable in most years for most of Georgia.
Materials and Methods
Post-larval prawns were received at 30 days post-hatch in shipping containers (5,000 post-larvae each) at 7 parts per thousand salinity under oxygen. Post-larvae were stocked into a recirculating water system at the rate of 5 post-larvae per liter (20/gal). Temperature in the nursery was between 25 and 27°C (77-80°F). Artemia was hatched in a separate container and fed to the post-larvae at the rate of 2 liters culture media per day along with shrimp post-larval diet at the rate of 15% of biomass per day.
Ponds were prepared by applying hydrated lime at the rate of 1,000 kg/ha to drained ponds, adding structure, filling to 3/4 full with well water (100 mg/L alkalinity), the filling to full with water from adjacent catfish fingerling ponds containing zooplankton and phytoplankton. Filling was completed in anticipation of stocking prawns within 5 to 7 d. Structure was orange plastic barricade fencing 1.1 m (4 ft) wide and enough to cover 50% of the pond bottom area. Each end of a fencing roll was anchored at opposite edges of the pond and a weight was place in the middle of the roll so that the structure was suspended 0.2 to 0.5 m (1-2 ft) above the pond bottom. Fertilizer was added as soon as the ponds were filled at the rate of 50 kg/ha 10-10-10 inorganic fertilizer and 50 kg/ha cotton seed meal. Cotton seed meal was reapplied at the rate of 30 kg/ha/day after prawns were stocked. Inorganic fertilizer was added at the rate of 3 kg/ha/week after prawns were stocked. The fertilization schedule ended after 60 days.
Four 0.04 ha (0.1 acre) ponds were stocked on April 19, 2001 with 38,000 juvenile prawns per hectare (16,000/A). These prawns had completed 20 days in the nursery and had begun to settle on the bottom and sides of the nursery tank. Prawns were transported in fresh water supplied with oxygen from a gas cylinder and tempered into each pond over a period of one hour. Ponds were within 3°C (5°F) of the nursery water at stocking time. In addition to the prawn juveniles, grass carp 7 cm (3 in) long were stocked at the rate of 200/ha (80/A) in order to control weed growth in the ponds. Each pond was monitored for dissolved oxygen twice daily and pH once daily in the afternoon. Aeration was provided by a single 0.5 HP axial flow aerator per pond. When pH approached 9.0, gypsum was added to that pond at the rate of 200 kg/ha and repeated if pH did not drop within 24 hr.
A 30% crude protein shrimp sinking pellet feed was applied starting on d 60 at the rate of 5% of the calculated weight of prawns. Prawns were sampled by seining between the structures. Feeding rate was reduced to 3% of the biomass when prawn average weight reached 22 g.
Rainbow trout were stocked at 2,400/ha (1,000 /A) within one month after removal of the prawns and structure and refilling the ponds. The average weight per trout was 75 g (0.16 lb) and the trout were obtained from a source using all female stock hatched in late spring in North Georgia. After trout stocking, ponds were aerated continuously and monitored for temperature and dissolved oxygen. The warm winter allowed pond water temperatures to reach 20°C (68°F). Trout were fed a commercial trout floating pellet feed throughout the winter.
Results and Discussion
Zooplankton numbers increased from an average of 2,025 cells/L to 4,600 cells/L during the second week after stocking prawn juveniles. By d 40, zooplankton number declined to 825/L due to prawn feeding, in spite of liberal fertilization. As prawns grew larger and became less dependent on the zooplankton for food, zooplankton numbers increased to 4,550/L by d 60. At that point, the prawns averaged 10 g each and feeding with a shrimp pellet was begun. Over the remainder of the production period, zooplankton numbers averaged 2,100/L. Prawn stocking should be delayed until approximately 2,000 zooplankton per L are present in the pond water.
Pond water quality was maintained with dissolved oxygen concentrations above 3.0 mg/L. Pond water pH was managed with addition of gypsum so that no pond exceeded pH 9.5. An average of 2,000 kg gypsum per ha was added to each pond during the course of the production cycle. The feeding rate on a unit of pond area basis was rather low as indicated by the relatively low accumulation of nitrogen compounds. Total ammonia concentration did not exceed 1.0 mg/L and nitrite did not exceed 0.5 mg/L during the study.
By August 21 many of the prawns had grown to 28 g (1 oz) or larger. With the pond full and by moving the substrate to the side of the pond, a 1.25 cm (½ in) mesh seine was pulled through the pond to capture the prawns. All prawns were sorted with a 48/64 bar-grader to remove harvestable sized prawns and replace smaller prawns. An average of 430 kg/ha (395 lb/acre) were harvested in this partial harvest. The number of orange claw males to blue claw males was 2.4:1 and the number of females to blue claw males was 5.5:1 in the harvest. Only 2.4% of the females were carrying eggs at this harvest. Between the partial harvest and the final harvest, a low dissolved oxygen event claimed one pond so that the harvest data after August 21 reflects the average of three ponds. Final prawn harvest was made on October 1, 217 d post-hatch or 167 d in ponds. An average of 487 kg/ha (447 lb/A) was recovered in the final harvest by completely draining the ponds. At this point, 48.1% of the females were carrying eggs and the ratio of orange claw males to blue claw males was 1.3:1 indicating a shift toward reproductive activity since the August harvest. Small males made up 11.6% of the total numbers. Total yield was 909 kg/ha (834 lb/A) on average for the three ponds carried till October.
Trout harvest yielded an average of 829 kg/ha (760 lb/A) between November 15 and March 15. Trout survival averaged 61.25%, with losses attributed to bacterial gill disease which was first identified in December during a warm weather period. Feed was converted 1.4:1 and the average size at harvest was 0.54 kg (1.2 lb). Total revenue was $20,395/ha ($8,498/A) with expenses of $9,648/ha ($4,020/A).
D'Abramo, L.R., M.W. Brunson, W.H. Daniels, and M.E. Fondren. 1998. Freshwater prawn hatchery and nursery management. MS Univ. Ext. Ser. Pub. 2002. 9 pp.
Tidwell, J. H., S. Coyle, C. Weibel, and J. Evans. 1999. Effects and Interactions of Stocking Density and Added Substrate on Production and Population Structure of Freshwater Prawns, Macrobrachium rosenbergii. J. WORLD AQUACULTURE SOC. 30(2): 174-179.