Charles Lutz | 6/3/2005 12:39:38 AM
C.R. “Chuck” Weirich and C. Greg Lutz
In Louisiana and other catfish-producing states, most growers focus on the production of market-ready fish and purchase fingerling catfish to restock their production ponds from a smaller number of farmers who specialize in fingerling production. In Louisiana, fewer than 20 fingerling producers satisfy the annual seed stock requirements for the state’s catfish industry. Production of seed stock is one of the most critical phases of the catfish production cycle.
To produce fingerling catfish, eggs are procured in the spring from ponds containing adult broodstock and transported to a hatchery where they are incubated under controlled conditions. At hatching, baby catfish (fry) possess a yolk from which they derive nourishment for the first four to five days of life and are called “sac fry.” After the yolk is absorbed, fry are called “swim-up fry” because they swim to the surface in search of food. Swim-up fry are typically held in the hatchery for up to 10 days and fed a finely ground commercial diet before they are stocked into nursery ponds. After stocking, fry feed mainly on natural food organisms such as zooplankton and insect larvae. These food sources normally become depleted within four to six weeks after stocking, and fish are then fed formulated pelleted feeds until sufficient growth is obtained. At a common stocking rate of 200,000 to 250,000 fry per surface acre, four to six months are required to produce fingerling catfish 4 to 6 inches long.
Early-age Stocking of Fry
Although most fingerling producers hold and feed swim-up fry in the hatchery for two to 10 days before stocking, several Louisiana catfish fingerling producers stock their nursery ponds with sac fry, usually within two days after they are hatched. This practice was initiated in an attempt to reduce hatchery operating expenses such as feed, labor and electricity. Although research has shown that fry can be stocked at the onset of yolk absorption with no detrimental effects on subsequent fingerling production, stocking sac fry has been reported to result in reduced survival rates and overall production. To further investigate this topic, production trials were conducted at the LSU AgCenter’s Aquaculture Research Station to determine the effect of stocking fry of three different ages on survival, weight, yield and feed conversion ratio.
Fingerling Production Trials
Two fingerling production trials were conducted over two growing seasons, 1998 (Trial 1) and 1999 (Trial 2). Trials were conducted under simulated nursery pond conditions using 12 earthen-bottom fiberglass pools (0.002 acre) for Trial 1 and 15 for Trial 2. Trial 1 included four replicates per treatment (age of fry at stocking), and Trial 2 included five. Fry were stocked at two (sac fry), seven or 14 days after hatching. Fry stocked at seven and 14 days after hatching were fed in the hatchery for two and nine days, respectively, before they were stocked. Fry were stocked at a rate of 250,000 (Trial 1) or 200,000 (Trial 2) per acre.
After pools were stocked, catfish starter was applied daily to each pool until fish were observed feeding at the surface. Fish were then fed progressively larger floating pelleted diets to satiation daily as they grew. Pools were drained at 110 (Trial 1) and 130 days (Trial 2) after stocking to allow harvest. At harvest, samples of fish from each pool were weighed. Remaining fish were counted and weighed to determine percentage of survival, yield and feed conversion ratio.
Stocking Sac Fry
Results indicate that the age at which fry were stocked had no effect on subsequent survival, yield or feed conversion ratio of fingerling catfish. Based on final weight, however, fingerlings reared from fry stocked at two and seven days after harvest were significantly larger than fingerlings reared from fry stocked at an age of 14 days after harvest. These findings suggest that sac fry can be stocked with no detrimental effect on subsequent fingerling production. Indeed, based on final weight at harvest, it may actually be beneficial to stock sac fry rather than to hold and feed fry in the hatchery before stocking. Sac fry stocked in properly managed nursery ponds have unlimited access to zooplankton and insect larvae, which comprise the bulk of the diet of recently stocked fry. Under hatchery conditions, feed is more restricted. Thus, fry stocked as sac fry may gain an initial size advantage over fry stocked at an older age, which may be retained over the course of production until harvest. But because of the reduced mobility of sac fry relative to older, larger fry, proper management procedures must ensure elimination of predators.
More Research Required
Although results of this preliminary study suggest that the practice of stocking sac fry may be an alternative to the traditional procedure of holding and feeding fry under hatchery conditions before stocking, experiments were conducted under simulated conditions. In a typical production pond, there is less control of certain variables such as predator populations, water quality parameters and disease organisms. Therefore, more experiments will be conducted in ponds at the Aquaculture Research Station and at two Louisiana catfish farms. These experiments will include comprehensive cost and return information to determine the economic impact of early-age stocking of catfish fry before recommendations can be made to the state’s commercial catfish industry.
This research was supported by the U.S. Department of Agriculture and the Louisiana Catfish Promotion and Research Board.