World Aquacluture Magazine - September 2020

WWW.WA S.ORG • WORLD AQUACULTURE • SEP TEMBER 2020 69 season, when secondary sexual characteristics become prominent (Fig. 2). The best morphological characteristic indicative of a ripe female stinging catfish broodfish is a bulging vent and a well-rounded abdomen, the fullness of which extends posteriorly to the pelvic fins. Males appear lean with a pale vent and a papilla with a pointed tip. In a mature female, the genital papilla remains in the form of a raised prominent structure, round and blunt with a slit-like opening in the middle. Induced Spawning Commercially available dehydrated carp pituitary gland extracts or a commercial synthetic hormone like Ovatide or Spawn Pro (salmon gonadotropin releasing hormone and domperidone) can be used. The body weight of each broodfish was weighed on an electronic balance to determine the required amount of inducing agents. Broodfish were injected with 0.3 mL/kg body weight for females and 0.1 mL/kg body weight for males. The hormone was administered by intramuscular injection beneath the dorsal fin slightly above the lateral line (Fig. 3). After injection, broodfish were released to a breeding hapa inside breeding tanks. Spawning and Egg Incubation Female broodfish ovulated 10-15 h after injection. Broodfish were then easily transferred from holding tanks after completion of ovulation to the breeding hapa (Fig. 4). Fertilized eggs were held in the same tank to avoid physical damage and fungal/bacterial contamination during egg collection. Each cement tank can accommodate approximately 10,000 eggs. After stocking eggs, a continuous water flow was maintained for aeration to guarantee that environmental conditions were optimal for hatching (Fig. 5). For large-scale incubation, an improved model was developed by SSKVK-RKMVERI. Hatching occurs within 16-22 h at 27-30 C in this tropical climate. After hatching, continuous water flow was maintained in the tank containing newly hatched larvae to make them free of eggshells and damage eggs. Hatchlings were transferred to circular or rectangular fiberglass or plastic containers for further rearing. Larval Rearing Hatch- lings of sting- ing catfish were not fed for three days, until the yolk-sac was fully absorbed. Then, post- larvae were fed on a paste of Tubifex worms at 10 percent body weight twice per day (Fig. 6). Suc- cessful rearing of post-larvae up to the stage suitable for stocking in nursery ponds remains the major challenge for expansion of cul- ture practice of stinging catfish at a commercial level. Suitable feed is the basic requirement for growth and survival of fish larvae. The transition from one type of food to another can be a challenge for larviculture. The period of transition, often referred to as the wean- ing period, may be between two types of live feed or from a live feed to an inert formulated diet. Fry and Fingerling Production Fingerling production is carried out with the aim to obtain a higher price and greater production when stocked for grow-out. The release of fry directly into ponds results in lower recovery related to natural mortality or predation. It is recommended to rear fry in small cement tanks of 10-20 m 2 for better survival and easy management (Fig. 7). Tanks are provided with date leaves for shade or shelters are provided on the tank bottom for fry. The shelters serve as hiding places for a large number of fry, which makes it easier to feed them near or around the shelter. Differential growth is found at this stage, so it is necessary to size grade at regular intervals to reduce competition for feed between smaller and larger fry, which also enhances survival. Filamentous algae occurs in the fingerling tank because of the shallow water depth and high nutrient leaching from uneaten feed and excreta of fish. Therefore, it is essential to remove filamentous algae at regular intervals for better growth and survival of fry. Fry usually grow to around 1 g in 50-55 d. Fry are segregated and stocked again at 50-100/m 2 for further rearing, until they reached 4-5 g in another 70-80 d (Fig. 8). Despite this, many farmers release fry to small nursery ponds because they lack a proper rearing system. Some seed growers also divide paddy fields into small plots to rear fry along with paddy rice for enhanced production and some are released into biofloc systems. ( C O N T I N U E D O N P A G E 7 0 ) FIGURE 7. Stinging catfish fry. FIGURE 8. Stinging catfish fry ready for stocking in grow-out ponds.