WWW.WAS.ORG • WORLD AQUACULTURE • MARCH 2018 43 (CONTINUED ON PAGE 44) At the end of the latency period (10 h), females are strip spawned by gently pressing the abdomen with a thumb from the pectoral fin towards the genital papilla (Fig. 3). Ovulated eggs will flow easily in a thick mass from the genital vent if females responded well to hormonal treatment. Ovulated eggs are brown and there are about 600 eggs in one gram of eggs. There is a strong correlation between female size and fecundity, with the average relative fecundity of 18,000-22,000 eggs/100 g fish. The mature ovary contains different sizes of eggs. Fully ripe ova are dull yellow/green and measure from 760 to 875 µm in diameter. Pabda males cannot be stripped and consequently sperm can only be obtained by dissecting the male and removing the testes. The dissected testes are then placed on a piece of clean and dry cotton cloth and milt is squeezed out and applied to an egg mass on a plastic tray. Eggs are fertilized by activating sperm with water and simultaneous mixing with a feather (Fig. 4). The water and egg mass are mixed by gently shaking the tray. After about one minute, fertilization has taken place and the sperm has lost its activity and fertilized eggs are ready for incubation in tanks. Egg incubation and hatching Incubation can be done in plastic tubs, cement tanks or aquaria. Plastic tubs are inexpensive, portable and easy to clean. A framed fine-mesh nylon net screen is spread horizontally in the container, 5-10 cm below the water surface and fertilized eggs are evenly spread over the screen (Fig. 5). Eggs maintained under flowing and well-aerated water survive better. Water in hatching tubs or cement cisterns must be clean and chlorine free. Embryos hatch 22-24 h after fertilization at 28-30 C. Hatchlings move to the bottom of the container through the nylon net (Fig. 6). Low hardness and alkalinity is conducive for better embryonic development, leading to better hatching rate. Newlyhatched larvae possess a large yolk-sac that is absorbed within 2-3 days. Non-fertilized eggs and shells remaining from hatched eggs must be removed immediately to avoid fouling of water. The screen and non-fertilized eggs should be removed from the hatching tub/cistern as soon as most eggs have hatched and fallen through the screen. FIGURE 5. Incubation of fertilized eggs in an aquarium. Note placement of eggs on horizontal screen below water surface. FIGURE 6. Newly hatched pabda larvae will aggregate at the tank bottom. FIGURE 7. Plastic tubs make excellent larval rearing units for pabda larval rearing. FIGURE 8. Tubifex larvae are an excellent larval food item.
RkJQdWJsaXNoZXIy MjExNDY=