World Aquaculture Magazine - March 2021

56 MARCH 2021 • WORLD AQUACULTURE • WWW.WA S .ORG conditions (environmental stress). A combination of high stocking density (25-100/m 2 ) and restricted feeding (1-3 percent of body weight) is the most commonly practiced technique for stunted seed production. Under stressed conditions — the stunting phase —weak and sick fry from ponds are eliminated, thus leading to the production of healthy and slightly larger yearling seed as stocking material for grow-out. Comparatively, stocking of stunted yearlings has more advantages than normal fingerling for grow-out. Compensatory Growth in Stunted Fish When stocked in grow- out ponds under optimal culture conditions, stunted seed will accelerate their growth rate, termed compensatory growth (Ali et al. 2003). The compensatory growth response in stunted seed is related to greater appetite and feed consumption. Based on the ability of stunted seed to recover lost growth during the stunting phase, three kinds of compensatory growth have been reported in fish. • Overgrowth compensation - stunted fish attain greater body weight gain than the continuously fed fish at the end of the refeeding phase. • Complete growth compensation - the body weight of both stunted and normal seed, will be same at the end of refeeding. • Partial growth compensation - stunted fish seed reach lower body weight gain than normal seed at the end of the refeeding phase. Generally, in fish compensatory growth response is identified by high growth rate, better feed utilization, hyperplasia and greater mitogen production. Growth Response of Stunted Fish Seed in Grow-out Ponds In general, growth in fish does not stop and continues with age. However, fish growth is also affected by nutrition and environmental conditions. Under captive conditions, fish depend primarily on supplementary feed for energy. Generally, when fish mature, somatic growth slows because energy is diverted towards gonadal development (Enberg et al. 2008). Therefore, stunting is not a viable option for fish with a short maturation period (less than 1.5 years). Stunted seed are larger (up to 5-150 g) than normal seed based on the duration of stunting. The weight lost by stunted seed during the stunting phase can be restored when fish are exposed to normal culture conditions (Jobling et al . 1994). However, faster growth is influenced by various factors, such as feed availability, environmental conditions, onset of maturation and duration of stunting. In the refeeding phase, stunted seed consume more feed because of hyperphagia and have a lower energy expenditure for metabolism and locomotion. Moreover, the stunted seed tends to accumulate more lean body mass, which additionally increase the feed utilization efficiency of stunted fish seed. Under normal feeding conditions, stunted fish express increased feed efficiency and protein efficiency (Ali et al . 2003, Won and Borski 2013). Feed typically represents 60-80 percent of operational costs. Compensatory growth induced by restricted feeding is considered to be an economic feeding strategy by fish farmers. In addition to normal practices, removal of weak and sick seed during stunted seed production leads to better survival of stunted seed at grow-out culture than normal seed. Physiological Responses of Stunted Fish Seed in Grow-out Ponds Stunted seed exhibits various physiological responses when they shift from unfavorable to favorable conditions. The important physiological responses are changes in blood metabolites, immune system, digestive enzymes and antioxidant enzymes. In general, stunted fish have better hematological, immune and digestive response in the refeeding phase. The immune system is influenced by the nutritional status of fish. Greater amounts of globulin (an immunoglobulin) in stunted fish helps in antibodies production, which enhance immune defenses of fish (Yengkokpam et al . 2016). Among physiological processes, immune cells production and maturation demands more energy. Commencement of feeding at grow-out culture increases nutrient availability for stunted fish which enhances immune cell production (Kiron 2012). Digestion plays a major role in feed utilization and determines the amount of energy available for all biological functions. Elevated digestive enzyme activities during the compensatory growth phase increases feed utilization (Yengkokpam et al . 2013). In general, increased digestive enzymes help in better digestion of feed which directly accelerates the growth of stunted fish. Stunted fish utilizes its endogenous energy reserves —muscle lipid and protein — in the restricted feeding phase. In general, Milkfish stunted for eight months showing compensatory growth after refeeding. Under stressed conditions — the stunting phase — weak and sick fry from ponds are eliminated, thus leading to the production of healthy and slightly larger yearling seed as stocking material for grow-out. The compensatory growth response in stunted seed is related to greater appetite and feed consumption. Compensatory growth response is identified by high growth rate, better feed utilization, hyperplasia and greater mitogen production.