Effects of lecithin and cholesterol supplementation to practical diets for Litopenaeus vannamei reared in low salinity waters

The culture of shrimp in inland low salinity waters is a developing industry in many regions of the world, including west Alabama. These inland low salinity waters are often deficient in key ions necessary for normal physiological function. In west Alabama, farmers normally remedy ionic deficiencies in the water profile through the addition of fertilizers containing K+ and Mg2+. It has been suggested that increasing phospholipids (lecithin) and cholesterol in excess of dietary requirement improve osmoregulatory capacity in Litopenaeus vannamei, thus leading to better survival and growth under low salinity conditions. Cholesterol is an essential sterol involved in the molting process in shrimp. Phospholipids are important in cholesterol transport, facilitate the storage of lipids in the hepatopancreas, an important energy reserve during the molting process and are an important component of cell membranes. In order to investigate the possibility of improving growth and survival under stressful (i.e. low K+ and Mg2+) rearing conditions, a series of lab and on-farm experiments were conducted. Two separate 35 day laboratory studies were conducted in reconstituted low salinity (4.0 ppt, low K+) waters. In both trials, five practical diets were formulated to contain 36% protein and 8% lipid, and supplemented with varying levels of cholesterol and lecithin. Three of these diets were utilized for an additional experiment carried out on-site at two different low salinity shrimp farms in west Alabama. Results from the lab trials indicated no significant differences in survival, growth, or percent weight gain among treatments. Survival, final weight, and percent weight gain ranged from 68% to 77%, 2.70-3.0 g, 415-471% in experiment 1, and 56-69%, 2.7-3.2 g, 1572-1913% in experiment 2. These results indicate that the shrimp were stressed in both experiments, and there were no apparent benefits to supplementing lecithin and cholesterol in excess of the dietary requirement. Two on farm trials were conducted in parallel using either a mediated water source (Farm 1) to produce low stress or waters. At farm 1, survival, final weight, percent weight gain, and FCR ranged from 93.8% to 98.8%, 4.48-5.23 g, 4273-4901%, and 1.79-2.06, respectively. At farm 2 shrimp had lower survival (37.5-47.5%), lower final weight (2.65-3.25 g), lower percent weight gain (2342-3088%), and higher FCRs (6.85-10.64). No benefits from lecithin and cholesterol supplementation in excess of the dietary requirement were observed when compared to the basal diet under any test conditions. Based on results of the present study, dietary supplementation of cholesterol and phospholipids in excess of the requirement is not warranted for L. vannamei reared in low salinity waters.