Effects of phytogenic compounds on growth and nutritional physiology of Nile tilapia (Oreochromis niloticus)

Abstract

With increasing world population, the demand for fish is growing thus there is a need to identify products with potential to increase the efficiency of fish production. Phytogenics are among the products being investigated as potential naturally derived growth promoters. The aim of this study was to identify phytogenic compounds and doses with growth-promoting effects in Nile tilapia and investigate relevant pathways underlying their growth promotion effects. The phytogenic compounds limonene, carvacrol and thymol, major constituents of essential oils from the plants citrus, oregano and thyme, respectively, were evaluated. Six Trials (Trials I, II, III, IV, V and VI) were carried out using diets supplemented with varying concentrations of the phytogenic compounds. In Trials I, II and III (Chapter 3), the effects of either limonene (Trial I), carvacrol (Trial II) or thymol (Trial III) on growth performance of Nile tilapia were investigated (objective 1) and performance parameters including final fish weight, daily growth coefficient, growth rate per metabolic body weight, percentage (%) weight gain, % survival, feed intake, feed conversion ratio and protein efficiency ratio were evaluated. Results from Trials I, II and III indicated that dietary supplementation of 400 and 500 ppm limonene and 750 ppm thymol had growth-promoting effects in Nile tilapia but the somatic growth was not associated with enhanced feed intake and feed utilisation efficiency. Trials IV and V (Chapter 4) investigated growth and nutritional physiology pathways in Nile tilapia regulated by individual phytogenic compounds (objective 2). This was accomplished by analysing the effects of limonene (Trial IV) and thymol (Trial V) supplemented diets on the expression of key genes participating in selected pathways of somatotropic axis-mediated growth, appetite regulation, nutrient digestion, absorption and transport, lipid metabolism, and antioxidant enzyme defence system. Limonene was supplemented in the diet at 0, 200, 400 and 600 ppm while thymol was supplemented at 0, 250 and 500 ppm. Trials IV and V found that growth-promoting effects of limonene (400 and 600 ppm) in Nile tilapia involved up-regulation of key genes within pathways including somatotropic axis-mediated growth, nutrient digestion, absorption and transport, lipid metabolism and antioxidant enzyme defence system. Dietary thymol at 250 and 500 ppm did not significantly enhance growth of Nile tilapia nor regulate the nutritional physiology pathways listed above. In Trial VI (Chapter 5), the effects of combined phytogenic compounds (limonene and thymol) on growth and nutritional physiology of Nile tilapia was tested (objective 3) to establish if the compounds had synergistic or additive effects on the growth of the fish as well as complementary effects on the selected nutritional physiology pathways. A candidate gene approach was also used for the selected pathways. Results from Trial VI showed that a diet supplemented with a combination of limonene (400 ppm) and thymol (500 ppm) has neither synergistic nor additive effects on the growth performance of Nile tilapia, with limonene mainly influencing the attained somatic growth. The analysed candidate genes involved in the pathways of nutrient digestion, absorption and transport, lipid metabolism, antioxidant enzymes and somatotropic axis growth also showed no synergistic or additive effects of a dietary combination of limonene and thymol in Nile tilapia. Overall, results from the study suggest approaches for developing functional diets for Nile tilapia using limonene and thymol growth promoters.