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. Phytogenies are among the products being investigated as potential naturally derived growth promoters
The aim of this study was to identify phytogcnic compounds and doses with growthpromoting
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 growthpromoting
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