Perera, Mallika Arachchilage D. A. D. creator Bhujel, Ram C. Chairperson Datta, Avishek Examination Committee Loc Thai Nguyen Examination committee Biomin (Test of Aqua Feed Supplement) Project Scholarship Donor text th Bangkok Asian Institute of Technology 2021 continuing eng 144 leaves : ill. Nile tilapia (Oreochromis niloticus) was chosen for the dissertation research as it has become an increasingly important farmed fish globally in terms of food and nutrition security of the people, potential of job creation and income generation. Farming of tilapia is expanding due mainly to the technique of producing mono-sex fry using hormonal sex-reversal which is done using the standard feed containing 60 mg 17u- methyl testosterone (MT) per kg of fishmeal mixed with 10-20 g of Vitamin C (L- ascorbic acid) or a mixture of all vitamins. However, use of fishmeal in aquaculture has been questioned due to its sustainability and high price. Similarly, use of MT has also been an issue in some countries. Therefore, there was a need to explore their alternatives with cost-effective and safe sex reversal feed. A series of three experiments were conducted to generate the data to support formulated hypotheses as a part of PhD dissertation. The first experiment (Chapter IV) was carried out to test field cricket (Gryllus bimaculatus) meal (FCM) as an alternative to fishmeal in the diet of the Nile tilapia fry using 18 aquaria (100 L) stocked with 300 swim-up fry (8.0±0.1 mg) each. Six diets were formulated replacing fish meal by 0, 20, 40, 60, 80 and 100% field cricket meal, maintaining the same crude protein level in all the diets (Avg 55.7±0.2% crude protein) to determine the optimum level in terms of survival, sex reversal, growth, and feed utilization. Results showed that there was no effect of replacement on sex-reversal efficiency. The average percentage of males remained high (range 97.2-98.6%). Most parameters showed improvement in the fishrneal replacement level indicatin,g up to 80% is possible. Quadratic models demonstrated that apparent digestibility % of protein and lipid were maximum at 76% and 90% ofFCM inclusion, respectively. Overall, the clear benefits of FCM inclusion started right from 20% replacement and declines only after 80%. In the second experiment (Chapter V), the best combination ofFCM (approx. 80%) and fishmeal (approx. 20%) determined by the first study was chosen to use as control, and five other diets were prepared by supplementing 10, 20, 30 40 and 50 g of vitamin C (L-ascorbic acid per kg diet) to determine its requirement as this vitamin is required to enhance immunity in the young fry. At the end of the feeding trial, gonad histology v showed an increasing number of spermatogonia with the increase in L-ascorbic was from the doses of 10 to 30g/kg diet but decreased thereafter. Results also indicated a significant increase (P<0.05) in growth, feed utilization and survival when fed with vitamin C at the dose of 10 and 20 g/kg diet during sex -reversal and nursing periods as compared to the control. Quadratic relationship showed the survival peaked when the dose was at 15 g/kg diet. As the survival of fry is the most important parameter at these stages, the dose which resulted in the highest survival is recommended. The third experiment (Chapter VI) was to use catfish testis, goat testis and bull urine as potential alternatives to the steroid hormone i.e. 17a methyl testosterone using the feed formulated, developed based on the first and second studies i.e. SO% FCM supplemented with vitamin C diets with MT and without MT (control). Results showed that the fry treated with bull urine+MT (9S.4±0.5%, T3), catfish testis+50%MT (96.6±1.0%, T5), goat testis+50%MT (94.S±0.4%, T4) and catfish testis (SS.1±O.1 %), had significantly higher (P<0.05) % of male as compared to control (5S.4±1.9). Results showed that while combining with 50% MT, dried goat testes and catfish testes can produce a reasonably high percentage of male indicating that 50% MT can be saved. Similarly, it showed that bull urine can replace 50% alcohol when the standard dose of MT (60mg/kg) is used dissolving in 50% alcohol. The present study concludes that FCM is good, but to replace only up to SO%fishmeal in the diets of young tilapia. Sex-reversal efficiency, growth and feed utilization parameters improved when FCM replaced fishmeal from 20 to SO%. However, complete replacement (100%) was detrimental. Field cricket meal (SO% FCM) requires 1 0-20g vitamin C/kg diet to improve fry survival. The best dose of vitamin C was found to be 15 g/kg diet. However, higher doses of vitamin C were not beneficial for mono- sex tilapia fry. The diet with SO% FCM and 20% fishmeal supplemented with vitamin C diet, bull urine was found to be equally effective when 50% alcohol was replaced indicating some cost saving. Reduction of 50% MT when combined with dried catfish and goat testes can result in a reasonably high percentage of males, indicating that 50% MT hormone as well as the cost could be saved. A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Aquaculture and Aquatic Resources Management Thesis (Ph.D.) - Asian Institute of Technology, 2021 Nile tilapia Fish-meal Fishery products Asian Institute of Technology. http://203.159.5.9/ait-thesis/detail.php?q=B15913 http://203.159.5.9/ait-thesis/detail.php?q=B15913 220228 20260817162151.0