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¹ Federal University Oye, Faculty name, Department of Fisheries and Aquaculture, Oye Ekiti/ Ekiti State, Nigeria DOI : 10.4194/AQUAST1461 Viewed : 299 - Downloaded : 207 Select this text, right click and click Merge Formatting (B) from the paste options. Two hundred (200) Coptodon zillii and Oreochromis niloticus (10g ±0.13) were subjected to an investigation to uncover the efficiency of crude and synthetic gossypol as reproduction bio-control agents. The treatment groups were five (5) altogether with twenty (20) fish per treatment (male and female 1:1). Each treatment was replicated twice and subjected to the same condition. The fish were fed with five isoproteic experimental diets of 37.5% crude protein for 90 days. In the diets, cottonseed meal (CSM) and synthetic gossypol (GSP) were used to replace soyabean meal (SBM) at 0%, 25%, 50%, 75% and 100% respectively. Histological sections of testes in O. niloticus and C. zillii showed a decline in spermatozoa with increasing inclusion of crude CSM and synthetic gossypol. Diets four and five which has the highest inclusion of both crude and synthetic gossypol (327g CSM/Kg diet, 436g CSM/Kg diet) exhibited spent and empty seminiferous lumina. Ovarian section in C. zillii and O. niloticus showed distortion with increasing inclusion rates of crude CSM and synthetic gossypol; to the point that most of the eggs were still in their early phases of embryonic formation after 90 days trial with prominent vitellogenic stages for 0% 0g (CSM/kg diet) and 25% (109 CSM/kg diet); atretic oocytes for 75% (327g CSM/Kg diet) and 100% (436g CSM/Kg diet). Milt and egg production of both O. niloticus and C. zillii all declined as the dietary inclusion of crude and synthetic gossypol increased. Overall results showed that dietary crude and synthetic gossypol destroyed the spermatocytes and oocytes; lumina was devoid of spermatozoa, subsequent reproduction was inhibited; the eggs were unable to finish their cytological cycles which would have led to fertilization. There were fewer offspring in crude gossypol-based diet treatments and there was no reproduction at all in all the synthetic gossypol-based diet treatments. Keywords : Gossypol Cottonseed Bio-control Thin layer chromatography Colony forming unit Reproduction inhibitor