Relative Contribution Of Rice Bran And Inorganic Fertilisers In Semi-intensive Tilapia (oreochromis Niloticus) And Catfish (clarias Gariepinus)polyculture In Kenya
Aquaculture development in Kenya has been hampered by lack of appropriate feeds. Available chemical fertilisers can enhance natural food production and thus indirectly provide protein to complement energy-rich rice bran. Consequently, a 20 week experiment was conducted at Sagana Fish Farm, Kenya to realise least-cost combinations of rice bran and fertiliser. Twelve 800m2 ponds were stocked with juvenile (32g) Oreochromis niloticus at 2 m-2 and Clarias gariepinus fingerlings (4.6g) at 0.2m-2. Four treatments were applied in triplicates as follows: 1. Urea and DAP to provide 16KgNha-'week-' and 4KgPha-'week-'; 2. Urea and DAP applied to give 8KgNha-'week' and 2KgPha-'week', plus rice bran fed at 60Kgha-'day"'; 3. Rice bran fed at 120Kgha-'day-'; 4. Rice bran as in treatment 3 and fertiliser as in treatment 2. Dissolved oxygen, temperature and pH were measured weekly in the morning and the afternoon while total alkalinity and total ammonia nitrogen were measured fortnightly. Ponds were sampled monthly to measure fish growth, and drained completely after 20 weeks. At harvest, average weight of tilapia was 89, 106, 106 and BIg while that of Clarias was 110,217,236 and 295g for treatments 1 through 4 respectively. Specific growth rate (SGR%d-') for tilapia was 0.6,0.8, 1.0 and 1.0 while that for Clarias was 2.26, 2.69, 2.72 and 2.90 for treatments 1 through 4 respectively. Combined Net fish yield (NFY Kgha-') was 1,127, 1,582, 1,607 and 2,098 for treatments 1 through 4 respectively. The tilap·r;-N~t fish yield (NFY Kgha-') was 894.0, 1200.5, 1241.8 and 1576.5 while that of Clarias was 232,"365, 382 and 522 for treatments 1 through 4 respectively. Combined Annual production (AP. Kghalyr".) was 2930 4113, 4179 and 5455 for treatments 1 through 4 respectively. The tilapia Annual production (AP Kghayr".) was 2,324.5, 3,121.1, 3,228.7 and 4,098.7 while that of Clarias was 605, 950, 993, and 1,357 for treatments 1 through 4 respectively. Apparent feed conversion ratio (AFCR) was 5.3, 10.3 and 8.0 for treatments 2 through 4 respectively. Feeding efficiency (FE) was 0.19, 0.10 and 0.13 for treatment 2 through 4 respectively. The Relative Condition Factor (Kn ) for tilapia was 1.03, 1.11, 1.13 and 1.15 while that for Clarias was 1.20, 1.34, 1.38 and 1.38 for treatments 1 through 4 respectively. A partial economics analysis indicated a Net profit (KSh) of 18,851, 9,895, 3,299 and 7,015 for treatments 1 through 4 respectively. Although treatment 4 had the highest SGR, NFY and AP and the second highest AFCR and FE, its high net expenditure was a serious setback since not many rural fish farmers can afford such an amount of recurrent costs. Even if they could, high unit production costs could jeopardise sales. Treatment 3 had the lowest net profit and was thus the worst treatment in terms of profit margins. This was due to high amount of rice bran inputs. Treatment 2 had the third highest SGR, NFY, AP and AFCR and the second highest Net profit. It had the highest FE while its figures for SGR, NFY and AP compare favourably and closely with those of treatment 3. Probable reasons are that its high FE gives it a higher output (fish production). Feeding efficiency (FE) increases with better diet and we can conclude that treatment 2 was the best diet. Treatment 1 had the highest Net profit but its growth rate began to level off at days 63 and 30 for both tilapia and Clarias which means that fish in treatment 1 may not have reached market sizes of 250-300g for tilapia and 500- 1000g for Clarias. A combination of fertiliser as in treatment 1 and rice bran as in treatment 2 could possibly have produced better results where Nand P could have been similar to those of treatment 4 but at much lower recurrent costs.