Investigations into the polyculture of two Indian major carps (Labeo rohita and Cirrhina mrigala) and Nile tilapia (Oreochromis niloticus) in fertilized ponds | |
| Author | Hossain, Md. Amjad |
| Call Number | AIT Diss. no.AE-95-04 |
| Subject(s) | Fish-culture Carps Nile tilapia |
| Note | A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Technical Science |
| Publisher | Asian Institute of Technology |
| Abstract | Polyculture of two Indian major carps (rohu, Labeo rohita and mrigal, Cirrhina mrigala) and Nile tilapia (Oreochromis niloticus) was investigated in a series of four sequential experiments on AIT campus. The effects of fertilization level and supplementary feed on various species combinations were first compared. The impacts of simulated tilapia recruitment and the prolonged stunting of carp fingerlings on growth of the two Major carps were subsequently investigated. The experiments were conducted in forty, 20m2 pens fixed in four 400m2 earthen ponds receiving a range of inorganic fertilization (1.5-3.0 kg N/ha/d) at a standard ratio of nitrogen to phosphorous (N:P = 2). The growth performance of Nile tilapia, rohu and mrigal in monoculture and polyculture, and their interactions in two and three species polycultures, were compared in an initial experiment. Treatments were compared at isodensities of 9000 fish/ha. Growth of all species was significantly different in monoculture and the individual growth of tilapia was significantly faster (p=<0.05) than rohu and mrigal (mean DWG over a range of fertilizer levels of 2.32, 1.85 and 1.69 g/fish/d respectively). Species combinations containing tilapia performed best in two species-polycultures. The rational of polyculture, the reduction in intraspecific competition, was demonstrated by reduced and increased individual growth rates of tilapia and the carps respectively as the ratio of tilapia in a three species-polyculture increased. The highest net fish yield of 3.4 g/m2/day was obtained in a tilapia-dominated polyculture (ratio 4:1:1; tilapia: rohu: mrigal) at a fertilization level of 2.5 kg N/ha/d. Mean weight gains of the three experimental fish were positively related to phytoplankton biomass (Tilapia: Y = 18.02 + 37.95x, r2 = 0.999; rohu: Y = 163.8 + 10.38x, r2 = 0.977 and mrigal: Y = 108.24 + 15.82, r2 = 0.971). Financial analysis showed the considerable risk to farmers if ponds do not respond predictably to high levels of fertilization. Negative returns were indicated for tilapia dominated-polyculture and monoculture in ponds responding poorly to inorganic fertilization. The effect of supplementary feed and four levels of fertilizer on the growth of Nile tilapia, rohu and mrigal at different levels of tilapia dominance within the polyculture were compared in a second experiment. Inorganic fertilizer had a similar impact on fish yields as supplementary feeding; a doubling in fertilizer (1.5 kg N/ha/d to 3 kg N/ha/d) increased mean net extrapolated yields by over 60% whereas feeding increased NFYs by between 33- 67% over unfed. Most of this increase was lower value tilapia at lower levels of natural feed, supplementary feed had greater impact on the growth of all the experimental fish. Supplementary feed did not have any significant impact on fish growth at lower densities of tilapia when fertilizer input was high but at higher densities some impact of feeding was observed, presumably due to more competition for natural food. A mean rate of supplementary feeding of 1.8% body weight/day over the experimental period increased fish yields by a mean of 29.5% but this did not cover the cost of feeding. The highest net returns were obtained in a highly fertilized, but unfed, polyculture in which tilapia constituted 60-70% of the fish stocked. The impact of tilapia recruitment on the individual growth of rohu and mrigal was simulated by stocking a range of MT-treated tilapia fingerlings with low densities of carps in pens over a range of fertilizer levels, with and without supplementary feeding. At low fertilizer levels tilapia competed with, and significantly reduced the individual growth of, rohu and mrigal for natural food at all recruitment levels and there was no significant impact of supplementary feed. At higher fertilizer input levels, densities of up to 5 recruits/m2 did not significantly affect the growth of rohu when given supplementary feed, or 5-10 recruits/m2 when unfed. Tilapia recruitment had no impact on the growth of mrigal in both fed and unfed systems at high levels of fertilizer use and even levels of recruitment found in normal mixed-sex tilapia culture (10 fish/m2) still allowed rohu to maintain growth rates of more than 2 g/fish/day. Monoculture of Indian major carps at low density was not profitable at even low fertilizer inputs. Recruitment of tilapia, however, negatively affected gross margins and the use of feeds worsened the situation. Appropriate use of inorganics at higher levels was the most practical and profitable method to overcome the problem. In Bangladesh, almost 50% of farmers interviewed in a survey of 100 households valued tilapia for their ability to produce seed on-farm. Farmers preferred tilapias for culture above any of the other introduced species; Indian Major carps were preferred most for both production and consumption over native catfish or any of the exotic species. Consumption preference was highly affected by size with large individual (> 1kg) riverine carps (Indian major and Chinese carps) being most marketable whereas catfish, silver barb and tilapia were favored at sizes of less than 300g. This has important repercussions for short season culture and suitable strategies for stocking seasonal culture systems. The stocking of overwintered fingerlings is a common practice in Bangladesh and the impacts of older seed in polycultures with tilapia was also investigated. The effect of carp nursing duration on their subsequent individual growth performance in polyculture with sex-reversed Nile tilapia was compared at four levels of fertilization. A longer period of nursing, or ‘stunting', increased subsequent mean carp growth (12 months, 2.3 g/fish/d;, 5 months 1.9 g/fish/d and 3 months 1.6 g/fish/d respectively). Carps grew relatively better than tilapia in less fertile systems. Stunting of fingerlings improved net returns of grow-out in well fertilized ponds by between 29% (5 month) and 54% (12 month) over younger fish (3 months). |
| Year | 1995 |
| Type | Dissertation |
| School | School of Environment, Resources, and Development (SERD) |
| Department | Department of Food, Agriculture and Natural Resources (Former title: Department of Food Agriculture, and BioResources (DFAB)) |
| Academic Program/FoS | Agricultural and Food Engineering (AE) |
| Chairperson(s) | Little, David; |
| Examination Committee(s) | Edwards, P.;Lin, C. Kwei;Demaine, Harvey; |
| Scholarship Donor(s) | Government of Bangladesh (IDA-Credit 1455 BD); |
| Degree | Thesis (Ph.D.) - Asian Institute of Technology, 1995 |