Contreras Sánchez and Mwau Defend

Sex differentiation in fish is a labile process that allows sex inversion in several species. The inherent capacity of fish germ cells to differentiate into oocytes or spermatocytes constitutes a key factor allowing for functional sex inversion. This thesis set out to determine the mechanism involved in steroid-induced sex differentiation of Nile tilapia, Oreochromis niloticus, by searching for differential expression of unique genes during the process. In addition, the studies documented the persistence of methyltestosterone (MT) in the environment after oral administration, and investigated the capabilities of short-term immersions in steroids for masculinizing tilapia fry as an alternative method. A significant leakage of MT to the rearing water and its subsequent accumulation in the sediments was detected after oral administration of the steroid. In addition, evidence was found for a significant effect of environmental conditions on the masculinizing efficacy of oral administration of steroids. Low levels of masculinization were obtained when MT was allowed to remain in the system. Results from short-term immersions in steroids indicated that the labile period for masculinization by immersion of Nile tilapia fry reared at 28°C occurs between 11 and 16 days post fertilization (dpf). Two three-hour immersions in trenbolone acetate at 11 and 13 dpf yielded the highest number of males. Time of immersion, length of the treatment, dosage, density, solvent vehicle, and number of immersions significantly affected the outcome of immersion trials. Heterogeneity of developmental stages, developmental rate, and sensitivity of progeny to steroids may play important roles in the efficacy of immersion treatments. The expression of unique mRNAs during the process of sex inversion was identified through the use of suppression subtractive hybridization. This technique allowed for the sequencing of 165 clones from which 61 proteins have been identified. A significant number of these genes seem related to the anabolic effects of trenbolone acetate. In addition, 12 genes were identified that are related to reproductive tissues; seven of which have unique or enriched expression in the testes. Some of the genes and protein products that have been identified are linked to gonadal development and testicular protein synthesis in other species.

Nutrient dynamics, with special reference to nitrogen and phosphorus in tilapia (Oreochromis niloticus)/catfish (Clarias gariepinus) polyculture ponds at Sagana Fish Farm, Central Kenya

Budgets for nitrogen and phosphorus were estimated over a growing period from Oct. 1997 to March 1998 in 12 research fish ponds randomly assigned four treatments in triplicates to assess the efficiency of utilization in these nutrients. In treatment 1, urea and DAP were added to provide 16kg N/ha/week and 4kg P/ha/week, in treatment 2 Urea and DAP were added to give 8kg N and 2kg P/ha/week plus rice bran feed at 60kg/ha/day. Treatment 3 was rice bran only at 120kg/ha/day, and treatment 4 was rice bran as in 3 and inorganic fertilizer as in 2. Each pond was stocked with 1,600 tilapia (Oreochromis niloticus) fingerlings averaging 32g body weight and 160 African catfish (Clarius gariepinus) fingerlings of 4g body weight. All inputs and outputs to and from the ponds were sampled and analyzed for nitrogen and phosphorus. Water column samples were taken monthly to monitor the nitrogen and phosphorus forms in the water column. Soil samples were also taken monthly to monitor the variation of the nutrients content of the soil over the experimental period. Nitrogen and phosphorus content in the water supply canal were monitored within this period and the quality of the effluent water discharged assessed.

Results indicated that, in the water column total nitrogen and phosphorus concentrations increased with time while the dissolved inorganic nutrients (soluble reactive phosphorus, nitrates, nitrites and total ammonia nitrogen) remained at relatively low levels despite weekly fertilization and daily feed inputs, an indication that these nutrients were taken up by phytoplankton and incorporated into the organic forms in the water column. Nitrogen and phosphorus concentration in the soil increased with time indicating nitrogen and phosphorus retention by bottom sediments. Fish harvest accounted for 11.2 to 18.7% of the nitrogen inputs and 6.2 to 9.1% of the phosphorus inputs. Soil adsorption accounted for 61.5 to 69.2% of the nitrogen inputs and 81.7 to 90.8% of the phosphorus inputs. Drainage and outflow accounted for 9 to 16.1% of the nitrogen inputs and 1.8 to 4.2% of the phosphorus inputs while seepage accounted for 3.2 to 8.3% of the nitrogen inputs and 0.7 to 2.4% of the phosphorus inputs.

Rain and inflow water from the supply canal are not of great importance to semi-intensive fish farming as nutrient (N & P) input sources. Adsorption by soil was a major nutrient sink especially for phosphorus. Uptake of nitrogen and phosphorus by phytoplankton with their subsequent conversion to nitrogen and phosphorus in fish flesh was the second major loss of nitrogen and phosphorus added to the ponds. Concentrations of nitrogen and phosphorus in effluent discharged from ponds would have a very low fertilization potential in the receiving waters.