small-scale farmers.
The aim of this activity was to examine the growth rates of GIFT and Chitralada strains of Nile tilapia to be used as seed stock for introduction. The second goal was to test culture regimes using fertilization only and fertilization with delayed feeding on growth rates of Nile tilapia in southern Nepal. Moreover, temperature in this sub-tropical region drops below 19¼C during the winter in December and January, possibly preventing growth of tilapia. Since overwinter conditions affect culture systems for tilapia, stocking in winter (December) was compared against stocking during the summer (August) to determine the optimal stocking period to achieve marketable-size fish (100 g) within four to five months of culture.
Methods and Materials
Two consecutive trials were conducted during this study period: 1) Comparisons between fertilization and fertilization only and feeding, and 2) comparisons between GIFT (6th generation select) and Chitralada strains. The three-week-old Chitralada strain from AIT and the GIFT-select strain acquired from NAGRI-Department of Fisheries, Thailand, were air transported to Nepal. They were conditioned in hapas for six days prior to stocking. Two ponds of 200 m2 each were partitioned into six equal compartments with bamboo frames and nylon screen nets. Spring water was pumped into the ponds after drying and liming. Ponds were fertilized with urea ammonium sulfate at a rate of 28 kg nitrogen ha-1 until green. This rate was then applied weekly to both ponds throughout the experiment. Mixed-sex fingerlings (30 d old) were stocked in respective ponds at 3 fish m-2.
Experimental Trial 1 (Winter Stocking)
Six experimental units were stocked during the winter
(14 December 2001) with 200 fish (Chitralada strain) each. Some mortality was observed on the second day of stocking. Dead fingerlings were removed and replaced with back-up stock. After 80 days of stocking, three of six randomly selected units were fed a prepared diet consisting of 67% rice bran and 33% fishmeal at 3% BWD. Every two weeks a sample of 50 fish was removed from each compartment to determine average weights.
Temperature was measured twice daily (at 0600 and 1600h), and other water quality parameters were monitored monthly. Dissolved oxygen, pH, ammonia, nitrite, and total hardness were measured at the National Agricultural Research Council (NARC) station laboratory following standard methods (APHA, 1985; Egna et al., 1987).
Fish were harvested after 190 d by seining and draining. Fish were individually counted and weighed. Final mean weight and survival rates were calculated. Total weight of recruitment was also determined. Significant differences in size at harvest among treatments were determined using a t-test
( = 0.05).
Experimental Trial 2 (Summer Stocking)
Ponds were drained and dried following harvest of the first experimental trial. During the summer (8 August), the same six experimental units were divided into two groups of three and stocked with Chitralada strain fingerlings from Experiment 1 (n = 200) in one group and GIFT-select strain fingerlings from Experiment 1 in the second group. Only fertilization was applied at the rate described earlier. Every month a sample of 50 fish was removed from each compartment to determine average weights.
