MATERIALS AND METHODS

The experiment was conducted for a period of 90 days from July to October 2004 in the earthen ponds each measuring 40 m² (1 decimal) area at the south-east corner of the Faculty of Fisheries Building, Bangladesh Agricultural University, Mymensingh. Before fish stocking water of the experimental ponds were drained out to eradicate all the undesirable fishes, renovated and liming was done in all the ponds at the rate of 1 kg decimal^-1. Ponds were filled up with DTW water and fertilized at the rate of poultry dropping 10 kg decimal^-1, urea 100 g decimal^-1 and TSP 100 g decimal^-1 as initial doses.

The experiment had two treatments each with three replications. In treatment-1 ponds were supplied with duckweed as supplementary fish feed and in treatment-2 ponds were kept as control (without supply of duckweed). Ponds were stocked at a stocking density of 80 fingerlings of tilapia per decimal. The initial average length and weight of tilapia were 10 cm and 30 g, respectively. The ponds were fertilized fortnightly with poultry dropping at the rate of 5 kg decimal^-1, urea 60 g decimal^-1 and TSP 90 g decimal^-1. Duckweeds were supplied to the fish culture ponds (treatment-1) at the rate of 60% of the total body weight (wet weight basis) of the fish every day.

Water Quality Parameters
Various physical, chemical and biological water quality parameters of ponds water such as water temperature (°C), transparency (cm), dissolved oxygen (mg L^-1), pH, free CO₂ (mg L^-1), total alkalinity (mg L^-1), PO₄-P (mg L^-1), NO₃-N (mg L^-1), phytoplankton density (cells L^-1) and zooplankton density (cells L^-1) were estimated fortnightly. Water temperature was recorded with a Celsius thermometer and transparency was measured with a secchi disc of 30 cm diameter. Dissolved oxygen was measured directly with a DO meter (Lutron, DO-5509) and a digital pH meter (CORNING pH meter 445) was used to measure pH. Free CO₂ and total alkalinity were determined by titrimetric method (APHA, 1981). PO₄-P (mg L^-1) and NO₃ N (mg L^-1) were determined by a Hach Kit (DR/2010, a direct reading Spectrophotometer). The counting of plankton (both phytoplankton and zooplankton) was done with the help of Sedgwick-Rafter Counting Cell (S-R cell) under a compound binocular microscope. The plankton population was determined by using the formula of Rahman (1992). Identification of plankton (phytoplankton and zooplankton) up to generic level were made according to Needham and Needham (1963) and Belcher and Swale (1978).

Survival, Growth and Production of Fish
Fish samples were collected with a cast net monthly to estimate the growth in length (cm) and in weight (g) and to check up the health condition of fish. At the end of the experiment, all fish were harvested through repeated netting by seine net.

The survival rate was estimated by the following formula:

Specific growth rate (SGR, % per day) was estimated by the following formula:

Where,

Statistical Analysis
T-test of net fish production of the ponds under T₁ and T₂ was done by a computer using SPSS package programme.

RESULTS

Water Quality Parameters
Throughout the study period, a number of physical, chemical and biological water quality parameters of the ponds such as water temperature (°C), transparency (cm), dissolved oxygen (mg L^-1), pH, free CO₂ (mg L^-1), total alkalinity (mg L^-1), PO₄-P (mg L^-1), NO₃-N (mg L^-1), phytoplankton density (cells L^-1) and zooplankton density (cells L^-1) were determined. The results of water quality parameters are shown in Table 1. All physical and chemical parameters of the ponds water were found to be within the acceptable ranges for the fish culture. The generic status of phytoplankton and zooplankton found during the tenure of experiment are shown in Table 2.

Table 1:	Water quality parameters (X±SD) of the experimental ponds during the experimental period

Table 2:	Generic status of phytoplankton and zooplankton found in the experimental ponds

Table 3:	Survival rate, growth and production (gross and net) of fish under T1 and T2
*: Treatment-2 was taken for 100%

Survival, Growth and Production of Fish
The details of survival rate, growth and production of fish are presented in Table 3. The survival rate of Oreochromis niloticus in T₁ and T₂ were 94.37 and 93.75%, respectively. In T1 SGR value recorded was 1.16% and in T2 SGR value recorded was 0.80%. The gross and net productions of fish of the ponds under treatment-1 were 25.88 and 16.28 kg dec.^-1 year^-1 and those of the ponds under treatment-2 were 18.52 and 8.92 kg dec.^-1 year^-1, respectively.

DISCUSSION

Water Quality Parameters
The physical and chemical water quality parameters of the ponds such as water temperature (°C), transparency (cm), dissolved oxygen (mg L^-1), pH, free CO₂ (mg L^-1), total alkalinity (mg L^-1), PO₄-P (mg L^-1) and NO₃-N (mg L^-1) of the ponds water were found to be within the acceptable ranges for fish culture and there was no abrupt change in any parameter of the pond water during the tenure of experiment (Table 1). The results were more or less similar to the findings of Wahab et al. (1995) and Kohinoor et al. (1998). Plankton (phytoplankton and zooplankton) population in number and genera were more or less similar to the findings of Tasneem (1998), Rashid (1999) and Israfil (2000).

Survival, Growth and Production of Fish
The survival rate of Oreochromis niloticus in T1 and T2 were 94.37 and 93.75%, respectively (Table 3). The survival rate was high because the initial length of fish was about 10 cm and tilapia can tolerate even a bad environmental condition. Kohinoor et al. (1993) obtained a survival rate of 86 to 94% in the monoculture of Thai sharpunti. In T1 SGR value recorded was 1.16% and in T2 SGR value recorded was 0.80% (Table 3). SGR value in T1 was higher than that in T2. SGR values obtained in the present study are similar to that obtained by Hossain et al. (1997). It can be said that the higher specific growth rate in T1 was due to use of duckweed as a supplementary feed for nile tilapia (O. niloticus). The gross and net productions of fish of the ponds under T1 were 25.88 and 16.28 kg dec.^-1 year^-1 and those of the ponds under T2 were 18.52 and 8.92 kg dec.^-1 year^-1, respectively (Table 3). Percent increase of net production of fish of T1 over T2 was 182.51%. Kohinoor et al. (1999) observed the effectiveness of duckweed as low cost supplementary feed through 6 months production trial of Thai sharpunti. Bornali (2004) found 4.99 ton/ha/year net production of tilapia (O. niloticus) where fresh duckweed was supplemental feed and the production was significantly higher in pond with supply of duckweed than that of the ponds without supply of duckweed.

Most of the water quality parameters of the ponds under T1 and T2 were more or less similar but the higher production of fish was recorded in T1. The reason behind the higher production in T1 was due to supply of duckweed as supplementary feed. T-test of net productions shows that difference between T1 and T2 is statistically significant (p<5%) i.e., influence of duckweed on production of fish is positively significant. Finally, it can be concluded that duckweed might be used as one of the most preferable supplementary feed items for nile tilapia (O. niloticus).