Nitrogen balance of intensive snakehead fish culture in earthen ponds under different water exchange frequencies: a case study of Ang Thong fisheries and processing cooperative
Abstract:
Background and Objectives : An intensive snakehead fish culture in earthen ponds in Ang Thong province are reared without any aeration system. Waste accumulation occurs according to the culture period. Therefore, the water must be exchanged with different volumes and frequencies. The discharged effluent is usually high in nitrogen compounds, especially ammonia and nitrate. These affected the water quality in natural water resources and the environment. Due to the lack of information on the optimal water exchange frequency for fish growth, in this study, the effect of 3 different water exchange frequencies on the total yield of snakehead fish and nitrogen balance changes in intensive snakehead fish culture in earthen ponds with different water exchange frequencies were studied.
Methodology: The completely randomized design consisted of 3 treatments, which were 3 systems of water exchange frequencies: Treatment 1, snakehead fish farming system with water exchange once a week (T1); Treatment 2, twice a week (T2); and Treatment 3, three times a week (T3). Each treatment contained 3 replicates. The experimental units were 1 rai (1,600 m2) earthen ponds, filled with water at 150 cm depth and a total water volume of 2,400 m3. The fish with an average initial weight (±SD) of 5.01 ± 0.87- 5.47 ± 0.69 g were stocked at the density of 10,000 fish/pond (6.25 fish/m2). The data of total fish weight and total feed were recorded. Water quality, sediment, and total nitrogen in feed, water, fish and sediment were analyzed; the data obtained were then used to calculate the nitrogen budget. Main Results : The result showed that the average final total fish weight (±SD) of T2 (water exchange 2 times/week) (9,000.00 ± 17.65 kg) and T3 (water exchange 3 times/week) (9,137.00 ± 11.25 kg) had significantly higher yield (p<0.05) than that of T1 (water exchange 1 time/week) (8,600.00 ± 15.14 kg). There was no statistically significant difference (p>0.05) in the total feed used in fish rearing in all three treatments. In addition, the average feed conversion ratio (FCR) in T2 (1.536 ± 0.001) and T3 (1.500 ± 0.001) were significantly lower (p<0.05) than that of T1 (1.603 ± 0.002). The nitrogen budget in snakehead fish culture with three different management systems showed that the nitrogen sources were fish feed, sediment and water, respectively. The main source of nitrogen in the fish ponds was fish feed. The percentages of nitrogen in T1 from water, sediment, and fish feed were 4.1 ± 1.12, 12.7 ± 2.61 and 83.2 ± 4.17%, respectively. Whereas in T2, the nitrogen from water, sediment, and fish feed were 5.7 ± 1.04, 14.0 ± 1.00 and 80.3 ± 2.18%, respectively. As well as, in T3, the nitrogen from water, sediment, and fish feed were 5.2 ± 1.32, 13.8 ± 2.15 and 81.0 ± 1.83%, respectively. The source of nitrogen from water filled into the ponds, was found to be that T2 and T3 had significantly higher (p<0.05) nitrogen percentages than T1, while there were no significant differences (p>0.05) among the nitrogen percentages from the sediment in all 3 treatments. As for the nitrogen from fish feed, T1 had a significantly higher (p<0.05) percentage of nitrogen than those of T2 and T 3. For nitrogen loss from the fish ponds, there was accumulation in fish, sediment, effluent, and others that was non-detected. The main loss of nitrogen from the fish ponds of all 3 treatments was accumulation in fish. As in T1, the nitrogen loss from effluent, fish, sediment and others were 22.1±2.83, 35.0±2.12, 30.1±3.61 and 12.8±3.80 %, respectively. For T2, the nitrogen loss from effluent, fish, sediment and others were 19.5 ± 2.33, 35.2±2.44, 28.6±1.65 and 16.7 ± 4.33 %, respectively. In addition, in T3 the nitrogen loss from effluent, fish, sediment and others were 17.8 ± 4.02, 38.4 ± 3.17, 27.4 ± 1.78 and 16.4 ± 4.12 %, respectively. Nitrogen loss from fish ponds by accumulation in effluent, it was found that in T1, the percentage of nitrogen loss was significantly higher (p<0.05) than those of T2 and T3. The same results were found in nitrogen loss by accumulation in sediment and others. On the other hand, by accumulation in fish, the nitrogen loss of T3 was significantly higher (p<0.05) than those of T1 and T2. Conclusions : The intensive culture of snakehead fish in earthen ponds with three different water exchange frequencies showed that the fish in T2 and T3 (the water exchange 2 and 3 times/week) had significantly (p<0.05) higher final total fish weight and lower feed conversion ratio than that of the fish in T1. The main source of nitrogen in the fish ponds was from the fish feed, which had an average nitrogen content (±SD) of between 80.3 ± 2.18 and 83.2 ± 4.17%. In addition, the main nitrogen loss from the fish ponds was the fish, which had an average nitrogen content (±SD) of between 35.0 ± 2.12 and 38.4 ± 3.1%. The optimal water exchange frequencies for an intensive snakehead fish culture in earthen ponds were twice a week.