Effect of the filter-feeder silver carp on the water quality of fertilized earthen ponds and Nile tilapia production

Abstract

The objective of this study is to test the effect of silver carp on water quality and tilapia yield. This work was conducted in six 1000 m 2 earthen ponds which randomly assigned to two treatments. The first treatment was the cultivation of Nile tilapia with silver carp and the second was the cultivation of Nile tilapia alone without silver carp. All ponds were fertilized for the first three months with cow manure, urea and superphosphate to maintain a level of 2 mg N/L and 0.5 mg P/L in pond waters. Water samples were collected weekly and analyzed for temperature, DO, pH, Chl. “a”, avail. P, NO3, NH4, NH3, T. Alk., and T. Hard. Two-way ANOVA results indicated that the effect of the experimental period was the most important source of variation for the water quality parameters and it was responsible for 47 – 100 % of the variability for most of water quality parameters. The results also revealed that the presence of silver carp led to about 57% reduction of the chlorophyll concentration in the water column. Silver carp also was significantly responsible for 38 % of the variability of the total alkalinity concentrations and for 5-26 % of the total variability for pH, available phosphorus, ammonium, ammonia and total hardness however it was not significant. The results of factor analysis indicated that combined two factors (photosynthesis vs. respiration and pond fertilization) accounted for 40% of the total variability explained by the ANOVA model. The presence of silver carp reduced tilapia production by about 10%, but it increased the total fish production which in turn resulted in 25% higher net profit (1009 LE/ha) than that in the case of absence of silver carp, which appeared also in the B/C ratio of about 28% greater with the presence of silver carp than its absence. The stocking density of silver carp used in this study was not enough to obtain all benefits of cultivating silver carp with other fish species, therefore further trials are needed with different stocking densities.