Investigating relationship between silver carp Mortality and some environmental factors in fish culture ponds of Azadegan Farming Complex

In the present study within nine months (from January to September) water of Azadegan freshwater ponds was sampled in every 15 days intervals (2 times monthly). Samples were taken from 4 ponds in which 3 ponds had mortality history and one pond was considered as control. Determination of physicochemical factors such as temperature, pH, dissolved oxygen, ammonia, nitrite and total phosphorus were performed and also recognition and counting of phytoplankton was done by using slide chamber and invert microscope. In this study a total of 28 genera belonging to 4 phyla of phytoplankton such as green algae (Chlorophyta), blue-green algae (Cyanophyta) , Diatoms (Bacillariophyta) and (Euglenophyta) were recognized. Most abundant phytoplankton was belonged to Cyanophyta phylum. But Chlorophyta phylum had the most variety of the genera. The one-way ANOVA test showed that there is no significant difference between phytoplankton densities in ponds. The mean NO2 concentration in wintering ponds was 0.12±0.19 that changed significantly during the season. In culture ponds the physico-chemical factors such as oxygen, pH, temperature, ammonia, nitrite and TP had changed significantly during the study. In culture ponds, the oxygen, temperature, pH, nitrite and ammonia ranges were 6-8 mg/l, 24-31 C, 8.5-9.2, 0.38-0.01 mg/l, 2.25-0.2 mg/l respectively. During the present study three spicies of cyanobacteria namely, Phormidium sp. ,  Aphanizomenon sp.  and Oscillatoria sp. were  identified which some other studies showed them as potential toxic or in the case of heavy bloom of  Oscillatoria, there has been massive fish mortality in the ponds.