Impact of Three Piscicides on Nitrogen-Mineralizing and Cellulose-Decomposing Bacterial Populations

Abstract

The effects of three selective piscicides: (1) mahua, Bassia latifolia, oil cake (at a concentration of 200 ppm); (2) dimethyl-dichlorovenyl phosphate (DDVP) (at a concentration of 5 ppm); and (3) a combination of bleaching powder and urea (at a concentration of 5 ppm Cl₂ and 5 ppm NH₃, respectively), on nitrogen-mineralizing and cellulose-decomposing bacterial populations were tested under simulated fish culture conditions in outdoor cement cisterns (180 L).Water samples were collected aseptically at 10-day intervals. Enumeration of denitrifying and ammonifying bacteria was done by spread-plate technique following the method as described by Alexander (1978); ammonia oxidizing bacteria by Drews (1974); and nitrogen-fixing and cellulose-decomposing bacteria by applying the procedures of APHA (1995). Bacterial populations declined immediately following application of any of the piscicides with a highest decrease in bacterial numbers associated with treatment with DDVP (80-96.5%). The recovery of the bacterial populations to pre-treatment level was faster in the mahua (25-30 days) treatment compared to the other treatments. Application of bleaching powder and urea resulted in a phosphorus-limited environment with high N:P ratio (10-45:1) whereas, DDVP application caused the environment to be nitrogen-limited (2-3:1). Nitrogen-fixing bacterial population decreased exponentially with increasing values of total inorganic nitrogen of water (y = 49.948e∼².^394x). Therefore, mahua oil cake, a natural plant derivative, appears to act as an effective piscicide in a more environmentally-sustainable way than the other two chemical piscicides tested in the present investigation.

Key Words:

• Nitrogen-mineralizing bacteria
• cellulose-decomposing bacteria
• Bassia latifolia
• urea
• bleaching powder
• dimethyl-dichlorovenyl phosphate