Effect of digested sewage sludge on the ventilation and coughing rates of two fresh water fish

Addition of digested sewage sludge at concentrations of 2% and 10% (v/v) to the water increased coughing rate in big head and tilapia (P < 0.05). Ventilation rate was significantly decreased (P < 0.05) in big head and tilapia at sludge concentrations of 6% and 2% (v/v) respectively. Copper (Cu), cadmium (Cd) and zinc (Zn) are trace metals which are commonly found in sludge. Cu caused a significant increase (P < 0.05) in coughing rate in both tilapia and big head at concentrations of 0.3 and 0.2 microgram/ml respectively. Zn caused significant increase (P < 0.05) in coughing rate only in big head at 2 microgram/ml. Neither fish responded to Cd of up to 2 micrograms/ml in the water. However, when the levels of these trace metals in the digested sludge were measured, they were below that which can cause significant changes in the respiratory movements. Therefore, the changes in ventilation and coughing rates after addition of sludge may be due to the presence of substances other than these metals. The results of this experiment provides a guideline to control the level of sludge that can be used in rearing these fresh water fish in ponds.