An investigative study was conducted to evaluate the effects of alpha-cypermethrin residues in cow manure used for pond fertilization on water quality, growth performance, and survival rate of Nile tilapia (Oreochromis niloticus). Cow manure samples were collected from livestock farms in Kilosa District, Tanzania, and transported in a cool box to the Chief Government Chemist Laboratory Agency (CGCLA) in Dar es Salaam for chemical residue analysis.
Chemical analysis revealed that the samples contained alpha-cypermethrin at concentrations ranging from 0.39 to 52 mg/kg. A pilot study to determine the lethal concentration (LC₅₀) showed that 50% mortality of Nile tilapia fingerlings (average weight 0.5 ± 0.01 g) occurred at 416 mg/kg after 12 hours of exposure, increasing to 90% by 96 hours. Based on these findings and the residue levels observed in the field, sub-lethal concentrations of 0.4, 0.8, and 1.6 mg/kg were selected for the main experiment to simulate environmentally relevant chronic exposure.
The experimental setup involved twelve 100 L glass tanks, each lined with a 3 cm layer of loam soil and filled with 80 L of water. Tanks were stabilized for 10 days before stocking with Nile tilapia fingerlings (average weight 0.608 g) at a density of 20 fish per tank. Fish were acclimated for 5 days before the experiment began. Cow manure either unspiked (control) or spiked with alpha-cypermethrin was applied every 10 days at a rate of 260 g/m³.
Survival rates were unaffected (p > 0.05), averaging 95% across all groups, consistent with expected natural mortality. However, significant reductions (p < 0.05) were observed in growth performance indicators such as weight gain, specific growth rate, condition factor, and hepatosomatic index in fish exposed to alpha-cypermethrin. Water quality analyses showed significant decreases in dissolved oxygen and pH (p < 0.05), while ammonia levels remained unaffected (p > 0.05).
These findings suggest that alpha-cypermethrin residues in cow manure, even at environmentally relevant levels, negatively impact water quality and the growth performance of Nile tilapia. The study underscores the need for educating fish farmers on the risks of pesticide contamination and promoting proper manure handling to protect fish health, ensure food safety, and improve aquaculture sustainability.