Egg surface decontamination with bronopol increases larval survival of Nile tilapia, Oreochromis niloticus

Intensive tilapia egg incubation techniques create favourable conditions for microbial proliferation which often leads to massive mortalities of fish larvae. The effects of Nile tilapia (Oreochromis niloticus) eggs exposure to bronopol on decontamination of their bacterial surface and survival of larvae were observed. Immersion treatments of fertilized eggs were applied at 10, 25, 50, 100, and 250 mg/l of bronopol for 10, 20, and 30 min. This treatment substantially reduced the number of bacteria on Nile tilapia eggs. The greatest reduction in bacterial numbers (1.58 × 103 colony forming units/g of egg) was observed at the maximum treatment dosage, i.e. 250 mg/l for 30 min, but this was not significant when compared with treatments of 100-250 mg/l bronopol for 10-30 min. Treatments of 50-250 mg/l bronopol provided better larval survival (89.33-94.67%) than those of < 50 mg/l (P < 0.05). The larval survival obtained from the Nile tilapia eggs disinfected with 10 and 25 mg/l bronopol for 10-30 min was similar to that of negative controls (79.33-80.97%). Additional in vitro test was performed to determine the inhibitory potency of bronopol against bacteria cultured from fertilized Nile tilapia eggs. Twenty-two bacterial isolates (Aeromonas hydrophila (n = 12), Staphylococcus aureus (n = 6), Escherichia coli (n = 2), and Micrococcus spp. (n = 2)) responded to minimum inhibitory concentrations of bronopol ranging from 64 to 128 µg/ml. The study demonstrates that the immersion treatment of Nile tilapia fertilized eggs with 250 mg/l for 10-30 min, the concentration proven effective against bacteria cultured from fertilized eggs, significantly reduced bacterial load and improved larval survival.