Experimental treatments with diflubenzuron and deltamethrin of sea bass, Dicentrarchus labrax L., infected with the isopod, Ceratothoa oestroides

Sea bass with approximate average weights of 5 and 20 g were treated against Ceratothoa oestroides infection with: (i) medicated pellets of diflubenzuron PC90 at a dosage of 3 mg kg^?1 body weight (BW) per day for 14 days. Lice were counted at the beginning of treatment and 19 days after treatment. The drug cleared all lice in the treated group; in the control group, infection remained high 30 days after beginning the experiment. It was concluded that medicated pellets containing 3 mg kg^?1 BW diflubenzuron effectively cleared pre‐adult and adult stages of the isopod parasite over a 14‐day period. No adverse effects were recorded in treated sea bass during the trials and no reinfection occurred 15 days after end of the treatment. (ii) Deltamethrin by means of bath treatments in infected sea bass kept in experimental tanks at 20°C. Before treatment, toxicity on healthy fish was preliminarily assessed by testing five fish from each size group at concentrations of 30, 10, 5, 3, 1, 0.1, 0.05 and 0.01 mg L^?1 for 30 min. The therapeutic concentrations tested were: 10, 5, 3, 0.15, 0.1, and 0.05 μg L^?1 and assessed at 1, 24 and 48 h. Best results were achieved with the 10 μg L^?1 (0.01 mg L^?1) dose, where prevalence was reduced from 100 to 0% over 24 h in both large and small fish. No parasite recovery was observed at 48 h. The dose of 5 μg L^?1 reduced prevalence from 100 to 11.7% and to 0% for small and large fish, respectively. Finally, with the 3 μg L^?1 dose, prevalence was reduced from 100 to 37.5% (small fish) and to 13.3% (large fish). Lower doses were ineffective on the parasites at either 24 or 48 h.