Effect of the pesticide deltamethrin as a treatment of Ceratothoa oestroides infestations of farmed sea bass Dicentrarchus labrax

Abstract

The haematophagic isopod parasite Ceratothoa oestroides (Risso, 1816) causes significant production losses in the commercial aquaculture of sea bass (Dicentrarchus labrax L. 1758). These are often expressed as reduced fish growth and lower survival rates of sea bass fingerlings <20 g mass. Deltamethrin is a synthetic pyrethrin with demonstrable positive results in the control of C. oestroides. The aim of this study was to determine its efficacy in the control of C. oestroides in commercial production conditions. The experiment was carried out at a commercial sea bass farm and in two net cages of 16 m diameter stocked with 200,000 sea bass fingerlings of the same origin and weight. After 5 months culture, a deltamethrin treatment was performed in one cage, while the other cage served as a non-treated control. The average sea bass mass during the antiparasitic bath was 19.70 ± 0.47 g and 18.48 ± 0.50 g (mean ± SEM) for the treated and the control cages, respectively. Two months after the treatment, visual examination of the entire population was carried out. The results revealed that the number of C. oestroides in the treated cage was significantly lower (P < .01) compared to those in the control cage. Also, the C. oestroides were smaller in length in the treated group compared to the control cage. No sexually mature females were found in the treated group whereas in the control cage 21.3% of the C. oestroides were sexually mature, containing average 39.7 pulli larvae. The monthly mortality rate during this study was <0.5%, but mortality in the treated cage, after treatment in June, was lower than in the control cage. The results indicate that the deltamethrin treatment was successful in sea bass infected with C. oestroides. After two months, however, a new generation of immature C. oestroides appeared. These results show that sea bass weighing over 20 g can be reinfected, but the absence of sexually mature females points to the importance of this treatment to reduce the dynamics of C. oestroides population on a farm site. Despite good antiparasitic results, there is a further need to improve fish farming in the Mediterranean through the introduction of changes in husbandry practices with a purpose of prevention of parasitic horizontal transmission.