Abstract
Two Argyrosomus regius nursery ponds suffered from high mortality during the summer season of 2020. Diseased fish showed signs of respiratory distress, mainly during hot days. Physiochemical pond water parameters were normal, except dissolved oxygen was lower than the optimum level. Livoneca redmanii (Cymotidae, Isopoda) was isolated from the gill chamber of infested fish. The infestation percent was 41 and 33%, and the intensity was 1.146 and 1.06 in the first and second ponds, respectively. The diseased fingerlings were infested in the natural habitat (Mediterranean Sea) before collection for culture. L. redmanii infestation was associated with high economic losses due to direct fish mortality, reduced growth rates and lower condition factor. The molecular study confirmed the presence of L. redmanii based on the phylogenetic analysis of 16S rRNA and COI gene sequences. Infested fish gills showed massive destruction in gill filaments, sloughing and curling of secondary gill lamellae with the detachment of the cartilaginous core supporting the primary gill filaments. Deltamethrin and trichlorfon effectively eradicated L. redmanii at concentrations of 12.5 mu g.l- 1 and 0.5 mg.l-1, respectively. The lowest detectable deltamethrin residue in treated pond water was 2.18 mu g.l- 1 at 120 h, with a half-life of 47.63 h.