Influence of infeed emamectin benzoate and winter on the bioresponses of Oreochromis niloticus

Antiparasitic drugs are vital for eliminating infectious fish parasites in aquaculture. However, medications used to control parasites can disrupt the physiological and biochemical processes of fish. This study explored the safety margin of administering emamectin benzoate (EB) at graded doses—0, 50, 125, 250, 375, and 500 μg/kg biomass/day—for 14 days during the onset of winter and the perturbations in the physiological and enzymatic activities of male Oreochromis niloticus fries compared to the control. Oral EB administration affected biological processes in a dose-, time-, and temperature-dependent manner. The graded doses of EB increased the mortality rate from 12.67% to 76.50%. EB administration during the onset of winter increased muscle glucose and alkaline phosphatase levels while decreasing calcium, chloride, superoxide dismutase, and brain acetylcholinesterase levels in a dose- and time-dependent manner, with all effects intensifying as water temperatures dropped. Even the recommended dose (50 μg) impacted the fries’ antioxidant capacity. Upon discontinuation of EB feeding, all measured parameters in the groups began to recuperate but remained different from the control at the end of the study. These results implied a cumulative effect of EB and low water temperature, which may adversely impact fish health and production in commercial aquaculture. Therefore, caution must be exercised while using veterinary medicinal products like EB in tropical aquaculture during the onset of winter.