Lethal and sublethal responses of Channel Catfish to formalin exposure

ABSTRACT Objective Formalin is used extensively in aquaculture as a disinfectant and for the control of external parasites. The objective of this study was to relate the recommended treatment levels to formalin concentrations that cause mortality and physiological dysfunction in Channel Catfish Ictalurus punctatus. Methods Median-lethal concentrations (LC50; the concentration of a substance that will kill half of exposed animals in a set period) and changes in plasma corticosteroid and chloride concentrations were determined for formalin-exposed Channel Catfish. Results After 3 h of exposure, the LC50 was 410 μL/L (95% CI [340–522 μL/L]). The value decreased rapidly to 139 μL/L (95% CI [118–158 μL/L]) by 24 h. Plasma corticosteroid concentrations increased significantly from 0.9 ± 0.1 μg/100 mL (mean ± SE) in the control fish to 2.3 ± 0.4 μg/100 mL in the fish that were exposed to 100 μL/L formalin for 24 h. Plasma chloride concentrations decreased significantly from 115 ± 3.1 milliequivalents (meq)/L in the control fish to 101 ± 3.3 meq/L in the fish that were exposed to 100 μL/L formalin for 24 h. Conclusions The current recommendation for treating warmwater fishes for external parasites is a maximum application of 250 μL/L formalin for up to 1 h. The lethal and sublethal results reported here indicate that the current recommendation for short-term treatment of external parasites is reasonable but that care needs to be taken when exceeding it.