Methemoglobin concentrations in three salmonid species following exposure to benzocaine or tricaine methanesulfonate.

Abstract

Methemoglobin is hemoglobin containing ferric iron rather than ferrous iron which renders it incapable of binding to oxygen. Blood sampling of fish is done under sedation or general anesthesia. Tricaine methanesulfonate (TMS) or benzocaine is commonly used but both can cause oxidation of hemoglobin to methemoglobin. Our objective was to determine if methemoglobin concentrations in healthy rainbow trout (Oncorhynchus mykiss), brook trout (Salvelinus fontinalis), or Atlantic salmon (Salmo salar) increase during sedation with 25mg/L of a 10% benzocaine solution or with repeated short anesthetizations by 65mg/L of 10% benzocaine solution or 65mg/L of TMS. Sedation by benzocaine caused a significant increase in methemoglobin in all species over time (P <0.05). The methemoglobin percentage in brook trout increased by 129%, rainbow trout by 42%, and Atlantic salmon by 49%. The methemoglobin in brook trout was significantly greater than the other species at multiple time points. Repeated brief anesthetizing by benzocaine and TMS caused significant methemoglobin by 60 (P <0.05), 90 (P <0.01), and 120min (P <0.001) in brook trout but no significant change in methemoglobin in rainbow trout or Atlantic salmon except at 120min in Atlantic salmon (P < 0.05) repeatedly anesthetized with benzocaine. For example, following multiple anesthetizations with benzocaine, the methemoglobin percentage in brook trout increased by 140%, whereas the rise in methemoglobin in rainbow trout and Atlantic salmon was more modest (37% increase in Rainbow trout and 53% increase in Atlantic salmon). Following multiple anesthetizations with TMS, the methemoglobin increased by 90%, 5%, and 1% in brook trout, rainbow trout, and Atlantic salmon, respectively. Methemoglobin may increase significantly over time in fish immersed in a sedating dose of benzocaine or repeatedly anesthetized with benzocaine or TMS. The susceptibility varies with the individual and species with brook trout being more susceptible than Atlantic salmon or rainbow trout.