Residues of benzocaine in rainbow trout, largemouth bass, and fish meal

Residues of the anesthetic benzocaine in muscle tissue of rainbow trout (Salmo gairdneri ) and largemouth bass (Micropterus salmoides ) were determined after exposure of the fish to 50 mg benzocaine/L for 15 min and withdrawal times of 0-24 h. The mean concentration of benzocaine residues in fish sampled immediately after exposure was 14.0 mu g/g in rainbow trout and 10.6 mu g/g in largemouth bass. Residues were below the control value after 8 h of withdrawal in largemouth bass and near the control value after 4 h of withdrawal in rainbow trout. Although residues of benzocaine were high in fish immediately after exposure, the concentration declined rapidly when the fish were held in flowing fresh water. Fish meal prepared from Pacific salmon (Oncorhynchus sp.) that had been anesthetized with benzocaine or tricaine (MS-222) contained residues of 45.1 mu g benzocaine/g or 47.7 mu g tricaine/g.