EUTHANASIA BY CONTINUOUS TOPICAL DELIVERY OF CLOVE OIL OVER THE GILLS RESULTS IN DELAYED CARDIAC ARREST IN COMPARISON TO IMMERSION BATH IN JUVENILE RED HEAD TAPAJOS EARTHEATERS (GEOPHAGUS PYROCEPHALUS).

Immersion bath in high doses of tricaine methanesulfonate (MS-222) and eugenol is a common euthanasia method for fish, causing rapid cessation of opercular movements. However, in previous studies where cardiac contractions persisted, despite dosages of MS-222 as high as 1000 mg/L, the hypothesis was advanced that apnea could prevent effective euthanasia. The objective of the study was to determine if continuous topical delivery (CTD) over the gills of MS-222 or clove oil (CO) would result in cardiac arrest at 60 minutes, without recovery after placement in fresh water for an additional 60-minute period. Thirty-six juvenile red head Tapajos eartheaters (Geophagus pyrocephalus) were randomly divided into six experimental groups (six fish/group, concentration of the anesthetics indicated in mg/L): (1) Bath MS 500; (2) CTD MS 500; (3) Bath CO 500; (4) CTD CO 500; (5) Bath CO 1000; (6) CTD CO 1000. The proportion of fish with a heartbeat at 60 minutes and the mean time to cardiac arrest (P < 0.001) were significantly different among groups. At 60 minutes, 6/6 and 5/6 of fish in the Bath CO 1000 group and the Bath CO 500 group experienced cardiac arrest, respectively, while only 1/6 and 3/6 of fish in the CTD CO 500 group and the CTD CO 1000, respectively, did. By 120 minutes, all fish except two fish in the CTD MS 500 group had undergone cardiac arrest. In contrast to our hypothesis, mean time to cardiac arrest occurred faster in immersion bath than in CTD at both concentrations of CO (P ≤ 0.022).