JAVMA • Vol 252 • No. 12 • June 15, 2018 1555 Aquatic Animals E uthanasia of fish is a common procedure and may be indicated for a variety of reasons, including poor quality of life or individual health concerns, as an endpoint for laboratory or field research, or for the purpose of zoo or aquarium population management. By definition, euthanasia must be rapid and result in minimal pain, stress, and distress. Current guidelines published by the AVMA 1 recommend both chemical and physical methods for finfish euthanasia. Chemical methods involve injecting an overdose of anesthetic directly into the fish (eg, into a vessel, muscle, or body cavity) or immersing the fish in a highly concentrated anesthetic solution for uptake by the gills, known as Use of tricaine methanesulfonate or propofol for immersion euthanasia of goldfish (Carassius auratus) Julie A. Balko VMD Ayako Oda DVM Lysa P. Posner DVM From the Department of Molecular and Biomedical Sciences, College of Veterinary Medicine, North Car- olina State University, Raleigh, NC 27607. Address correspondence to Dr. Balko (jbalkovmd@ gmail.com). OBJECTIVE To substantiate current AVMA guidelines for immersion euthanasia of gold- fsh (Carassius auratus) with tricaine methanesulfonate (TMS), determine whether immersion in propofol at 5 times its immersion anesthesia concen- tration for 30 minutes is suf fcient for euthanasia of goldfsh, and quantify the duration of myocardial contraction following immersion of goldfsh in TMS and decapitation. DESIGN Prospective clinical trial. ANIMALS 36 healthy, adult goldfsh. PROCEDURES Goldfsh were randomly assigned to be immersed in 1 of 6 test solution treatments (n = 6/treatment): TMS (500 mg/L) for 15 minutes followed by placement in anesthetic agent–free water (T15W), placement out of water (T15A), or decapitation (T15D); TMS (1,000 mg/L) for 15 minutes followed by placement in anesthetic agent–free water (T15XW); TMS (500 mg/L) for 30 minutes followed by placement in anesthetic agent–free water (T30W); or propofol (25 mg/L) for 30 minutes followed by placement in anesthetic agent–free water (P30W). Any fsh that resumed operculation in group T15A was returned to anesthetic agent–free water. Times from onset of immersion to induction of anesthesia, cessation and resumption of opercu- lation, and recovery (T15W, T15A, T15XW, T30W, P30W) or cessation of Doppler ultrasounds (T15D) were recorded. RESULTS Overall, 5 of 6, 6 of 6, 6 of 6, 6 of 6, and 5 of 6 fsh survived in the T15W, T15A, T15XW, T30W, and P30W groups, respectively. Median time to cessation of Doppler ultrasounds in group T15D was 77.5 minutes (range, 30 to 240 minutes). CONCLUSIONS AND CLINICAL RELEVANCE Timed immersion in test solutions (TMS at 500 mg/L or 1,000 mg/L or propofol at 25 mg/L) resulted in death in only 7% (2/30) of immersed gold- fsh. Myocardial contractions continued for up to 4 hours in decapitated goldfsh. ( J Am Vet Med Assoc 2018;252:1555–1561) immersion euthanasia. There are currently no FDA- approved drugs for immersion euthanasia of aquatic spe- cies; however, several are considered acceptable by the AVMA. 1 Among them, tricaine methanesulfonate (MS- 222) is one of the most commonly used. It is currently the only FDA-approved anesthetic for use in food fish 2 and, thus, is widely used as an immersion anesthetic and euthanasia agent in US aquaculture. 3,4 In light of the 21-day withdrawal period following its use, TMS is not recommended for euthanasia of fish intended for human consumption. Tricaine methanesulfonate is classified as a local anesthetic and blocks sodium chan- nels in the peripheral and central nervous systems, de- creasing neuronal activity. 5 However, despite its wide use as an anesthetic, the exact mechanism by which it produces general anesthesia is currently unknown. 6 Similarly, the mechanism by which it causes death in fish has not been evaluated, although TMS has been suggested to cause death as a result of decreased ner- ABBREVIATIONS HR Heart rate OR Opercular rate TMS Tricaine methanesulfonate