Propofol Immersion As a Euthanasia Method for Adult Zebrafish (Danio Rerio)

The exponential rise of the zebrafish (Danio rerio) as a model organism in biomedical research has far outstripped our un- derstanding of basic husbandry and welfare for this species. As a case in point, here we investigate the efficacy and welfare impact of different euthanasia methods for zebrafish. Not only is a humane death central to welfare and the 3Rs, but stress during euthanasia can change scientific outcomes. However, the most frequently used methods of euthanasia have multiple shortcomings with regard to animal welfare and human safety. In this study, we propose the use of propofol for immersion euthanasia of adult zebrafish. Propofol has been known to rapidly induce anesthesia in many species, including zebrafish, but its efficacy as a euthanasia agent for zebrafish has not fully been explored. In this study, adult zebrafish were euthanized by immersion on one of 5 different preparations: ice bath, 250 ppm MS222, 600 ppm lidocaine hydrochloride, 100 ppm propofol, or 150 ppm propofol for 20 or 30 min. Display of aversive behaviors, time to loss of righting reflex, time to cessation of opercular movement, and time to recovery after transfer to clean tank water were assessed and recorded. Propofol at both concentrations induced loss of righting reflex and loss of opercular movement more quickly than did MS222 or lidocaine hydrochloride and caused no display of aversive behaviors as seen with ice bath or lidocaine exposure. However, fish exposed to propofol at either concentration for 20 min sometimes recovered, whereas a 30-min exposure was sufficient for euthanasia of all fish tested. These findings suggest that exposure to propofol for a duration of at least 30 min quickly and effectively euthanizes adult zebrafish without compromising end-of-life welfare.