The use of fish as a substitute for mammals in laboratories has skyrocketed into the millions. Little research has been done on the effects of anesthetics that are used on fish during surgical procedures and euthanasia. This study tested nine anesthetics on Danio zebrafish for reactions that indicated discomfort, distress, or pain. Seven of the anesthetics, including two of the most widely used, caused aversive reactions. The authors conclude that they are inhumane, and should be discontinued. They also call for similar tests for other types of fish, since reactions can vary from species to species.

[Abstract excerpted from original source.]

“This study addresses a fundamental question in fish welfare: are the anaesthetics used for fish aversive? Despite years of routine general use of many agents, within both scientific research and aquaculture, there is a paucity of information regarding their tolerance and associated behavioural responses by fish. This study examined nine of the most commonly used fish anaesthetic agents, and performed preference tests using adult mixed sex zebrafish (Danio rerio), the most commonly held laboratory fish. Video tracking software quantified swimming behaviour related to aversion for each anaesthetic at 50% of its standard recommended dose compared with clean water in a flow-through chemotaxic choice chamber. Results suggest that several commonly used anaesthetics were aversive, including two of the most commonly recommended and used: MS222 (ethyl 3-aminobenzoate methanesulphate) and benzocaine. For ethical best practice, it is recommended that compounds that are aversive, even at low concentration, should no longer be used routinely for anaesthesia or indeed the first step of humane euthanasia of adult zebrafish. Two agents were found not to induce aversive behavioural responses: etomidate and 2,2,2 tribromoethanol. For the millions of adult zebrafish used in laboratories and breeding worldwide, etomidate appears best suited for future routine humane use.”