Assessment of Gaseous CO 2 and AQUI‐S as Anesthetics when Surgically Implanting Radio Transmitters into Cutthroat Trout

Tricaine methanesulfonate (MS‐222) and CO 2 are anesthetics that can be legally used in fisheries work in the United States, but they are limited in their field applications. A mandatory 21‐d withdrawal period is required for fish exposed to MS‐222. Carbon dioxide is not approved by the U.S. Food and Drug Administration, but it is a “low regulatory priority drug” that can be used legally for fish anesthesia. However, stressful induction and lengthy recovery times have been associated with CO 2 . AQUI‐S is a clove oil derivative that has the potential to become an approved anesthetic without the limitations of MS‐222 or CO 2 . We compared the efficacy of CO 2 with that of AQUI‐S when surgically implanting radio transmitters into cutthroat trout Oncorhynchus clarkii. A 20% survival rate was observed when CO 2 was used in combination with silk sutures, but a 100% survival rate was observed when CO 2 was used in combination with surgical staples to shorten the duration of the surgical procedure. A 100% survival rate was observed when AQUI‐S was used in combination with either silk sutures or surgical staples. Carbon dioxide in combination with surgical staples seemed to provide a reasonable option when surgically implanting radio transmitters into cutthroat trout, but AQUI‐S may be the preferred anesthesia because high pH and dissolved oxygen levels and low free‐CO 2 concentrations are maintained during surgical procedures.