Concentration effects of three common fish anesthetics on Pacific hagfish (Eptatretus stoutii)

The efficacy of three common fish anesthetics (clove oil, 2-phenoxyethanol, and tricaine methanesulfonate) was evaluated in the Pacific hagfish ( Eptatretus stoutii ). The overarching aim of our study was to identify the best anesthetic and concentration for the purposes of routine laboratory use of Pacific hagfish (i.e., short and consistent induction and recovery times and minimized stress and safety risk to hagfish). The objectives of our study were fourfold: (1) identify anesthetic stages of Pacific hagfish using clove oil anesthesia; (2) establish standardized anesthesia preparation procedures; (3) determine the optimal anesthetic and concentration for safely achieving stage V anesthesia; and (4) investigate the effects of repeatedly exposing Pacific hagfish to anesthesia. Experimental concentrations, ranging from 50 to 400 mg/L, of each anesthetic were tested on at least three Pacific hagfish individuals. We found the following: (1) Pacific hagfish exhibited similar stages of anesthesia to those described for bony fishes; (2) sufficient mixing of clove oil with seawater had a considerable effect on the consistency and timing of anesthetic induction; (3) concentration and anesthetic significantly impacted induction and recovery timing, whereas body mass had no impact on anesthetic trends; and (4) repeatedly exposing Pacific hagfish to optimal concentrations of clove oil or MS-222 had no effect on induction or recovery timing, whereas exposure number significantly impacted induction timing when using 2-PE. Due to consistent induction and recovery times, low risk of accidental overdose, and high safety margins for both handler and hagfish, we recommend 175 mg/L of clove oil as the ideal anesthetic and concentration for the routine laboratory use of Pacific hagfish.