Influence of time held in anaesthesia on stress and survival of surgically implanted juvenile salmonids

The length of time a fish is exposed to “knock down” anaesthesia in preparation for surgery could influence the survival or behaviour of the fish. Currently, the amount of time fish are held between reaching stage 4 anaesthesia and the time surgical implantation of a transmitter begins varies among studies and research groups, which may lead to adverse effects on survival and an inability to directly compare survival results. Therefore, there is a need to determine the amount of time fish can be held in knock down anesthesia without negatively influencing survival models. In this study, juvenile Chinook salmon, Oncorhynchus tshawytscha, were anesthetized in a dose of MS-222 that placed them in stage 4 anesthesia after 2-3 minutes of exposure. They were held for 3, 5, 6, 7, 8, 9, 10, or 15 minutes in this solution and then had a transmitter surgically implanted using standardized surgical procedures.  Mortality and opercular beat rates were monitored during the anaesthesia exposures. Hematological and plasma parameters were also measured. The results of this study will provide a guideline for the length of time that fish can be held in knock down anaesthesia prior to the beginning of surgical implantation.