All fish kept by humans must be stunned before slaughter. However, how to do this in an ethically defensible manner is unclear. The lack of knowledge when it comes to stunning methods for fish is based on the difficulty of verifying whether a fish is unconscious or not.
Link to the article
Bowman J, Hjelmstedt P, Gräns A. 2019, Non‐invasive recording of brain function in rainbow trout: Evaluations of the effects of MS‐222 anaesthesia induction. Aquaculture Research 50, 3420–3428.