There is various information on the internet, much of which seems to be more what people have done, rather than basing it on updated knowledge of fish biology. I would like to know what combination of stunning, killing, dissecting is recommended.
This is a difficult question to answer because there is no clear definition of "humanely."
Working in the commercial fish industry, we used to put trout in tanks where we pumped carbon dioxide into the water. After a short period of time the fish tipped over and we ran them through the processing equipment. In our salmon and steelhead spawning sites, we use a pneumatic hammer that euthanizes the fish with a sharp blow to the head. In other states, they rip gill arches and let the fish bleed out. If fish are not being consumed by humans, there are several approved anesthetics that can be used to put the fish to sleep and then kill the fish.