It seems like the consensus here is that I am wrong about canned tuna. I will try to relisten to the shiur I heard about the subject. I definitely heard that salmon can be identified without scales due to its unique look (assuming you know exactly what raw salmon is supposed to look like.)
I heard the direct opposite.
See more in the link below.
https://sites.google.com/view/kashrusbeintheknow/audio-classes/fish-facts-part-2-scaleless-proof-less
I listened to that shiur. He did say the OU did not require a mashgiach to check the tuna before they canned it (He said that about tilapia also so I am not sure what OUs reasoning is). He seemed to say salmon would be OK but said he was going to talk about it in the next shiur but the link wasnt working for that shiur.
If the facts were that there is nothing else that looks and smells liked canned tuna, why is that not a good enough siman?
My father did hashgacha at tuna plants when I was a kid, and iirc, he had to examine every single fish that was going to be canned as it was possible for non kosher species to be mixed in.
That is for sure lichatchila and a hashgacha should be doing its best job to make sure everything is mehadrin min hamihadrin.
The question is does it need to be checked meikar hadin?