I am able to taste a flavor in some fish (usually white fish) that most other people do not (I also can smell the asparagus pee smell, and am a strong taster ... but not supertaster?... of bitter flavors). What I would like is a way to predict whether a given piece of fish will have the flavor, so I can avoid it. I don't think that I can detect it as a smell. I find it unpleasant, but not intolerable. It doesn't seem to be a result of spoiling. At a guess, I'd say I encounter it in less than 20 percent of white fish. It occurs across species, so I might find it in cod, haddock, halibut, sole, etc. If I'm cooking up some fillets from the supermarket, that flavor might be in one piece, but not the next. My wife and I will often swap portions if I detect it in mine, and she notices no difference, where to me it's as obvious as if I'd popped a chunk of 80% dark chocolate in with a bite of fish. Not to imply that I dislike choco - I love it.
I don't ask everybody that I meet, so there might be others who detect this flavor, but I can't describe it, except... hmm... one-dimensional, the way a taste of metal would be one-dimensional... though it doesn't seem to be a metallic flavor either.
Anyway, would be obliged if somebody else knew what I meant, and especially if there's a way to tell if a piece of fish will be affected BEFORE I buy. Oh, and it's only in fish. No other food that I've encountered. Not affected by method of cooking. Not affected by seasoning. Anybody?