when fish spoils, some turn into fish morsel but some turn into rot

We all know not all fish are the same. When killed, you can get raw fish, fish morsel, corn, popcorn, or leafy meat, and normally that's what you get when those fishes spoiled (i.e. when living fish left to die). However, for those leafy meat fishes, this is not true. Fishes like Bloomfin Tuna will become rot when left to die. I would expect them to be leafy meat instead.