When I was a commercial salmon troller back in the 70's, I only saw belly burn in coho; not in other species.
That's because of coho's lifecycle; they double or triple their size in their last year at sea. That means they eat and therefore digest, immense amounts of food. To quickly break down and absorb all that food in their guts, they produce huge amounts of powerful digestive enzymes. When they are alive, that's not a problem to them. But as soon as they are killed, digestion stops, and all those unused enzymes then start to dissolve that thin, clearish membrane we see lining the gut cavity, separating the meat from the guts. With belly burn, we see gaps in that membrane, which allow digestive juices to dissolve the meat. Ribs stick out, the meat becomes mushy, and buyers would either give us a lower price, or if the belly burn was bad, refuse the fish.
As a newbie deckhand, it was made very clear to me to clean coho ASAP. And of course, to clean all salmon as soon as possible, because seafood is much, much more perishable than land animal meat.
Another lesson I learned about how powerful coho digestive juices are, was on those days of huge coho catches. Those days meant many, many hours of gutting, and several times I ended up with "coho poisoning", or "coho burn", which is when the thin skin on the back of your hands is basically dissolved. It's painful, red, raw, and oozes pus; like a bad chemical burn.
I shake my head at sport anglers leaving the guts and gills in their salmon for so long. At the very least, remove the gills and let them bleed out, because blood goes bad faster than flesh.