The short answer to the OP is that people get the good parts of the tuna; the cats get the guts, the backbone steamed and ground up, the skin…Cats don’t normally eat fish in the wild. They eat small rodents. Tuna cat food is meant to appeal to you, the consumer, with your 50 years of folklore and Warner Brothers cartoons that tells you, “Cats love fish.”
I have here in my hand a can of Kroger Cat’s Choice Gourmet Ocean Whitefish Dinner. (No, we don’t have a cat, it’s a long story).
Ingredients (as per FDA requirements, in descending order of important in the mix):
water sufficient for processing
And then there’s a bunch of vitamins and minerals.
Dunno how much you know about the pet food industry, so I’ll summarize. “By-products” means the parts people won’t eat. Poultry by-products is the parts that Louis Rich is’t allowed to grind up into Ground Turkey Sausage, but it can be nothing more than ground-up feathers–it’s perfectly legal for pet food. “Meat” by-products can be, literally, anything from a pig, sheep, or cow that people won’t eat, the body parts that the meat processors aren’t allowed to grind up into hamburger. “Meat” means “whatever fell off the truck from the slaughterhouse this week.” If it’s from a cow, they have to at least call it “beef” by-products. This is the guts–the anuses, the lips, the uteri, the penises, the spleens, etc.
“Liver” could be liver from anything–turkeys, chickens, pork, beef. Americans eat very little liver. Most of it goes into dog and cat food.
And for pet food, just because it says “ocean whitefish” or “tuna” doesn’t mean it has to be the nice filets. It’s just the opposite–it’s the skin and guts mainly. But hey, fish guts is still protein, and your cat can survive perfectly well on a diet of fish guts, with all the necessary vitamins and minerals added.
Guar gum and carrageenan are what they make instant pudding out of. It’s to hold the glop together so it looks nice in the cat’s food dish.
The onion salt is in there to make it smell not quite so appalling to you, the consumer. The cat doesn’t care.