Catfood actually comes in three flavours: Gills and Fins, Beaks and Feet, Lips and arseholes.

Edited for autocorrect

Why feather free? I think it's also good for the digestive system.

There was! There was a brand of mouse-based cat food.

I am in no place or have the qualification to give a legal answer, so this is just as far as I know from talking with people who own reptiles which are fed mice.

It is not allowed to use living animals as food, and the animals have to be put down in a humane and painless/stressless matter.

So it sounds a lot like in the Uk, yes

"Maybe them cats wanna turkey and cheese sammich!"

"OK, grandpa, let's get you to bed..."

"Tarnations, boy! Im tellin' ya they want a dat gummed sammich! Turkey and cheddar! Hold the bread! "

Tuna are saltwater fish. In what environment can cats hunt saltwater fish? I don't think they could ever realistically interact.

The real reason is farming and phytosanitary needs. The animals used need to be cheap enough to make sense to feed a pet, and farmed in a way where you can be sure your product won't give cats worms or other parasites. Cats also don't really eat the hair and hide, or feathers, of animals, so you ultimately have mammal, birds, and fish as categories. We already farm mammals, birds, and fish for humans, so the systems are already there.

That being said, I used to feed my little house panther raw food, which was a little frozen gore nugget of rabbit or chicken or duck. She loved them. Loved, loved, loved them. She's caught pigeons and rats before, so it's really not that far off. It's not like either would present vastly different nutrition as compared to a mouse or a rat or song bird anyway.

Because I would buy them from an Asian supermarket in a frozen 12 pack.
I would also buy mature chickens for $3 a bird and cut them in quarters and hand feed those to my dog. It was very good for teaching him to not bite me when crushing the bones right next to my hand.

Now with added barf!

Cats that live near ocean shallows. Egyptian Maus are known to catch them.

"Oops! all cloaca"

My cat is allergic to seafood or something. She vomits every time she eats tuna cat food, multiple different brands. Land meat is never an issue though.

It mimics their natural diet in the wild of hunting roaming wheels of parmesan and violent flightless birds 10 times their size.