Earlier quoted context omitted.
Real tuna is delicious. Canned tuna is worse than pet food. (perhaps this explains the slip in your last sentence?) We probably shouldn't be eating tuna at all, for environmental reasons. GP's point that there are lots of non-apex-predator fish in both fresh and salt water still holds.
What's wrong with pet food?
Dry feed is linked to kidney failure in cars and small dogs.
Veterinarian food is marginally better but it still isn't comparable to a close-to-natural diet. Pretty much the same business model used by toothpaste brands sold by dentists (piggy-backing on profession trust).
There are higher quality cans and options like BARF which cost a bit more but are far better (and tastier) for pets. It can also be done at home at a lower cost but it's pretty messy. Time-wise takes around 8 man-hours per month for us (recepie calculation, getting supplements, grinding and cutting meat, getting it all together and packaging).
We've been feeding our cats with it for over 2y now. Their blood tests are far better and there's a visible increase in fur quality. Partner is a vet tech and has easy access to the clinic to checkup on our pets. She also daily sees effects of cheap dry stuff on patients.