When you're cooking a piece of fish, you're not changing its structure using heat and pressure so that globular plant cell structures are transformed into stuff resembling fibrous animal cell structures.
https://youtu.be/uTHCqjIspKU?si=pFqoQOuGbIPNnJox
Around the 1:30 mark.
You are if the fish ate any plants before they died (and you cooked with heat, I'm vegetarian so am unfamiliar with the other poster's a la Ceviche).
And even if the fish didn't eat any plants, you're still changing the protein structures — this is even part of the point of cooking, as amongst other things this does, changing the proteins inside the food is a process that kills any bacteria and other parasites.
Try it with dried cranberries and sunflower seeds for a great salad! (Don’t forget the salt, pepper, and garlic)