For a food that is i) cheap, ii) extremely healthy, iii) non-perishable, iv) easy to transport, move and store, it surprises me that mercury free tuna cannot be

For a food that is i) cheap, ii) extremely healthy, iii) non-perishable, iv) easy to transport, move and store, it surprises me that mercury free tuna cannot be purchased somewhere. The small amounts of mercury that tuna contains typically come from what it eats, and despite tuna farms existing, they are typically located in the ocean, and even when they aren't, the fish are fed food from the ocean, and so they end up consuming mercury. Therefore, health experts recommend extremely small quantities of tuna. Coming up with a low cost source of mercury free (or nearly mercury free) tuna seems like an obvious way to make the world a healthier, better place. EDIT: according to this 2014 report [1] the difficulty is finding something to feed to farmed tuna that isn't fish (which either has to be bred at significant cost, or caught from the ocean, which is self-defeating since it means the tuna contains mercury). [1] https://www.youtube.com/watch?v=m4r9XC6Ta-s https://www.youtube.com/watch?v=m4r9XC6Ta-s