Several years ago I drove to the coast (hour and a half) and bought a large hunk of tuna "right off the boat" to bring home to can. Looked in my canning books and, like Bonnie said, it was recommended to pack it in water, though I don't remember the pre-cooking bit. The fellow I bought it from told me his wife canned it without pre-cooking & without additional water, so I followed his advice (first time canning fish). Worked beautifully; the fish produced it's own little bit of broth in the canning process; tastes good, stayed moist. Only "problem" - I way overestimated how much tuna this family of two might use, and still have quite a lot left LOL