Tuna really isn't that great a source of protien on the list of quality protien sources. If it's tuna or nothing, go for the tuna of course. This isn't a flame, just a fact.
You can also include other fish like salmon and actual tuna fillets or steaks (the pouched or canned stuff just don't cut it), seafood like crab, lobster, shrimp, LEAN ground beef, turkey, cheeses (you'll need a little extra fat while on low carb high protien), wild game meats like deer, moose, elk, caribou, etc., etc.
If you're having trouble getting in enough protien through solid foods, just kick up how many protien shakes you're taking.