Fattier than tuna but with a milder flavor than sardines, mackerel offers the best of both worlds, and it's typically canned ready to eat with the skin and bones removed.
Moreover, canned mackerel flakes just as easily as tuna, but its extra fat means it has a better texture and flavor, making it the perfect substitute for tuna in any recipe.
You can find canned mackerel in most grocery stores, whether packed in oil, tomato sauce, spices, or water. Common brands include Patagonia Provisions and Wild Planet Foods.