The tuna fish generally eats smaller fish that range in size from 1.5 inches to 6 inches, although they will also eat crustaceans, crab larvae and squid from time to time. The diet of the tuna fish is opportunistic by nature, with tuna eating whatever fish happen to live in the waters around them.
Although diet varies by region, some of the fish that are consumed by tuna include flying fish, skipjack, puffer fish, rabbit fish, triggerfish, surgeon fish and lancet fish. Generally, the fish preyed upon by the tuna are those that travel in schools, although area of the ocean has a lot to do with what the tuna eats. For example, the Bluefin tuna population in the north Atlantic feed mainly on sand lance, while herring and mackerel are big parts of the diet of tuna in the Gulf of Maine.
Tuna have been known to swim incredible distances to feed. In fact, some tuna originating in the Gulf of Mexico swim across the Atlantic Ocean to the coast of Europe to feed before swimming back to their birthplace to breed.
Researchers say that Bluefin tuna have huge appetites and require a large quantity of fish to survive. Atlantic menhaden are the most common prey of the Bluefin in its most populous region, the western Atlantic, although populations of both the Bluefin and its favorite food are dwindling in the face of commercialized fishing.