Tuna being Hobbes' favorite food was made apparent in the very first strip, where Calvin uses a tuna fish sandwich to lure and trap Hobbes, and subsequently became something of a running gag.
As Hobbes is unable to use a can opener, he must rely on other means in order to obtain tuna. He is liable to pounce on Calvin when the latter opens a can of any sort, as the can could contain tuna, as proved in a strip where Calvin attempts to open a can of pineapple and Hobbes pounces on him, claiming "all cans sound the same". Calvin also once offered salmon to Hobbes, indicating Hobbes' preference for seafood.
In his sleep, Hobbes is heard dreaming of tuna. This once led him to sleepwalk downstairs in order to reach the tuna cans. As he was sleepwalking, he may not have realized that he could not open them. Calvin tends to distance himself from Hobbes when he dreams of tuna, fearing that he may be mistaken for a meal.
Hobbes stopped eating tuna when he found out that the fishing industry kills dolphins to get it. His new favorite food became fresh swordfish steaks grilled outside.