In the story Rumble Fish, what exactly is the term "rumble fish" referring to?
In Chapter 10, Motorcycle Boy goes to a pet store, and is fascinated by the brightly-colored Siamese fighting fish. These fish, called "rumble fish", are known for their undiscriminating ferocity - if they are left together in the same tank they will kill each other, and Motorcycle Boy notes that "if you lean a mirror against the bowl they'd kill themselves fighting their own reflection". The situation of the rumble fish parallels the violent, purposeless existence of youths like Motorcycle Boy and Rusty James. Mired in poverty and alienation with anger and pent-up energy, they are trapped in a cycle of destruction from which there is no escape.
In a symbolic attempt to free the frustrated captives from their stifling environment, Motorcycle Boy, in Chapter 11, breaks into the pet store to steal the rumble fish and set them free in the river. He is pursued by the police and killed, and in a scene representative of the overriding motif of futility, he lies dead along with the rumble fish, just a little too far from the river which is too polluted to have allowed them to survive anyway.