The rumble fish in Hinton's novel of the same name is symbolic of the violence and the possibility for violence that exists throughout the narrative. Motorcycle Boy looks to them at the end of the novel and wonders if the Siamese fighting fish would be less violent if they were freed in the river instead of in a cage in a pet store. For Motorcycle Boy, the fish are symbolic of his own violent past that he tried to free himself from but ultimately, in the end of the novel, is unable to escape from.
The Siamese fighting fish end up being analogous to Motorcycle Boy's own story. He was violent in the place he was stuck in as a young man, but when he was finally free to leave, his behavior changed. The inherent violence that surrounded Motorcycle Boy went away with his own freedom and possibility for exploration. The Rumble Fish symbolize the potential for violence when we are trapped; they also represent the possibility for change and redemption when we are free.
A "rumble fish" is a nickname...
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