This sentence might have come from a novel or poem where one person compared himself to a bird and the other to a fish to express the differences but similarities between them. This metaphor could be a rhetorical device to emphasize the friendship or feelings between two people by comparing their similarities and differences. It could also be a plot arrangement that used this metaphor to draw out the interaction and story between the two people. The specific application of this sentence in the novel needed to be analyzed according to the plot and content of the novel.