I would first establish that the fish is a symbol of nature by quoting the imagery the narrator uses that shows its intimate connection to nature:
He was speckled with barnacles,
fine rosettes of lime,
with tiny white sea-lice,
and underneath two or three
rags of green weed hung down.
Second, I would show that Bishop introduces a conflict between nature and civilization through also having the narrator use imagery about the fish that associates it with civilization. The narrator struggles between her understanding of the fish as a natural creature and her desire to deny the fish's essential nature as a free creature by domesticating it, making it an artifact of human culture:
his brown skin hung in strips
like ancient wallpaper,
and its pattern of darker brown
was like wallpaper:
shapes like full-blown roses
Finally, I would note that the narrator's encounter with looking into the eyes of the fish decides her attitude toward the fish. She goes from the triumph of having caught it as...
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