What is the theme in The Fish by Marianne Moore?

Nature is the main theme in the poem. Moore's poem attests to the fact that although humanity may attempt to shape nature to fit its needs, it is ultimately a futile endeavor. Time itself effaces humanity's attempt to control nature. Moore threads images of the human world—culture—throughout the poem. In almost all cases they are negative images, suggesting the worst that human beings have to offer. In the opening stanza, she refers to the ocean's sand as "ash heaps," creating the sense, ironically, of a landscape decimated by fire. Later, she describes the sun as moving with "spotlight swiftness" as it slices through the water.