[By the midpoint of the poem] [t]he poet does not simply relinquish her desire for imaginative contact with the fish. But her attention shifts from spatial to historical imagining. History is no longer distant and figurative but "still attached" in the form of "five old pieces of fish-line, / or four and a wire leader / … with all five big hooks / grown firmly in his mouth." Five wounds on a fish make him a Christ figure, but the epiphany he brings the poet has nothing otherwordly about it. The domestic images at the beginning of the poem, followed by the battered body of the fish, evoke the poet’s unconscious life, the uncanny return of the repressed which can "cut so badly." But Bishop can entertain such self-reflection now within the larger context of the life of nature and the beholder’s tentative grasp of it. She no longer has to define a discrete interior space through dream or symbolic abstraction in order to explore her subjectivity; she has brought the self out of nocturnal seclusion and explored its relation to everything under the sun.
There is also a pervasive but ambiguous sexual quality to the fish. An untamable, corporeal energy violates the domestic world of wallpaper and roses. The fish, a he, hangs like a giant phallus, yet as the beholder imagines his interior, its "pink swim-bladder / like a big peony,? He takes on a female aspect. Indeed, the hooks in his mouth suggest that phallic aggression is the fisherman’s (woman this time) part. This hermaphroditic fish challenges the conventional hierarchical antithesis of female nature and male culture. Here there is no struggle, and the victory is not exclusive.
For Bishop, nature mastered as static knowledge is a fish out of water. Its beauty and venerability belong to time. Yet it can be entertained, with a certain humility and lightness (such as simile registers), for its figurative possibilities. The poet "stared and stared" even though the fish did not return her stare. Her imagination transforms a "pool of bilge / where oil had spread a rainbow" into an ecstatic (and perhaps deliberately excessive) "rainbow, rainbow, rainbow!" Such an epiphany, set as iut is in the highly ephemeral space of the rented boat with its rusted engine, must be of mortality. The grotesque is the style of mortality not because it makes us turn away in horror but because it challenges the rigid frames of thought and perception through which we attempt to master life. All the conceptual and emotional contradictions that emerge within the description of the fish point to the letting go.
from Bonnie Costello, "Attractive Mortality," Chapter 2 in Elizabeth Bishop: Questions of Mastery (Cambridge: Harvard University Press, 1991), 63-64.