Described with astounding deatail, Elizabeth Bishop's "tremendous fish" may well be a Northern Pike (also known as Pike-perch) who has been most likely been caught with steel leaders and treble hooks that yet hand from his old lip after he managed to break the lines. In the description of this brown fish who is so old that barnacles have grown on him, the poet employs several figures of speech, among them similes and metaphors and synedoche.
Two metaphors that are essential to understanding the poem are as follows: When the speaker looks into the eyes of the "venerable" old fish, she notices that they are larger than hers,
the irises back and packed
with tarnished tinfoil
seen through the lenses
of old scratched isinglass.
1. The "tinfoil" and "scratched isinglass" create comparisons that extend beyond as metaphors for the eyes of the fish that connote the description of natural phenomena as well as the self-referential nature of the speaker's interpretation. For, as she reflects upon the dignity of this creature of nature, the speaker realizes its significance.
2. In another metaphor, this realization culminates in "victory [that]filled up/the little rented boat." This "victory" is the celebration of the nobility of nature that can come from destruction:
was rainbow, rainbow, rainbow!
The metaphor "rainbow" is the victory of both the fish and its capturer as the promise of hope and beauty is experienced. And, herein lies the theme of Bishop's poem: Respect for Nature that reveres and renews life.