Elizabeth Bishop provides a great deal of descriptive information about the fish because she wants the reader to have a very clear understanding of what has been caught. This is no ordinary fish. This creature is "tremendous" in size and amazing in appearance. Instead of smooth scales, his age was apparent.
his brown skin hung in strips
like ancient wallpaper,
and its pattern of darker brown
was like wallpaper:
shapes like full-blown roses
stained and lost through age
Most notable of all, however, was the evidence that this fish was a fighter. It had been caught and had escaped from other anglers in battles from long ago.
from his lower lip...
hung five old pieces of fish-line,
or four and a wire leader
with the swivel still attached,
with all their five big hooks
grown firmly in his mouth.
Recognizing what these remains indicate about the fish now caught, the poet at first is "filled up" with pride at having succeeded in catching this prize. Upon further consideration, however, the poet concludes that the fish is too magnificent, too deserving of respect in homage to his age and experience, to become a stuffed trophy on a wall. "And I let the fish go."
Bishop wanted readers to understand the wonder of the fish, and the reasons for letting it go. Lots of details were needed to convey this information effectively.