Elizabeth Bishop's poem The Fish displays her ecological awareness that leads her to accept a relationship of co-existence between human beings and non human beings. This ecological awareness in the poem is reflected when she leaves the fish free.

      The poem in the beginning is simply a narration about what happen in a particular day. The speaker narrates her catching of a big fish and other small fishes too. In her narration about the fish the commercial attitudes are reflected. She begins with the description of the skin and includes other parts like ‘white flesh’, ‘gills’, ‘bones’, ‘lip’, ‘jaw’ and so on. Reflecting the commercial attitude, she in her imagination unskins the flesh of the fish for commercial sale/benefit. The description of the fish interms of its parts can be related to cutting of the fish into parts that can be used for different purposes.
The poet attempts to look into the eyes of the fish marks a crucial point in the poem. Perhaps this is an attempt to identify the human existence with the existence of the fish. As she continuously stares at the fish, she becomes aware of interdependent existence. The awareness of the speaker is the awareness of the transitoriness of human glory, human domination over animal or even of commercial benefit. The rust in the engine and rainbow can be related to this awareness. The killing of the fish can rust the ecology. And rainbow reinforces the awareness of transitoriness of that achievement. When the poet or speaker decides to set the fish free interms of ecology, she not only saves of life of that fish but also shaves many lives. In that sense the poet gains big things by losing some small thing.

The Fish - Poem by Elizabeth Bishop

I caught a tremendous fish
and held him beside the boat
half out of water, with my hook
fast in a corner of his mouth.
He didn't fight.
He hadn't fought at all.
He hung a grunting weight,
battered and venerable
and homely. Here and there
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.
He was speckled and barnacles,
fine rosettes of lime,
and infested
with tiny white sea-lice,
and underneath two or three
rags of green weed hung down.
While his gills were breathing in
the terrible oxygen
--the frightening gills,
fresh and crisp with blood,
that can cut so badly--
I thought of the coarse white flesh
packed in like feathers,
the big bones and the little bones,
the dramatic reds and blacks
of his shiny entrails,
and the pink swim-bladder
like a big peony.
I looked into his eyes
which were far larger than mine
but shallower, and yellowed,
the irises backed and packed
with tarnished tinfoil
seen through the lenses
of old scratched isinglass.
They shifted a little, but not
to return my stare.
--It was more like the tipping
of an object toward the light.
I admired his sullen face,
the mechanism of his jaw,
and then I saw
that from his lower lip
--if you could call it a lip
grim, wet, and weaponlike,
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.
A green line, frayed at the end
where he broke it, two heavier lines,
and a fine black thread
still crimped from the strain and snap
when it broke and he got away.
Like medals with their ribbons
frayed and wavering,
a five-haired beard of wisdom
trailing from his aching jaw.
I stared and stared
and victory filled up
the little rented boat,
from the pool of bilge
where oil had spread a rainbow
around the rusted engine
to the bailer rusted orange,
the sun-cracked thwarts,
the oarlocks on their strings,
the gunnels--until everything
was rainbow, rainbow, rainbow!
And I let the fish go.