One of the interesting things about "The Fish" is that the reader discovers the significant aspects of the fish as the speaker of the poem does. The other interesting device of the poem, in terms of style, is that the speaker reveals her understanding and appreciation for the fish through imagery. She, the speaker, withholds her thoughts and emotions until the very end of the poem.
First, note that the poem is not about the "victory" of catching the fish. The entire poem is of the objective (eventually becoming subjective) perception of the fish. In objectively describing the fish, the speaker and the reader discover things which lead to a more subjective appreciation.
Although the fish doesn't put up a fight, he continues to fight, to breathe that "terrible oxygen." Upon catching the fish, the speaker describes it as "battered and venerable / and homely." She has respect ("venerable") for this battered fish and feels pity for its plain, unattractive ("homely") look. This is the objective perspective becoming subjective and more personal.
The speaker describes the fish's skin/scales like armor rusted over time, "shapes like full-blown roses / stained and lost through age." She describes the fish like an old warrior or an old warship adorned with barnacles and sea weed. Following these subtle praises of the evidence of the fish's history of survival, the speaker imagines the fish's colorful insides. She tries to intuit some meaning by looking into the fish's eyes but sees only a semi-transparent surface ("isinglass") which shows how she is struggling to achieve a personal understanding of the fish by making objective observations.
Acknowledging the five hooks and fishing lines dangling from the fish's mouth, the speaker thinks of them as signs of wisdom and medals of valor:
Like medals with their ribbons
frayed and wavering,
a five-haired beard of wisdom
trailing from his aching jaw.
Realizing that this battered, rusted fish has battled to stay alive so many times, the speaker appreciates the fish's perseverance and appreciation for life. Seeing the rainbow dispersed on the surface of the oil, the speaker translates this abundance of color to vitality and symbolic vibrancy with which she thinks the fish must have lived. The fish might look like old brown wallpaper but it has come to symbolize the fullness of life which is comparable to the fullness of colors in a rainbow. It is now with this subjective perception (appreciating life in its full splendor of color and vitality) that she looks upon everything: the rusted engine, oarlocks, gunnels, and of course, the fish.