Throughout the novel, life in Spain is presented as more natural, less hectic, somehow more real, than the decadence and phoniness of urban existence in Paris. The unique, primal rhythms of Spain are also given to us a welcome contrast to the puritanical restrictions of life in mid-twenties America. Jake and Bill's fishing trip is a development of this central theme.

Out in the wilderness they find peace; they find their true selves. Here they can be authentic to themselves and to each other. Their natural surroundings energize the soul, establishing a connection between the men and their environment. As the men are in nature, so they can also be more natural with each other, free from all the complex dynamics of their social circle.

If the fishing trip symbolizes anything, it is the enormous gulf that exists between town and country, nature and civilization, authentic and inauthentic existence. This gulf cannot be closed, nor can the numerous tensions it generates ever fully be resolved. At some point, Jake and Bill must return to the world from which they seek refuge. However, for an all-too-brief interlude, it's possible for the men to retreat from the ceaseless flux of a world which is sometimes just too complicated for both of them, and learn more about themselves and each other.

When Bill and Jake go fishing together, the entire mood of the story changes for a brief interlude. When the men are out in the midst of nature, a type of peace transcends them. Jake temporarily can escape from all of his problems and time seems to almost stand still. You can see how much more relaxed Jake is, and he shares things with Bill that have been difficult to discuss prior to this scene. It is a very spiritual scene of male bonding.

So, if you are looking for something that the fishing scene symbolizes, I would say it symbolizes peace, or perhaps escape. Occuring as it does half-way through the novel, it is juxtaposed with the hectic scenes of Paris, and drinking, and women, and the sordid existence of their past activities.