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.