In Paris Sketches: The Pilot Fish, Hemingway relies on an extended metaphor to express a profound sense of regret and, ironically, betrayal over the way his marriage with Hadley ended:
The pilot fish [...] is always going somewhere, or coming from somewhere, and he is never around for very long. He enters and leaves politics or the theater in the same way he enters and leaves countries and people’s lives in his early days [...] Nothing ever catches him and it is only those who trust him who are caught and killed. He has the irreplaceable early training of the bastard and a latent and long denied love of money. He ends up rich himself, having moved one dollar’s width to the right with every dollar that he made.
Throughout this chapter, Hemingway mourns the way his and Hadley's lives changed once rich vacationers discovered the skiing slopes in Austria. If Paris was an affordable refuge for the struggling writer in the 1920s, the mountains were the refuge from the refuge. He, Hadley, and their son could escape there when winter in the city became cold, dreary, and unbearable. They could afford warmer, more abundant meals and could appreciate one another's company. Once their refuge was discovered and Hemingway met the woman who would become his second wife, the simple "early days" of Hemingway's European life were complicated forever. His paradise began to rot from within.
Hemingway feels betrayed not by the rich (who acted only according to their nature), but by the "pilot fish" that told them all they should come meet Hemingway in the mountains. The pilot fish is a tropical fish that is often spotted traveling near larger sea creatures like sharks. It is a carnivore, but it does not hunt the animals it follows. Rather, it feeds on the parasites carried by these animals. This symbiotic relationship can look strange and has led to the misconception that the fish "pilots" sharks to potential prey. This idea seems to be the basis of Hemingway's metaphor: Hemingway feels that the rich "sharks" would never have found or destroyed his and Hadley's wintertime sanctuary if not for the humble, not-particularly-wealthy "fish" who came first, built a rapport with Hemingway, and finally made the fatal introduction.
What Hemingway is most angry about still, at the end of his life, is that "it is only those who trust [the pilot fish] who are caught and killed" by the sharks. Where he once held himself open to every new connection he made in Paris, the pilot fish shattered his sense of safety among fellow writers and artists. Some "friends," it seems, are willing to throw him to the "sharks" for their own gain. Ironically, the most humble-seeming writers and artists can turn out to be the most ruthless. These people have "the irreplaceable early training of the bastard;" born without an automatic claim to "old money" and prestige, they have been fighting their whole lives for what they believe they deserve. They will do anything to get it.
Many critics believe that the "pilot fish" Hemingway writes about so scathingly is the ambitious and well-connected novelist John Dos Passos. Hemingway made many social connections through Dos Passos before they had a falling-out over Dos Passos's increasing loyalty to wealthy, conservative politicians. Regardless of whether or not Dos Passos is the person Hemingway has in mind in this passage, the metaphor of the pilot fish reveals his lifelong anxiety about social climbing and the way it can both make and break a person.