Why was talking only when necessary at sea considered a virtue in The Old Man and the Sea?

Fishing is a profession taken very seriously in The Old Man and the Sea. The old man, Santiago, and his friend Manolin, a young boy, live in a fishing village with many other fishermen, some more successful and some less. Santiago has gone 84 days without a single catch, and Manolin's parents refuse to let Manolin accompany him after the first 40 days without any catches, since his boat is now considered unlucky. Manolin hates to leave Santiago, but must listen to his parents. Now Santiago fishes alone, and feels lonely as a result. Even thought he and Manolin did not speak often when they fished together (as shown in the quote below), his company comforted and strengthened Santiago, as is evidenced by how often Santiago mentions Manolin and how he wishes he was there to help him.

When he and the boy fished together they usually spoke only when it was necessary. They talked at night or when they were storm-bound by bad weather. It was considered a virtue not to talk unnecessarily at sea and the old man had always considered it so and respected it. But now he said his thoughts aloud many times since there was no one that they could annoy.

It was probably considered a virtue not to speak more than you had to at sea so as to not alert the fish to your presence. Throughout the book, we see Santiago's sensitivity and awareness to the perspective and behavior of the fish, and keeping quiet could very well be an extension of that awareness. It is also something that developed as a habit or custom among the fishermen, because general silence while fishing was something that all of the fishermen followed.

Sometimes someone would speak in a boat. But most of the boats were silent except for the dip of the oars.

In contrast, when at the docks, the fishermen spoke to each other freely and to Santiago freely, whether politely or to denigrate his fishing skills.

They sat on the Terrace and many of the fishermen made fun of the old man and he was not angry. Others, of the older fishermen, looked at him and were sad. But they did not show it and they spoke politely about the current and the depths they had drifted their lines at and the steady good weather and of what they had seen.