Answer :
The Colomber is defined in the text as "the fish that sailors fear above all others, in every sea in the world. It is a tremendous, mysterious shark, more clever than man. For reasons that perhaps no one will ever know, it chooses its victim, and when it has chosen it pursues him for years and years, for his entire life, until it has succeeded in devouring him. And the strange thing is this: no one can see the colomber except the victim and his blood relations."
The Colomber has been a source of torment for Stefano even when he is far away from it as the Colomber follows him everywhere he goes. It keeps on tormenting him in his thoughts and actions and his mind is never at peace. Stefano is afraid of the Colomber following him every time he goes out to sea.
Stefano has been driven to the edge by the Colomber throughout his life. He takes many precautions in order to avoid the Colomber after he first sees it. Stefano's father sent him to study at an inland city which was very far off from the coast. Stefano used many distractions like work, friends, diversions and love affairs to put the thoughts of the Colomber out of his mind but the Colomber's thoughts still tortured him like a mirage. As time passed, gradually the thoughts became more and more insistent. Whenever he returned from school during vacations, he could see the Colomber as a small black point surfacing on the water now and then, waiting for him to come to the sea.
Once Stefano completed his education his father declared "Seafaring is not for you, my son. You must resign yourself. After all, you will be able to make your fortune on land too." and opened a shop for him inland. But deep inside, Stefano longed to continue his family legacy and pursue his dreams of being a sailor. In the end, Stefano decides that it is not right to keep the Colomber waiting and becomes a sailor. He decides to struggle with all his power once he encounters the Colomber.