Is it permissible for me to sell fish and trade in them after they have multiplied and been cared for, and what is the right of the worker who brought them the first time?

The fish left by the worker belongs to him. If you cultivate them until they multiply, then you both are partners in them, and you share their produce by agreement. Sheikh al-Islam Ibn Taymiyyah justified this by stating that the acquired wealth resulted from the benefit of the worker's body and the benefit of the capital, and that the growth is generated from both origins. He cited the story of Umar ibn al-Khattab with his two sons, Abdullah and Ubaydullah, in which Umar ruled that the profit should be shared between them. When the partnership is dissolved, the worker has the right to dispose of the original fish by selling them, entering into a new partnership, or raising them in a place provided by your father.