The discoverer, or his assigns, being citizens of the United States, may be allowed, at the pleasure of Congress, the exclusive right of occupying such island, rocks, or keys, for the purpose of obtaining guano, and of selling and delivering the same to citizens of the United States, to be used therein, and may be allowed to charge and receive for every ton thereof delivered alongside a vessel, in proper tubs, within reach of ship's tackle, a sum not exceeding $8 per ton for the best quality, or $4 for every ton taken while in its native place of deposit.