Pearls and gold were still among the leading productions of the Isthmus, and the most valuable fisheries were at the old Pearl Islands of Vasco Nuñez de Balboa.[XXII‑18] Diving for pearls was performed by negroes chosen by their masters on account of their dexterity as swimmers, and their ability to hold their breath under water. From twelve to twenty under charge of an overseer usually formed a gang. Anchoring in twelve to fifteen fathoms of water, they would dive in succession, bringing up as many shells as they could gather or carry. It was a laborious calling, and attended with great danger because of the sharks that swarmed around the islands and with which they had many a fierce struggle, often losing limb or life in the encounter. The divers were required to collect a certain quantity of pearls, and any surplus they were at liberty to sell, but only to their own masters and at a price fixed by them.[XXII‑19] Ever since their first discovery these fisheries had maintained their fame, and there was obtained the largest pearl then known in the world; one that became the property of Philip II., and was described by Sir Richard Hawkins[XXII‑20] as being "the size of a pommel of a ponyard;" its weight being two hundred and fifty carats, and its value one hundred and fifty thousand pesos. It was presented by the king to his daughter Elizabeth, wife of Albertus, duke of Austria. The number and variety of pearls were such that this trade became one of the most prolific sources of wealth to Panamá, Seville alone importing in 1587 some six hundred pounds weight, many of them rivalling the choicest specimens found in Ceylon and the East Indies. From this time there occurred a marked falling-off both in quantity and quality, and in consequence a series of restrictions was put upon the industry. Notwithstanding these precautionary measures the pearl-beds became rapidly exhausted; diving proved a profitless labor,[XXII‑21] and not until several decades later was this industry revived.