A noticeable feature of these fisheries is their uncertainty, a prosperous season being followed by an absence of fishing sometimes extending over ten years or more. This is not of recent development. Over eight hundred years ago a total cessation of yield for a considerable period was recorded[132] by Albyrouni, who served under Mahmud of Ghazni. He stated that, in the eleventh century, the oysters which formerly existed in the Gulf of Serendib (Ceylon) disappeared simultaneously with the appearance of a fishery at Sofala in the country of the Zends, where previously the existence of pearls had been unknown; hence it was conjectured that the pearl-oysters of Serendib had migrated to Sofala. In the 249 years since Ceylon passed from the dominion of the Portuguese in 1658, there have been only sixty-nine years in which the pearl fisheries were prosecuted. During the last century there were only thirty-six regularly authorized fisheries. Enormous quantities of oysters have appeared on the reefs, giving rise to hopes of great results, only to end in disappointment, owing to their complete disappearance. In the fall of 1887, for instance, examination of one of the reefs revealed an enormous quantity of oysters, covering an area five miles in length by one and a half miles in width, with “600 to 700 oysters to the square yard” in places. It was estimated by the inspection officials that there were 164,000,000 oysters, which exceeded the total number taken in the preceding sixty years, and which should have yielded several million dollars’ worth of pearls in the following season, according to the usual returns. But some months later not an oyster was to be found on this large reef, the great host presumably having been destroyed by action of the sea. Numerous reasons are assigned for the failure of promising reefs. Those most frequently heard are that the currents sweep the oysters away, that they are devoured by predaceous enemies, that they are covered by the shifting bottom, or that they voluntarily move to new grounds.