Manama's economy in the early 20th century relied heavily on pearling; in 1907, the pearling industry was estimated to include 917 boats providing employment for up to 18,000 people. Shipbuilding also employed several hundred in both Manama and Muharraq. The estimated income earned from pearling in 1926 and subsequent years prior to the Great Depression was £1.5 million annually. Custom duties and tariffs served as the prime source of revenue for the government. With the onset of the Great Depression, the collapse of the pearling industry and the discovery of oil in 1932, the country's economy began to shift towards oil.