At the time of the World War I the price of mother-of-pearl plummeted with the invention and expanded use of plastics for buttons and other articles which had previously been made of shell. Broome had been the centre of an industry that supplied up to 70% of global demand for the shell. Concerns regarding over-harvesting by the industry led to the voluntary "Northern Territory Pearling Ordinance" in 1931. Pearlers such as Jiro Muramats continued to operate out of Cossack. By 1939 only 73 luggers and 565 people were left in the industry and during the World War II, pearling virtually stopped. Japanese divers discreetly went home or were interned and Broome was bombed, destroying many of the remaining luggers. After the war, as few as 15 boats employing around 200 people remained.