NEW YORK (AP) - A mysterious illness is killing tens of thousands of lobsters in Long Island Sound, alarming lobstermen and conservation officials. The die-off is the Sound's worst in nearly a decade, officials said. About eight of every 100 lobsters caught are coming up dead, said Byron H. Young of the state Department of Environmental Conservation. Tests on blood taken from the lobsters have not pinpointed the cause. Water quality tests have ruled out pollution and low oxygen levels. New York has 900 commercially licensed lobstermen. New York was the nation's No. 3 lobster producer in 1998, behind Maine and Massachusetts, with 8.5 million pounds of lobsters worth nearly $30 million. The fall harvest normally accounts for half of the annual income for lobstermen.