MONTEREY, Calif. (AP) — Leatherback sea turtles have been plying the world’s oceans for tens of millions of years, but scientists say a rapid decline in their numbers means they could disappear from U.S. West Coast waters within decades. All seven distinct populations of leatherbacks in the world are troubled, but a new study shows an 80% population drop in just 30 years for one extraordinary sub-group that migrates 7,000 miles across the Pacific Ocean to feed on jellyfish in cold waters off California. Scientists say international fishing and the harvest of eggs from nesting beaches in the western Pacific are to blame.