The U.S. market for ivory has virtually collapsed since President Bush banned imports a year ago to help protect the African elephant, and sales have dropped in some other countries, the World Wildlife Fund said yesterday.

But the group was worried that ivory carvers may turn to using hippopotamus teeth and suggested instead that they ply their art on the tusks of extinct mammoths being dug from the frozen tundra of Siberia.

Bush announced a moratorium on ivory imports June 5, 1989, and a ban is also in effect in Japan, the world's biggest market for ivory.

"Dealers report that wholesale prices for jewelry and simple carvings, which historically accounted for most of the U.S. market, have been discounted by 40 to 70 percent," the group's report said. "One major ivory trader in New York stated firmly that price is irrelevant in the current U.S. market, since demand does not exist."

Results in other areas are mixed but the report saw encouragement for the fund's efforts. It estimated that Africa's elephant population decreased from 1,194,331 in 1981 to about 610,000 last year.

One substitute for elephant ivory: the tusks of prehistoric mammoths dug up by the Soviet Union. "We don't try to protect the mammoth -- it's been extinct for 10,000 years," said Michael Sutton, one of the authors of the report.

He said he was more concerned about carvers who are shifting to hippopotamus teeth. Many hippos have to be killed to get the same weight in teeth as comes from the tusks of one elephant.