OPEC ministers meeting in Vienna vowed to try to keep a choke on oil supply in the hope that prices will stay around current 19-month highs.

Ending talks late Thursday, cartel leaders acknowledged that the next three months will probably see a seasonal dip in surprisingly strong world demand for petroleum products.They also agreed that developments like the Valdez oil spill that cut Alaskan supply and given prices their latest boost are one-time affairs. Still, said Algerian Oil Minister Saddek Boussena, OPEC should be able to defend its target price of $18 a barrel.

The average for cartel crudes (which vary in quality) has only just climbed back to that level, last touched in a Persian Gulf war flare-up in August 1987, after a glut last year took it down almost to $10.

But Boussena and others said everything depended on OPEC sellers sticking to output quotas assigned from Jan. 1 to erase the glut.

Secretary-general Subroto, however, said that Abu Dhabi in the United Arab Emirates, which has been a leading source of excess supply, had telexed to say it was cutting back.

Meanwhile, the Canadian government threatened Thursday to retaliate against a superfund oil tax imposed by the United States that discriminates against imported oil.

International Trade Minister John Crosbie said the tax, which was implemented under the 1986 "Superfund" legislation to provide $8.5 billion to clean up environmental problems such as toxic waste dumps, has cost Canadian oil exporters more than $8 million.