The Senate yesterday ratified an international agreement to ban ocean dumping of plastics, which kills hundreds of thousands of marine mammals every year through strangulation or intestinal damage.

U.S. ratification was considered essential to trigger the agreement, which becomes effective one year after nations representing half the world's gross shipping tonnage sign on. The Senate's 93-to-0 vote pledging U.S. support adds enough weight to satisfy the ratification requirement, according to Senate aides.

"We must dispel the notion that the ocean is a convenient place to dump wastes," said Sen. Bill Bradley (D-N.J.).

The agreement, an annex to the Marine Pollution Treaty of 1973, represents the first effort to stop littering of the seas with plastic nets from commercial fishing vessels, plastic sheeting and ropes from offshore oil rigs and the plastic household products of merchant vessels, pleasure ships and navies.

Durable, strong and often invisible to sea creatures, plastic entangles seals, whales, turtles and birds. It can wind around them tightly enough to strangle or prevent feeding. Often mistaken for food, the indigestible synthetic can lead to starvation or intestinal blockage.

The pact ratified yesterday will "go a long way" toward sparing marine animals from the death and suffering caused by plastic, said Sally Ann Lentz, staff attorney for the Oceanic Society. Other environmentalists question the immediate benefit of an agreement that is almost impossible to enforce and makes no provisions for the tons of plastic debris already in the oceans.

A provision exempting "public vessels" from the ban opens the way for continued dumping by navies, major contributors to the problem.

In prohibiting ocean disposal of plastics within 25 miles of shore, the agreement requires nations to provide port facilities for the debris. Few U.S. ports have such facilities, giving ship captains little choice but to dump their garbage overboard.

The House passed a bill Oct. 13 to implement the agreement, requiring the Navy to comply in five years and leaving responsibilty for debris collection to port authorities. A similar bill is in the Senate.

Lentz urged Congress to desposit the instrument of ratification with the International Maratime Organization even before it passes legislation. The deposit is necessary to start the countdown to activation, which Lentz said should proceed quickly. She said a recount of the world's shipping tonnage is expected to result in a ratification shortfall -- even with the Senate vote.

"It might be prudent to expedite the process so that the whole thing doesn't fall through," she said.