THE GOOD NEWS is that the International Whaling Commission last weekend approved what amounts to a ban on the killing of sperm whales. The bad news is that it once again defeated a proposal for an indefinite moratorium on all commercial whaling. To its credit, the Reagan administration continued the U.S. support for the moratorium that now extends through four presidencies. Interior Secretary Watt was responsible for appointing and backing a dedicated conservationist and whale expert to head the U.S. delegation.

Since the IWC began setting quotas for commercial whaling, the allowed worldwide catch has fallen from near 50,000 to below 15,000. Whaling from immense factory ships has been banned. sanctuary has been established in the Indian Ocean where whales may not be killed for 10 years. There are significant achievements requiring, as they do, the cooperation of many nations. But whaling is still allowed for several species that are on the international endangered species list. Of what were once the many species of the world's largest -- and, don't forget it, largest-brained -- creature, only the small minke whale has not been hunted by man to the point of scarcity or near extinction.

Now that there are inexpensive substitutes for all whale products, including the use of whale meat for human consumption, there is little excuse left for continued commercial killing. Yet endangered industries die hard. Despite the fact that sperm whales cannot be eaten because of their dangerously high levels of mercury, and despite their precariously small numbers, Japan fought hard against the complete ban on killing sperm whales that was recommended by the IWC's scientific advisers. The partial ban that was finally adopted included an exemption for the western Pacific and an offsetting increase in Japan's quota of minke whales. Yet the Japanese delegation made no secret of its dissatisfaction with the outcome.

Little by little the pressure of international opinion is eroding the now unnecessary whale trade. Only the Japanese market keeps it alive. Ultimately a ban seems inevitable. ythe danger is that it will come too late. Among all animal species, the whale's survival seems to be peculiarly chancy. Once a decline in the numbers of a particular type of whale can be documented, the process often seems to be irreversible. Many whale species -- including the largest, the Great Blue -- have not yet recovered after decades of protection. The reasons for this unusual behavior are not known. It may have to do with the small number of whale offspring, their long infancy, and the whales' wide range.

Until the answers are known, a moratorium, not the let's-wait-and-see-what-happens justification for annual quotas, is the only responsible policy. Perhaps next year the IWC will take the last needed step.