LONDON—Humans have studied the seas for centuries. But the publication Monday of the first global marine census suggests that the golden age of oceanic discovery still lies ahead.

Researchers participating in the census say they have now pinpointed about 250,000 species that live in the sea, but estimate that another 750,000 species still elude human discovery. And that's without counting millions of microbe species, which constitute 90% of the ocean's biomass.

"Diversity is an indicator of health in the oceans," said Ian Poiner, who chaired the census steering committee, in an interview here, where the census was unveiled. Dr. Poiner added that because of increasing human impact on the oceans—in the form of pollution, over-fishing and acidification—"we need to understand how sea life is being altered."

In their decade of trolling the seas, the census takers added 1,200 new species to the known tally a decade ago, and have yet to formally identify another 5,000 or so species collected over the same period. The most common additions were crustaceans, followed by mollusks. Scores of new species were discovered even in the well-studied fish group.

Several findings were rich and strange: a hairy new species dubbed the yeti crab; a 21-foot-long squid; a new species of lobster weighing 8.8 pounds; an ancient shrimp thought to have become extinct 50 million years ago.