THE DIRE ECOLOGICAL condition of the world's oceans -- and the role that overfishing plays in their degradation -- has been the subject of numerous studies and blue-ribbon commissions. The message is clear: The oceans are far more fragile ecosystems than people have believed, and only dramatic changes in human interaction with them can prevent irreversible degradation. A recent study of big-fish diversity worldwide is the latest to reinforce this depressing conclusion.

The study, released recently by Science magazine, examined the diversity of big predator fish species caught by Japanese fishing boats over the past five decades. An international team of scientists led by biologists Boris Worm and Ransom A. Myers at Dalhousie University in Nova Scotia studied records of the Japanese catch on what are called long lines -- that is, fishing lines stretching many miles and baited with multiple hooks. They corroborated these records with other sources, and they show that certain areas of the oceans -- generally temperate regions -- form particular "hot spots" of marine life. Yet as industrial fishing ramped up in the second half of the 20th century, these hot spots became much cooler, barely hotter, in fact, than the rest of the oceans. As the total catch of tuna, billfish and other big predator fish exploded over the past few decades, the diversity of species caught in any given area -- that is, the number of species caught on a given number of hooks -- plummeted by as much as 50 percent. While climate and other factors play a role, the authors wrote that they "could not identify a factor other than fishing that may plausibly explain long-term, global-scale declines."

These results are particularly disturbing because they deal with the open ocean, not with coastal waters, where depletion of fish species was already well established. A decline of species diversity could make oceanic ecosystems more vulnerable to climate change and other environmental shifts.

The good news in the study is that a few hot spots remain -- though they are dramatically less vibrant than they were. One of these is off the southeastern coast of the United States. Another is south of Hawaii. These areas desperately need protection. More broadly, commercial fishing needs to be brought down to levels that will be, in the long-term, sustainable and will permit whatever recovery of species diversity is still possible.