HUMANITY DEPENDS on the oceans, but their worsening state gets little attention. Good for Secretary of State John F. Kerry, then, for trying to elevate the issue last week in an international oceans conference in Washington. The conference produced a billion dollars in pledges for ocean programs, promises from other nations to better protect their marine ecosystems and the news that President Obama will set aside a vast portion of U.S. waters in the central Pacific for ecological conservation. That’s all to the good. But the health of the oceans — sources of employment, recreation and food for billions — depends on what Mr. Kerry and those like him can get other nations to do.

Mr. Obama’s plan is to protect nearly 700,000 square miles of Pacific habitat adjacent to islands and atolls the United States controls. Because the president has authority to preserve precious natural land- and aqua-scapes, and because the United States controls more of the planet’s oceans than anyone else, Mr. Obama is perhaps the person on the planet with the most power to protect the seas with a stroke of a pen. The White House will solicit feedback over the next few months, after which Mr. Obama might amend his ocean plan. But, like Presidents Bill Clinton and George W. Bush before him, Mr. Obama should not shrink from using his powers to preserve pristine ecological treasures.

Even with the preservation of some habitat, though, people will need to do a better job of managing the exploitation of ocean resources that will continue. According to a 2012 analysis from California Environmental Associates, “over 40 percent of fisheries have crashed or are overfished, producing economic losses in excess of $50 billion per year.” Some 80 percent of the world’s catch is pulled in from unmonitored fisheries. Desperate fishermen, many in developing countries, are spending more time and traveling further to catch fewer fish. About two-thirds of unmonitored stocks could provide more fish on a sustainable basis — with the right oversight.

The United States has gone a long way to fixing its regulation, deploying a system that gives fishermen ownership interests in the long-term health of their fisheries. This model could work in many other places, if those nations could enforce the rules on which the system depends. The United States can and should help other governments develop this capacity, though it ultimately will be up to foreign leaders to act in their nations’ best interest.

Then there is the issue of ocean acidification, driven by the release of carbon dioxide into the atmosphere. The ocean is absorbing huge amounts of the greenhouse gas, which will, over time, kill coral, shellfish and other creatures sensitive to a lower pH. The problem demands worldwide effort, led by the United States, to reduce the amount of carbon dioxide pumped into the air.

Efforts to curtail overfishing or reduce carbon emissions will not be easy, not least because both require many countries to work together. But the consequences of inaction are frightful.