The world’s oceans generate more than half the oxygen we breathe and half the food we eat. Plus, they regulate our weather and climate. Yet they are perhaps the planet’s most vulnerable ecosystem.

“You see this massive body of water with its power, its waves, and its depth, and you just say, ‘Man, nothing could touch that.’ But we’re wrong. It’s fragile,” U.S. Secretary of State John Kerry said in an interview with National Geographic.

Overfishing has depleted 90 percent of large fish, such as tuna, swordfish, marlin, cod, halibut, skates, and flounder. Agricultural runoff and sewage have polluted coastal waters with nitrogen, phosphorous, and other algae-feeding nutrients, creating more than 500 large dead zones globally. Warming ocean water is 26 percent more acidic than before the industrial revolution, killing coral reefs and shellfish.

Meanwhile, eight million tons of plastic trash flows into the oceans every year—an amount that doubles every decade. This debris entangles and kills seals and whales, has been ingested by 90 percent of seabirds, and has spread into every continental nook and cranny, including Arctic ice.

Kerry, who grew up in Massachusetts, wants to keep the world’s focus on these perils. This week, he is hosting the third annual Our Oceans Conference in Washington, D.C., where scientists, activists, fishermen, and diplomats from around the world have gathered to consider how to make fishing more sustainable and clean up pollutants.

He has prodded 45 nations into signing onto a global network to monitor and combat illegal fishing, and worked to assure that the conference, which will convene in Malta next year, keeps ocean health at the forefront of international talks. Participants in the first two conferences committed more than four billion dollars to conservation programs and promised to set aside almost 2.3 million square miles of ocean for protection. Kerry says he expects another $1.8 billion in pledges to be made this year.

Kerry discussed efforts to restore ocean health with National Geographic.

You’ve got a lot on your plate. The ceasefire in Syria, keeping North Korea and Iran from acquiring nuclear weapons, 15 years of war in Afghanistan. Why do you make time for oceans?

Every one of those are life and death issues. And the oceans are, too. If we don’t preserve the oceans from nitrate runoff and plastic and chemicals, and if we don’t preserve it from acidification, and if we don’t preserve it from grotesque overfishing—too much money chasing too few fish—we’re going to have the most massive ecosystem on the planet in peril. It is in peril. And I view this as important as any of those other issues.

Others in the Obama Administration could have tackled oceans. Is there something about oceans that you have a connection to?

I have been passionate about the oceans since I dipped my toes in the water at the first age of three. If you come from Massachusetts, you are connected to the sea. And you go to Cape Cod, you go to the islands, you grow up near the water or on the water, I think you understand this. I spent a long time on the water. I went into the Navy because I love the ocean and I am still privileged to have a place that I can go to that’s by the water.

Manufacturers make about 360 million pounds of resin every year. What responsibility do manufacturers have to pay for managing their material that is distributed widely in developing nations that lack the capacity to collect it up? The Europeans and Canadians are moving toward producer-responsibility laws—they pay a tax on what they produce that creates a fund. Is there room in America for that to happen?

I believe so. I think yes. We’ve proven our willingness to do that. We made people put seatbelts in automobiles, and it added to the cost of the car. You’ve got to acknowledge that ultimately consumers wind up paying for it all, because any manufacturer is going to pass the cost down in the cost of goods. But, am I for making sure that the downstream consequence is taken care of? Absolutely. And can we do that? Yes, we can. And should we do it? Yeah, because we need responsibility to pull us back from the brink. Literally, by 2050, at the current rate, we will have one ton of plastic for every three tons of fish that are in the water. That’s just staggering when you think about it.

What do you consider your greatest ocean achievements?

One of the things I’m proudest of is an idea to push [for] a global accountability enforcement network [for illegal fishing] in areas of the ocean where we know certain countries are sending vessels that go in and still drift net and strip mine, using GPS and other technology to trace and track. When I began this endeavor in 2014, there were 10 countries signed up for the Port State Measures Agreement [a treaty that prevents boats with illegally caught fish to dock and off-load those fish in participating ports]. Now we’re up to 60 countries that have joined Port State Measures. That is a huge step forward in terms of accountability as to where fish are coming from and how they’re being sold and who caught them.

Should the high seas be closed to fishing?

They ought to be monitored and regulated. Certain seas like the Ross Sea [in Antarctica] ought to be closed, and we took steps to close the Arctic. We all agreed, a group of countries that border on it, to not exploit fishing, and Russia joined in that. So that’s a big deal. We have the same thing we’re fighting for now with the Ross Sea.

People who feel their fishing industry will suffer will resist efforts to regulate or close areas. How do you get around that?

Not all of these avenues of remediation or prevention or regulation are fully exploited yet. We’re making progress, but we still have unanswered questions. I can’t tell you how countries will behave as the pressure mounts. We’ve taken a lot of steps—days at sea limited, equipment changes in fishing practices, limitations on catch. And the fishermen are under great pressure today as a result, so there’s a lot going on.

If you could do one big thing before January, what would that be?

The biggest thing I’d like to see is to bring the Paris [climate] agreement into full force. Every meeting we’ve gone to, we are pushing nations to sign up, join up. My hope is that we get the Paris agreement in place because that will have a profound impact. The faster we get that out there, the bigger the impact is on the acidification of the oceans, the warming and melting, sea level rise. It’s all wrapped up in that.

And your legacy?

I leave it up to others. I’m just trying to maximize what I can do every day here and when I’m gone, I’ll sit back and reflect. I want to be able to tell my grandchildren I tried. I gave it my all.

This interview has been edited for length and clarity.