We can save the oceans — and it’s easier than you might expect.

“How inappropriate to call this planet Earth,” Arthur C. Clarke wrote, “when it is clearly Ocean.” Covering 71 percent of the planet’s surface, our oceans are almost indescribably vast — and incredibly important.

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They serve as economic engines: hundreds of millions of people rely on seafood for income or nourishment, and about

90 percent of world trade is carried by international shipping

. But their ecological importance is even more significant, as our oceans are also home to startling biodiversity, unique habitats, and most of the life on the planet. Protecting the oceans is, therefore, both a monumental and vital task.

But how to protect something that is so large and used in so many varied ways? Any observer of global affairs might note that broad, multinational agreements are difficult to negotiate and yet frequently toothless. This World Oceans Day, I want to offer some optimism and explain why saving the oceans can be tackled on a country-by-country basis.

It turns out that the most productive parts of the ocean are, broadly speaking, shallower. Sunlight provides energy for plankton and seagrass and other marine plant life, ultimately supporting a broad and diverse food web.

Nutrients are also recycled from the ocean floor more easily in shallower waters. And coastal regions — most shallow parts of the ocean — were effectively nationalized in 1982 with the designation of Exclusive Economic Zones, which gave countries control over the waters up to 200 nautical miles off their shores.

So forget international agreements for a moment. The most productive parts of the ocean are largely under national jurisdiction. If we enact policies in these countries that will protect, restore, and responsibly manage their waters, we can make a significant impact on the health of the global oceans.

And we can narrow our focus even further. Let’s look at seafood: if we add up just the top 10 fishing nations (by weight of catch), it turns out they account for 63 percent of the world’s wild fish catch, according to data from the United Nations.

If we look at the top 30 nations, we can account for 90 percent. Suddenly, we don’t need a global compact to make real, in-the-water change. We just need 30 countries to get their acts together.

Implementing responsible policies in these countries can make a tremendous difference — and we already know what works. There are four key measures that will help restore healthy oceans.

First, and most importantly, we must prevent overfishing. Fish — from sardines to tuna to sharks — are vital to the food web, and we cannot take them out of the water faster than they can reproduce.

Second, we must protect habitat. Fragile reefs need to be protected from damaging fishing gear, for example, and important spawning grounds or migratory routes should be safeguarded for the sake of the species we hope to save.

Third, we must curb pollution. Reducing plastic waste and taking measures to prevent oil spills — especially through the cessation of offshore oil drilling — will help ensure the long-term health of the seas.

And finally, we need governmental transparency to ensure these policies are effectively implemented and enforced.

When these measures are put in place and backed by science, they work. Fish populations that have been devastated by overfishing can rebound, providing more food and jobs to coastal communities. And revitalized ecosystems mean healthier oceans for all the creatures that live there. Environmental protection, feeding people, and growing the economy — imperatives that are so frequently at odds — are all advanced by healthier, vibrant oceans.

Around the world, we’re seeing leaders from across the political spectrum embrace policies that advance the cause.

Belize, home to the second largest barrier reef in the world, recently banned offshore oil and gas from its waters.

In May, Brazil implemented scientific management policies for tainha, an important fishery for both commercial and artisanal fishermen, for the first time.

Earlier this year, the U.S. closed areas in the Pacific to destructive bottom trawling to protect fragile seafloor habitat.

These victories show that we are making progress. It’s possible to restore a healthy, vibrant ocean — an ocean that will be more resilient in the face of a changing climate and an abundant source of healthy seafood that can help feed more than a billion people around the world.

Andy Sharpless is CEO of Oceana, the largest international organization dedicated solely to ocean conservation.