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The return of sea otters, a top predator, to a California estuary is helping slow erosion and restore the estuary’s degraded geology.

In recent years, “30 by 30” has become a rallying cry in international marine conservation. It’s the idea that to safeguard marine biodiversity and limit environmental damage caused by future sea-floor mining and other industrial-scale human activities, we need to place at least 30% of the world’s oceans within marine protected areas (MPAs) by 2030.