Everywhere sea otters have been studied, from Alaska to California, where sea otter populations fully or partially recovered, researchers observed sea otters effectively controlling urchins; including purple urchins, red, and greens.
In the California mainland, purple urchin density is consistently lower in areas with sea otters, though other factors such as storms and wave energy can also strongly affect urchin abundance in the absence of sea otters.
While this predator/prey relationship is well documented, some nuance exists. At San Nicolas Island (California), recovering sea otters focused most foraging effort on red urchins during the first years of recovery (through the 1990s and early 2000s). However, after sea otter abundance increased and red urchin density had been greatly depleted, sea otters then began consuming purples and now effectively control purple urchins in the areas of the Island where most foraging occurs.
In the Monterey Peninsula area, after decades of sea otters controlling purple urchin abundance, there was a boom in urchin abundance after 2014 in many areas, including Monterey. Mark Carr, Tim TInker, and collaborators initiated a NSF-funded study to investigate this phenomenon. Preliminary results indicate that sea otters show a strong preference for urchins in kelp areas over those in urchin barrens, however urchin consumption by sea otters increased after the 2014 urchin boom, and was focused in the kelp areas around the edges of urchin barrens: this would suggest that otter predation is serving to halt the spread of urchin barrens and “protect” kelp areas. And indeed, the complete deforestation of kelp by purple urchins that occurred in northern CA (no sea otters) has not occurred anywhere within the sea otter range, as the patchy urchin barres that did emerge in 2014 have not spread.