The number of fish caught just outside a recently expanded marine protected area in Hawai’i has risen, a sign that quadrupling the size of the reserve in 2016 may have bolstered fish populations in the region.

When President Barack Obama enlarged the Papahānaumokuākea Marine National Monument around Hawai’i to 1,510,000 square kilometers, marine conservationists around the world rejoiced.

Fishers may have felt differently, however, as fishing inside the area is not allowed. Yet by creating a space for dwindling tuna populations to recover, proponents argued, the reserve would benefit fisheries as well.

As populations inside the reserve boundaries steadily increased, they predicted, the fish would spill over into the surrounding areas, increasing the amount of tuna available to catch.

Proving that is tricky,