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The number of fish caught just outside a recently expanded marine (海洋) protected area in Hawaii has risen. It is a sign that quadrupling (四倍) the size of the reserve in 2016 may have shored fish populations in the region.

When the Marine National Monument around Hawaii was enlarged 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, supporters argued, the reserve would benefit fisheries as well.

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

Proving that is tricky, however, as tuna can’t be counted directly. Their numbers may rise or fall for a variety of reasons other than the expansion of a reserve. But the new study, published in Science this week, strongly suggests the number of fish caught just outside the MPA is higher now than it used to be.

Alan Friedlander, chief scientist for the National Geographic Society’s Pristine Seas project, calls the study a “very careful and strict test of spillover from marine protected areas.”

Importantly, says John Lynham, an environmental economist at the University of Hawaii and one of the study’s authors, the increase in tuna catches near the reserve held up even when looking at the average numbers caught by particular fishers. This shows the effect is not due to more effective crews now fishing local waters, he explains. To account for effort, catch numbers were divided by the ever-increasing number of fishing hooks in the area.

Lynham and colleagues found the catch per hook increased over the 10 years of the study. Fishers were catching on average six more yellowfin and five more bigeye tuna per year after the expansion than before.

“That last one, especially, was a surprise,” says Lynham, “because it is economically much more important, and there were fewer indications of an increase.”