HONOLULU — Terry Kerby has been piloting deep-sea submarines for four decades, but nothing prepared him for the devastation he observed recently on several underwater mountains called seamounts in the middle of the Pacific Ocean.

“It was a biological desert,” he said. Where normally fish and crabs dart about forests of coral and sponges, “all we could can see was a parking lot full of nets and lines, with no life at all.”

Mr. Kerby and Brendan Roark, a geographer at Texas A&M University, are comparing seamounts that have been fished to those in pristine, protected areas. This month, they surveyed the upper reaches of four seamounts, one of which, Hancock, lies inside Papahānaumokuākea Marine National Monument, which includes the Northwestern Hawaiian Islands.