Next stop: Socorro Island. At 10 miles long, it’s the largest in the Revillagigedo chain of islands. Mount Cerro Evermann, its top point, soars to 3,707 feet. This shield volcano island has erupted at least five times since 1848, and most recently in 1993. After presenting our documents to authorities from Socorro’s Naval station, we headed for the fish. We fish mostly with live bait and circle hooks.Those fish we catch, we scoop up from the water with the use of a rigid-framed heavy-gauge aluminum dip net with knotless webbing.

We place small fish in a tagging cradle, large fish on a foam pad covered with a wet synthetic chamois and remove the hook.

If the fish is in good condition (no eye or gill damage and no significant bleeding), we implant an archival tag. We make a tiny incision less than an inch long, insert a sterile tag, and close it with two sutures. We cover the eyes of the fish with a soft wet cloth so that it stays calm. The whole process takes under two minutes.

Next we release the fish back to the ocean. If a commercial fisherman catches a tuna with an archival tag in its belly, he’ll be paid $500 for turning it in. The data on those tags are precious. Once every minute, the tag records the temperature of the tuna and the water, depth and light levels. We use the info to reconstruct the tuna’s journey from the time we release it to the time it’s caught.

Busy doesn’t begin to describe our day at Socorro Island. We tagged 119 yellowfin – 91 with conventional plastic dart tags and 28 with archival tags that we implanted surgically in their abdomens.