A team of National Geographic researchers and filmmakers have become eyewitnesses to what many scientists consider to be among the major threats to marine biodiversity at Costa Rica's treasured Isla del Coco, a national park 365 miles off the Pacific coast.

The group, which arrived in Costa Rica three weeks ago to film part of National Geographic's “Ocean Now” series, filmed a green sea turtle and a yellowfin tuna wrapped in illegal fishing line. The group was able to save the turtle, but when they untangled the tuna, the fish sank helplessly to the ocean floor.

“The last three days, we have witnessed firsthand what is killing the oceans,” team researcher Enric Sala wrote on the project's blog. “It was like waking up from the most wonderful dream to the crudest reality.”

Fishing is prohibited inside the park's boundaries. Still, hundreds of miles of illegal fishing lines and thousands of hooks are found inside the protected area every year, according to the National Geographic group.

The Coast Guard and non-governmental groups operate patrol boats to protect the island from poachers and illegal fisherman, but conservationists have consistently criticized the Costa Rican government for not doing enough to protect the park.

Ocean Now is a “project to study the last healthy, undisturbed places in the ocean.” Follow the team's Isla del Coco experience at http://ocean.nationalgeographic.com.