"It was really, really hard to see these huge animals swimming, they swim majestically, they are like underwater torpedoes, they are so powerful … and suddenly you see them hanging from a hook, fighting really hard to get released," Sala said. "It really breaks your heart."
The sharks are of special concern to scientists because they're at the top of the food chain; if too many are killed, it could upset the entire ecosystem.
Sala's team tagged a shark with a satellite tracking device in hopes of observing how the predator comes and goes, and watching whether it swims into harm's way. The goal was to learn enough to protect the population.
After the three-week expedition, Sala returned to the National Geographic Headquarters in Washington, D.C., where the one shark they tagged was being tracked from space. At first, it was alive and well, migrating outside the protection zone through the waters of Central and South America, looking for food. About 20 days later, however, researchers stopped hearing from it. While they never knew what happened, Sala believed fishermen killed it.
"It shows a problem," he said. "It was very clear. Inside the park, there are so many fish but outside the park, they are catching almost nothing and as soon as the sharks leave the park, they run the risk of being caught.
"And then the shark fins go to Asia so somebody can show off at a wedding."
Researchers found that the pristine island now has one of the highest fish densities in this part of the world. Massive amounts of sea life have returned, evidence of just how fast the ocean and its sea life can regenerate itself.
It's proof, Sala said, that protection like this is needed in waters all around the globe.