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Sporting snorkels and wetsuits, a team of researchers plunged into the waters off two isolated coral islands in the Pacific Ocean to take stock of the marine life. The coral islands — known as atolls – are separated from each other by only few hundred miles, but they are worlds apart in terms of the impact they feel from humans, the researchers say.

An atoll is an island consisting of a circular coral reef surrounding a lagoon. The two atolls that were surveyed, Palmyra and Tabuaeran (or Fanning Island), are located about 1,000 miles (1,600 kilometers) south of Hawaii.

While coral reefs around the world are threatened by climate change — rising water temperatures bleach corals, killing them off — other human activities, such as overfishing, are wiping out marine life as well. Palmyra and Tabuaeran are ideal natural settings for researchers to uncover how fishing impacts atolls.

Palmyra is a protected U.S. wildlife refuge that is virtually uninhabited and bans fishing along its shores. Beginning in 2007, the team took trips to swim with amazing and frightening creatures such as bumphead parrotfish and sharks to count their numbers and reveal the cost of overfishing at the atolls.

The result of the conservation efforts is obvious to those that journey to Palmyra's waters.

"Palmyra has some of the highest densities of sharks and other large fish of any coral reef in the world. That's clear within seconds of jumping in the water there," said graduate student Douglas McCauley of Stanford University, who was part of the research team involved in the study.

Nearly 250 miles (400 km) away from Palmyra is Tabuaeran, part of the island nation of Kiribati, Tabuaeran is home to about 2,500 people that rely on fishing for food and income.

Fishing is a way of life on Tabuaeran, and the marine life is a far cry from Palmyra. Large predators, such as the sharks and parrotfish are scarce in this atoll. These large sea creatures take a long time to grow and reproduce, so very little overfishing can have a very large impact.

"By contrasting near-pristine Palmyra with inhabited and fished Tabuaeran, we are in a unique position to gather data that will ultimately help reef managers protect these vibrant and vulnerable habitats," said marine biologist Fiorenza Micheli of Stamford, who was also involved in the study.

Parrotfish have bottomless appetites — they chomp on whole branches of coral — so even slight overfishing can throw a coral reef ecosystem out of whack. Stopping people from reeling in these trophy fish, however, is a daunting task.

Even when protected waters are set aside, enforcing fishing limits is nearly impossible in such remote and sparsely populated territory. Also, the big fish don't always stay home. Radio-tagged sharks from the protected waters of Palmyra have been caught by fisherman at reefs hundreds of miles away, McCauley said.

Shark meat is an important part of local diets, and shark fins garner large sums of money from traders who re-sell them to soup manufacturers.

Millions of people inhabit coral reefs around the world, putting additional pressure on reef animals. Establishing sustainable fisheries, even at remote islands and atolls, could significantly slow the decline of many reefs, say marine ecologists.

Because the livelihoods of so many Tabuaerans depend on healthy fisheries, locals are eager to preserve fish numbers, McCauley said. "Those who depend most on the environment can and should be its best stewards," he added.

"We know that fishing can dramatically change the composition of a reef ecosystem," Micheli said. "By confronting overfishing immediately, we may increase the resilience of coral reefs to global warming and other threats."

The researchers’ results are preliminary and have yet to be published in a scientific journal.

This article was provided by OurAmazingPlanet, a sister site to LiveScience.