Impact of fishing on remote coral reefs

Coral reefs - kaleidoscopes of pink anemones and silver sharks - are the planet's most colorful ecosystems and among its most endangered, say marine scientists.

As global warming raises ocean temperatures, a number of corals blanch and die, a phenomenon called "coral bleaching." And pumping large amounts of carbon dioxide into the atmosphere could make the ocean more acidic, further decimating corals and the fish that depend on them for food and shelter.

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

"We know that fishing can dramatically change the composition of a reef ecosystem," said Fiorenza Micheli, a professor of biology at Stanford University's Hopkins Marine Station. "By confronting overfishing immediately, we may increase the resilience of coral reefs to global warming and other threats".

To gain new insights on the ecology of reef fishing, Micheli and a team of Stanford scientists are taking advantage of an ongoing "natural experiment" at two isolated Pacific atolls - Palmyra and Tabuaeran (or Fanning Island) - located about 1,000 miles south of Hawaii. The project is funded by Stanford's Woods Institute for the Environment.

Separated by just 250 miles of ocean, the two atolls are worlds apart in terms of fishing pressure. Palmyra, a protected U.S. wildlife refuge, is virtually uninhabited and bars fishing along its shores. But Tabuaeran, part of the island nation of Kiribati (pron. "kee-ree-bahs"), is home to about 2,500 people who depend on the reef for food and income.

With support from a Woods Institute Environmental Venture Projects grant, a team of marine ecologists, oceanographers and anthropologists has been working alongside residents of Tabuaeran to better understand their fishing techniques and priorities. At the same time, the scientists are conducting underwater surveys to assess the populations and diversity of marine life at both atolls.

"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," Micheli said.

A tale of two atolls.

Fieldwork at Palmyra and Tabuaeran began in 2007. On one trip, scientists -including several Stanford undergraduates - donned snorkeling gear and counted the number and variety of fish species along sample areas at various reefs. Preliminary results from the underwater census suggest that the two atolls host very different communities of animals, in part because of the impact of fishing.

"Palmyra has some of the highest densities of sharks and other large fish of any coral reef in the world," said Douglas McCauley, a graduate student working with Micheli. "That's clear within seconds of jumping in the water there".

But at Tabuaeran, where fishing is a way of life, sharks and other large species are in short supply, McCauley said. "That was surprising, because Tabuaeran is a somewhat lightly populated island," he explained. "Most people arrived only a few decades ago, and fishing there is still very artisanal in nature".

Big fish grow and reproduce slowly, so their populations take longer to recover, he added. "It appears that it takes very little harvesting to reduce populations of these sensitive, large reef fish," McCauley said.

Trophy catches like sharks and the 100-pound bumphead parrotfish were the first to decline, he said. Highly prized by Tabuaerans, parrotfish have bottomless appetites that can alter the architecture of their coral homes. "The parrotfish's large size allows it to break off and crunch up whole branches of coral," McCauley said. "It plays a unique and important role in reef ecology that's simply not achieved by other fish species".

By spending hours in the water making detailed observations of bumphead parrotfish eating habits, the team is trying to piece together what a reef without these heavy eaters would look like.

Shark ecology.

Sharks are also important for healthy coral reef ecosystems. For decades, conservationists have tried to protect reef sharks by setting aside reserves like Palmyra that provide a safe space to grow and reproduce. But sharks tagged at Palmyra have been caught by fishermen at reefs hundreds of miles away, McCauley said.

Shark tissue also contains unique ratios of carbon and nitrogen isotopes that identify its reef of origin. By sampling shark tissue, Stanford marine scientist Rob Dunbar confirmed that these top predators have been straying far from their home reefs.

"At Palmyra, we're finding that some sharks don't stay at home like we thought, so managers can't protect them outside the sanctuary borders," McCauley said. "It seems that effective management strategies for gray reef sharks and other similarly wide-ranging species will need to be thought out at much larger scales".

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. In 2009, Stanford anthropologists Bill Durham and Doug Bird, along with graduate student Eleanor Power, monitored the activities of Tabuaeran fishermen on daily forays for reef animals and conducted interviews with atoll elders on the history of local fishing. The results of these surveys will be used to assess fishing patterns and provide information to Tabuaeran leaders looking to achieve sustainable harvests.

BY DANIEL STRAIN.


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