Reef sharks and humans don’t mix

The New York Times
By Douglas M. Main
May 2, 2012

Sharks may have more reason to fear people than ever. In reefs surrounding Pacific islands inhabited by people, there are 90 to 97 percent fewer sharks than in similar areas without people, according to a study in the journal Conservation Biology.

“Wherever there are humans, the reef shark numbers are much, much lower,” said one of the study’s co-authors, Julia Baum, a researcher at the University of Victoria in British Columbia.

It’s the first study to examine populations of reef sharks throughout the Pacific, drawing on data collected over the last decade across 46 Pacific islands and atolls. Shark estimates were collected in more than 1,600 “tower dive” surveys, which involve towing two divers behind a boat.

This is considered a better way to count reef sharks than the previous prevailing method, in which divers counted fish while swimming under their own power. While that is fine for counting small species, it’s not adequate for larger animals that move quickly because they tend to be counted multiple times during such surveys, Dr. Baum said.

The study also found that shark numbers were higher in areas with warmer water temperatures and with higher levels of phytoplankton. “Sharks like it warm and productive,” Dr. Baum said. Still, the absence of humans was by far the largest factor predicting shark abundance.

“This is one of few studies that tells us what we should be seeing, but cannot: the missing predators,” said Nicholas K. Dulvy, co-chairman of the International Union for Conservation of Nature’s shark specialist group, who was not involved in the study.

Dr. Dulvy, a researcher at Simon Fraser University in Burnaby, B.C., said he found the results alarming, especially in conjunction with his own work, which shows that humans remove about 40 percent more fish than can be replenished worldwide. “Before climate change bleaches and destroys these reefs, they’re going to be empty,” he said.

Although the study did not look at the reason that a human presence depresses reef shark populations, Dr. Dulvy said the cause was simple. “To paraphrase others, ‘It’s fishing, dummy,’ ” both by locals and foreign industrial fishing boats, he said. The vessels usually ply the open ocean but have been known to anchor at reefs at night and cast lines for sharks, especially because populations of oceanic sharks have plummeted.

Dr. Baum said that unintentional bycatch was probably a contributing factor, as well as the deliberate fishing of sharks for their highly valued fins.

Dr. Dulvy said that in response to the study, he planned to convene a group of shark specialists and fisheries managers in the next year to focus on drafting an action plan to protect the sharks’ future. Species most heavily affected by the presence of humans include the gray reef shark and the whitetip reef shark, both of which are listed by the I.U.C.N.’s red list as “near threatened.”

The loss of apex predators in the reefs can destabilize the ecosystems, in some cases setting off a cascade of changes that can turn coral reefs into algal reefs, Dr. Dulvy explained.

Researchers said they were worried that unregulated fishing by international fleets could even cause declines of sharks in reefs surrounding uninhabited islands and atolls in the near future.

“A lot of really remote uninhabited islands were decreed national monuments in the last few years,” Dr. Baum said. On paper this may sound great, she said, given that they are among the last reefs in the world with apex predators.

“But how do you go out and actually enforce those protections?” she said. “My fear is that distant water fleets are going to fin sharks on these islands in the next decade.”

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