Pacific Reef Sharks Vanishing Near Populated Islands

Stephanie Pappas, LiveScience Yahoo News 27 Apr 12;

As many as 90 percent of reef sharks have disappeared from reefs near populated islands, a new study finds.

The research is the first to provide a large-scale estimate of reef sharks in the Pacific, a group of species that includes the gray reef shark, the whitetip reef shark and the tawny nurse shark.

"We estimate that reef shark numbers have dropped substantially around populated islands, generally by more than 90 percent compared to those at the most untouched reefs," said study leader Marc Nadon, a doctoral candidate at the University of Miami's Rosenstiel School of Marine & Atmospheric Science. "In short, people and sharks don't mix."

Nadon and his colleagues pulled shark sighting data from more than 1,607 dives at 46 reefs in the central-western Pacific, which included reefs near the Hawaiian islands and American Samoa as well as extremely isolated reefs nearly devoid of human influence. Though eight species of shark were seen on the dives, the researchers excluded sharks, such as hammerheads, that aren't dependent on reefs. That left them with five shark species to tally: gray reef sharks, blacktip reef sharks, whitetip reef sharks, Galapagos sharks and tawny nurse sharks. [On the Brink: A Gallery of Wild Sharks]

Combining that data with information on human population, habitat complexity, availability of food and sea-surface temperatures, the researchers created models comparing the numbers of sharks at pristine versus human-impacted reefs.

"Around each of the heavily populated areas we surveyed — in the main Hawaiian Islands, the Mariana Archipelago, and American Samoa — reef shark numbers were greatly depressed compared to reefs in the same regions that were simply [farther] away from humans." Nadon said in a statement. "We estimate that less than 10 percent of the baseline numbers remain in these areas."

The devastation of sharks in areas near human civilization could be the result of illegal fishing, incidental killing or fishing for sport, the researchers report Friday (April 27) in the journal Conservation Biology. Human impact on the reef fish that sharks call dinner could also play a role. Human influences were shown to outweigh natural influences, such as warmer water temperatures, the researchers found.

"Our findings underscore the importance of long-term monitoring across gradients of human impacts, biogeographic, and oceanic conditions, for understanding how humans are altering our oceans," said Rusty Brainard, head of the coral reef ecosystem division at NOAA's Pacific Islands Fisheries Science Center, which conducted the shark surveys.

Scientists Provide First Large-Scale Estimate of Reef Shark Losses in the Pacific Ocean
ScienceDaily 27 Apr 12;

Many shark populations have plummeted in the past three decades as a result of excessive harvesting -- for their fins, as an incidental catch of fisheries targeting other species, and in recreational fisheries. This is particularly true for oceanic species. However, until now, a lack of data prevented scientists from properly quantifying the status of Pacific reef sharks at a large geographic scale.

In a study published online April 27 in the journal Conservation Biology, an international team of marine scientists provide the first estimates of reef shark losses in the Pacific Ocean. Using underwater surveys conducted over the past decade across 46 U.S. Pacific islands and atolls, as part of NOAA's extensive Pacific Reef Assessment and Monitoring Program the team compared reef shark numbers at reefs spanning from heavily impacted ones to those among the world's most pristine.

The numbers are sobering.

"We estimate that reef shark numbers have dropped substantially around populated islands, generally by more than 90 percent compared to those at the most untouched reefs," said Marc Nadon, lead author of the study and a scientist at the Joint Institute for Marine and Atmospheric Research (JIMAR) located at the University of Hawaii, as well as a PhD candidate with Dr. Jerry Ault at the University of Miami's Rosenstiel School of Marine & Atmospheric Science. "In short, people and sharks don't mix."

To obtain these estimates, Nadon and his colleagues used an innovative survey method, called 'towed-diver surveys,' which were designed specifically for the census of large, highly mobile reef fishes like sharks. The surveys involve paired SCUBA divers recording shark sightings while towed behind a small boat.

"Towed-diver surveys are key to our effort to quantify reef shark abundance," said Ivor Williams, head of the team responsible for these surveys. "Unlike other underwater census methods, which are typically at an insufficient spatial scale to properly count large, mobile species, these surveys allowed our scientists to quickly record shark numbers over large areas of reef."

The team crunched the numbers from over 1,600 towed-diver surveys, combining them with information on human population, habitat complexity, reef area, and satellite-derived data on sea surface temperature and oceanographic productivity.

The models showed the enormous detrimental effect that humans have on reef sharks.

"Around each of the heavily populated areas we surveyed -- in the main Hawaiian Islands, the Mariana Archipelago, and American Samoa -- reef shark numbers were greatly depressed compared to reefs in the same regions that were simply further away from humans." Nadon said. "We estimate that less than 10% of the baseline numbers remain in these areas."

Like all fishes, reef sharks are influenced by their environment. "They like it warm, and they like it productive," said Julia Baum, Assistant Professor at the University of Victoria, Canada, referring to the increase in reef sharks the team found in areas with higher water temperatures and productivity. "Yet our study clearly shows that human influences now greatly outweigh natural ones."

"The pattern -- of very low reef shark numbers near inhabited islands -- was remarkably consistent, irrespective of ocean conditions or region," added Williams.

"Our findings underscore the importance of long-term monitoring across gradients of human impacts, biogeographic, and oceanic conditions, for understanding how humans are altering our oceans," concluded Rusty Brainard, head of the Coral Reef Ecosystem Division at NOAA's Pacific Islands Fisheries Science Center, which conducted the surveys.

Journal Reference:

Marc O. Nadon, Julia K. Baum, Ivor D. Williams, Jana M. Mcpherson, Brian J. Zgliczynski, Benjamin L. Richards, Robert E. Schroeder, Russell E. Brainard. Re-Creating Missing Population Baselines for Pacific Reef Sharks. Conservation Biology, 2012; DOI: 10.1111/j.1523-1739.2012.01835.x