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Oceans: Some reef shark populations drop 90 percent

‘People and sharks don’t mix’

Curious gray reef sharks (Carcharhinus amlyrhynchos) at Kure Atoll in the Papahanaumokuakea Marine National Monument, Hawaii were studied as part of a study published April 25 in the journal Conservation Biology. PHOTO COURTESY P. AYOTTE.

By Summit Voice

SUMMIT COUNTY — Reef shark numbers have plummeted by more than 90 percent around some populated islands in the Pacific, according to an international research team that surveyed 46 U.S. Pacific islands and atolls during the past decade.

The numbers are sobering, said Marc Nadon, a researcher at the Joint Institute for Marine and Atmospheric Research at the University of Hawaii.

“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. In short, people and sharks don’t mix,” he said.

Using an innovative survey method, called "towed-diver surveys," involve paired SCUBA divers recording shark sightings while towed behind a small boat. This approach was designed specifically for the census of large, highly mobile reef fishes including sharks. PHOTO COURTESY MARC NADON.

“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 percent of the baseline numbers remain in these areas.”

Many shark populations, especially oceanic species, have plummeted in the past three decades. Harvesting sharks for their fins, incidental in fisheries targeting other species and recreational fishing are all factors in the decline.

Until now, a lack of data prevented scientists from properly quantifying the status of Pacific reef sharks at a large geographic scale. The research was part of NOAA’s extensive Pacific Reef Assessment and Monitoring Program.

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.

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.

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2 Responses

  1. [...] many as 90 percent of reef sharks have disappeared from reefs near …LiveScience.comOceans: Some reef shark populations drop 90 percentSummit County Citizens VoiceGlobalPost -Outcome Magazineall 71 news articles » [...]

  2. [...] from: Oceans: Some reef shark populations drop 90 percent – Summit County Citizens Voice [...]

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