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Reef shark populations in steep decline

Numbers drastically lower in inhabited areas

Blacktip reef sharks swimming in tropical waters over coral reef.

Blacktip reef sharks swimming in tropical waters over coral reef.

Reef sharks, which are often killed for their fins or caught in fishing nets, have declined to 10 per cent of historic levels near populated islands in the Pacific Ocean, US scientists said.

Our study clearly shows that human influences now greatly outweigh natural ones

The survey by the University of Hawaii showed that the numbers were drastically lower near populated islands in Hawaii, the Mariana Archipelago and American Samoa, compared to more pristine, remote areas in the ocean.

“We estimate that reef shark numbers have dropped substantially around populated islands, generally by more than 90 per cent compared to those at the most untouched reefs,” said Marc Nadon, in the journal Conservation Biology.

“In short, people and sharks don’t mix,” added Mr Nadon, a scientist at the university’s Joint Institute for Marine and Atmospheric Research.

Researchers believe the shark numbers began to decline about three decades ago but counting the sharks − which tend to move around quickly and are particularly mobile at night − has proven difficult.

The latest research was based on a method called “towed-diver surveys,” in which paired Scuba divers record shark sightings while being 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 the 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.”

Researchers analysed data from over 1,600 towed-diver surveys taken from 2004-2010, in combination with information on human population growth and reef area, as well as satellite-data on sea surface temperature and ocean traits.

“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,” Mr Nadon said.

“We estimate that less than 10 per cent of the baseline numbers remain in these areas.”

More reef sharks were spotted in areas with higher water temperatures and what scientists call “increased productivity,” meaning areas with more algae and oxygen.

“They like it warm and they like it productive,” said co-author Julia Baum, assistant professor at the University of Victoria, Canada.

“Yet our study clearly shows that human influences now greatly outweigh natural ones,” she added.

“The pattern − of very low reef shark numbers near inhabited islands − was remarkably consistent, irrespective of ocean conditions or region.”

Researchers counted five types of reef sharks for the study, including the most common types − grey and whitetip reef sharks − as well as blacktip reef sharks, Galapagos sharks and nurse sharks.

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