Reef Sharks Vanishing Around Populated Islands

Gray reefs sharks: Albert kok via Wikimedia CommonsGray reefs sharks: Albert kok via Wikimedia Commons

A new study finds that sharks living on reefs near areas populated by people have declined by between 90 and 97 percent compared to relatively pristine reefs where few or no people live.

“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,” says Marc Nadon, lead author of the study.

The authors of the paper in early view at Conservation Biology deployed 1,607 towed-diver surveys—that’s where scientists are used as shark bait (kidding, sort of)—to count sharks at 46 reefs in the central-western Pacific Ocean. They combined those data with information on human population, habitat complexity, and reef area, as well as with satellite-derived measurements of sea surface temperature and oceanographic productivity.

These methods allowed them to fill in the blanks on the numbers of missing sharks. Their models showed that:

  • Densities of gray reef sharks (Carcharhinus amblyrhynchos), whitetip reef sharks (Triaenodon obesus), and other reef sharks increased substantially as human population decreased. 
  • Densities of reef sharks increased substantially as primary productivity and minimum sea surface temperature (which correlates to reef area) increased.

From the paper:

Simulated baseline densities of reef sharks under the absence of humans were 1.1–2.4 [per hectare] for the main Hawaiian Islands, 1.2–2.4 [per hectare] for inhabited islands of American Samoa, and 0.9–2.1 [per hectare] for inhabited islands in the Mariana Archipelago, which suggests that density of reef sharks has declined to 3–10% of baseline levels in these areas.

 

Blacktip reef sharks: Jon Rawlinson via Wikimedia CommonsBlacktip reef sharks: Jon Rawlinson via Wikimedia Commons

“[Sharks] 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 paper:

  • NADON, M. O., BAUM, J. K., WILLIAMS, I. D., MCPHERSON, J. M., ZGLICZYNSKI, B. J., RICHARDS, B. L., SCHROEDER, R. E. and BRAINARD, R. E. (2012), Re-Creating Missing Population Baselines for Pacific Reef Sharks. Conservation Biology. doi: 10.1111/j.1523-1739.2012.01835.x

 

ONE MORE QUICK THING:

Or at least we hope. It’s fall fundraising time, and we’re trying to raise $250,000 to help fund Mother Jones’ journalism during a shorter than normal three-week push.

If you’re reading this, a fundraising pitch at the bottom of an article, you must find our team’s reporting valuable and we hope you’ll consider supporting it with a donation of any amount right now if you can.

It’s really that simple. But if you’d like to read a bit more, our membership lead, Brian Hiatt, has a post for you highlighting some of our newsroom's impressive, impactful work of late—including two big investigations in just one day and covering voting rights the way it needs to be done—that we hope you'll agree is worth supporting.

payment methods

ONE MORE QUICK THING:

Or at least we hope. It’s fall fundraising time, and we’re trying to raise $250,000 to help fund Mother Jones’ journalism during a shorter than normal three-week push.

If you’re reading this, a fundraising pitch at the bottom of an article, you must find our team’s reporting valuable and we hope you’ll consider supporting it with a donation of any amount right now if you can.

It’s really that simple. But if you’d like to read a bit more, our membership lead, Brian Hiatt, has a post for you highlighting some of our newsroom's impressive, impactful work of late—including two big investigations in just one day and covering voting rights the way it needs to be done—that we hope you’ll agree is worth supporting.

payment methods

We Recommend

Latest

Sign up for our free newsletter

Subscribe to the Mother Jones Daily to have our top stories delivered directly to your inbox.

Get our award-winning magazine

Save big on a full year of investigations, ideas, and insights.

Subscribe

Support our journalism

Help Mother Jones' reporters dig deep with a tax-deductible donation.

Donate