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Mounting Evidence Suggests Sharks Are In Serious Trouble

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Can you imagine oceans without sharks? We may soon have to, as new research suggests may already be 90% of the way there.

Studying shark populations can be tricky. As David Shiffman explains well, while there are a number of methods that can be used to study shark populations, quantifying just how far their numbers have fallen can be difficult. However, recent research out of the University of Hawaii suggests that the presence of humans has a severe and strong negative impact on sharks, driving down numbers by over 90%.

Sharks play a vital role in coral reef ecosystems. Yet every year, millions are killed for asian delicacies and disproven cancer cures. There is no question our shark fishing habits have devastated their populations; the only questions that remain are how much of an effect are we having, and can the sharks recover.

In an effort to answer the first, the research team crunched data from 1607 surveys from the NOAA Coastal Reef Ecosystem Division (CRED) to calculate the effect of human habitation on shark populations. The CRED team counted sharks throughout the Pacific using towed diver surveys, the most efficient and effective way to study open ocean creatures on a large spatial scale, and compared their counts with local human population numbers. Their results were clear - and sobering.

“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,” said Marc Nadon, lead author of the study. "We estimate that less than 10% of the baseline numbers remain in these areas.”

The team also looked at other factors that might be affecting shark populations, including temperature and reef productivity. However, while sharks preferred warmer waters full of potential prey, the negative impact of humans dwarfed these effects. “Our results suggest humans now exert a stronger influence on the abundance of reef sharks than either habitat quality or oceanographic factors,” the authors wrote.

The team estimated that less than 100 people is enough to cut shark populations by 20%. Even 1,000 people - which is much less than the population of many small islands in the Pacific - was enough to decrease shark populations by 60%. As Nadon put it, "In short, people and sharks don't mix."

The findings are consistent with other research in the field. A 2003 paper, for example, found that shark populations in the Northwestern Atlantic dropped over 65% between 1986 and 2000. Similarly, a 2010 paper estimated that shark populations in the Chagos Archipelago had declined 90% since the 1970s. The more we study sharks, the worse the picture becomes, and the stronger the case becomes for conservation efforts. We simply cannot continue to treat these animals the way we do now, for all scientific evidence suggests the day is fast approaching when there will be no sharks left to exploit.

Reference: Nolan et al. Re-Creating Missing Population Baselines for Pacific Reef Sharks. Conservation Biology; DOI: 10.1111/j.1523-1739.2012.01835.x

The views expressed are those of the author and are not necessarily those of Scientific American.

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