Monday, July 21, 2014

Quick, Somebody Get The Name Of That Shark!

On Saturday morning, July 5, 2014, Steve Robles was out for a long distance swim. It was much like his usual Saturday swims, except something about this swim tasted different. Oh, yeah - it was him! A great white shark took a bite out of Steve off Manhattan Beach in Los Angeles, California.

Steven Spielberg was given a stinker when he signed on to
direct Jaws. The script wasn’t finished, Richard Dreyfus hated
his character’s development, and the first mechanical shark
sank as soon as it was put in water. But because of the movie’s
success, he became powerful enough to make any movie he
wanted. Jaws is the reason we were given Schindler’s List and
Close Encounters of the Third Kind.
The movie Jaws was released in 1975, and great white attacks have been on our collective mind ever since. Robles’ wasn’t the first great white attack this year, and it won’t be the last. Other recent news stories have discussed the sightings of great whites off Cape Cod (July 3), and the third sighting of them off of the New York, New Jersey shore (July 3).

Every time there is a sighting or an attack, the media goes out of its way to tell us how rare the attacks are. Every news report includes a disclaimer that attacks occur once in a blue moon. They usually make some comparisons: you’re more likely to be killed by a cow than by a great white shark; you’re more likely to be bitten by Luis Suarez – you know, things like that.

But is it true? Are sightings and attacks by great white sharks rare and staying rare? Or has the faster news cycle led to the need for more sensational stories and therefore more coverage? It may be a bit of both.

More people swim in the oceans now than in the 1700’s or 1800’s. The reasons are many – more leisure time, more information about open water (lack of sea monsters), and a big increase in world population. So maybe the attacks reflect an increase in pruny people.

The International Shark Attack File (ISAF) at the University of Florida states that attacks have been increasing each decade since the early 1900’s, although there was a dip in the late 70’s and early 80’s. The ISAF says because they had a lapse in their record keeping, but I think it was due to the Jaws effect.

People used to sail within sight of land because they feared
falling off the Earth and the sea monsters that lived in open
water. They weren’t going to swim out there! It wasn’t until
1875 that someone swam the English Channel, and it didn’t
happen again for 31 years. Of course sea bathing, or wading,
was popular, but most often the sea water was
brought to pools on land.
Another reason for more unprovoked attacks may be that there are more sharks. Recent studies indicate that there are more great white sharks than previously counted off the coast of California and New York.

On the west coast of North America, an older study severely underestimated the number of great whites because of sampling biases, so a 2014 study re-evaluated this study and generated a number of more than 2000 for just those around California. Another 2014 study on the east coast of North America showed a dip in 1970’s but a steady increase in the number of great whites since the 1980’s.

A third 2014 study in Australia linked bite risk to increased shark numbers and suggested that this increase was due to an increase in whale numbers. Add to this upshot in population a study that used radiocarbon dating of vertebral columns. The authors showed that male great white sharks can live up to 73 years, much longer than previously assumed. More great whites living longer lives than we thought? Sounds like a recipe for more attacks.

The reason for this –  humans probably. In 1972, the Marine Mammal Protection Act was passed. This made the killing of any marine mammal (dolphins, seals, whales, etc.) illegal. This was joined by the international moratorium on commercial whaling passed in 1982. As a consequence, marine mammal numbers have been on a steady rise. Yum, more food for sharks. More food means more surviving sharks; more adult sharks means more baby sharks.

Helping this survival rate was a 1997 ban on the hunting of great white sharks. I don’t know a better way to increase the number of sharks than to stop killing them. So – there are more sharks, is it too weird to think that there might be more attacks?

But there are problems in counting attacks. The perpetrators eat the evidence in most cases. We may be significantly underestimating the number of fatal attacks just because we don’t have bodies to count.

The great white shark (1, Carcharodon carcharias), the bull shark
(2, Carcharhinus leucas), and the tiger shark (3, Galeocerdo cuvier)
are responsible for the majority of unprovoked attacks on humans
in open water. The great white swims in temperate and tropical
water around the world. The bull is in coastal water and rivers
around the world, but the tiger shark lives in tropical and
subtropical waters. Tigers only come about halfway north in the
US. Could you tell which one was gnawing your leg off?
Another problem with the ISAF; they list attacks by various species of shark. I can’t believe that a person can tell you what kind of shark attacked them, and physical evidence such as a tooth is very rare. Am I going to take the time to ID the species of shark that is gnawing my leg off? Shark scientists often argue about what species they’re looking at – you expect John Q. to know the difference between a nurse shark and a reef shark?

Get hit by a car – sure, try to get the license plate number and you might be able to get some satisfaction. But unless it’s a really unusual looking shark, I don’t think you’re going to get a call saying the private detective you hired has found him. Moby Dick, maybe, but a shark that looks like every other shark – nope.

But there’s an exception. Were you chewed on in a coastal river or river-fed lake? Then it was probably a bull shark.   Bull sharks are exceptions amongst sharks. Most sharks are able to live in saltwater because they have a way of stopping the loss of cellular water to the high salt environment they live in. Most sharks accomplish this by storing a huge amount of chemical called urea. See this post for more information.

There are very rare sharks of the Glyphis genus that can
live in brackish waters as well. They live in southeast Asia
and Australia, but they are very mysterious and aren’t
sighted often. This is a bull shark found in a golf course
lake in Australia. If bitten while in a river or lake, think
bull shark, not Glyphis.
Having cells with a lot of urea keeps the water from moving out of their cells and into the saltwater. In this way, sharks don’t dry up in saltwater. But bull sharks go one better, they have kidneys that can ramp up urea and salt elimination if they swim into freshwater, so they can survive in both low salt and high salt environments. Good for them, bad for rubber tubers and swimmers in rivers near the oceans.

This is important because bull shark attacks are the most likely to be fatal -if you can believe the statistics. So the coastal rivers aren’t as safe as you thought, but at least you’ll know who’s gnawing your leg off.

Contributed by Mark E. Lasbury, MS, MSEd, PhD
Mark is writer and educator in the areas of science and history

Burgess GH, Bruce BD, Cailliet GM, Goldman KJ, Grubbs RD, Lowe CG, MacNeil MA, Mollet HF, Weng KC, & O'Sullivan JB (2014). A Re-Evaluation of the Size of the White Shark (Carcharodon carcharias) Population off California, USA. PloS one, 9 (6) PMID: 24932483

Curtis TH, McCandless CT, Carlson JK, Skomal GB, Kohler NE, Natanson LJ, Burgess GH, Hoey JJ, & Pratt HL Jr (2014). Seasonal Distribution and Historic Trends in Abundance of White Sharks, Carcharodon carcharias, in the Western North Atlantic Ocean. PloS one, 9 (6) PMID: 24918579

Sprivulis P (2014). Western Australia coastal shark bites: A risk assessment. The Australasian medical journal, 7 (2), 137-42 PMID: 24611078

Hamady LL, Natanson LJ, Skomal GB, & Thorrold SR (2014). Vertebral bomb radiocarbon suggests extreme longevity in white sharks. PloS one, 9 (1) PMID: 24416189

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