Tuesday, June 30, 2015         


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No 'smoking gun' in cluster of shark attacks

By Carl Meyer and Kim Holland


The recent spate of shark attacks in Hawaii is unusual.

So far this year, there have been nine unprovoked attacks, whereas the average over the past 20 years was three to four per year.

We crave "smoking gun" explanations, but spikes in attack numbers may simply reflect natural variability and arise purely through chance. In most Hawaii cases the species of shark could not be identified, but tiger sharks are the suspected culprit in most severe attacks.

This year's apparent up-tick is difficult to explain, but one frequently heard explanation —the recovery of the sea turtle population — does not add up because the number of turtles has increased dramatically over the last 20 years whereas the number of attacks on humans has increased only slightly. Plus, turtles are only one part of the very varied diet of tiger sharks.

The slow, long-term increase in shark attack rates probably reflects gradually increasing numbers of people using the ocean. Scientists believe that the number of people rather than the number of sharks is what determines attack rates.

Our tracking studies indicate that tiger sharks visit shallow waters during both day and night, yet more than 60 percent of shark bites on people in Hawaii occur between the hours of 10 a.m. and 4 p.m., reflecting our preference for daytime water activities. There has never been any evidence of a "rogue" shark focusing on attacking humans.

We don't know the size of the population, but tiger sharks are common around all the Hawaiian Islands and are found over all parts of the reef — from the deep outer slopes to shallow bays — although they seem to be less frequent in very shallow waters used by swimmers. Of the 430 tiger sharks that we have caught and tagged, the largest was 15 feet long and this is probably about the maximum size for this species in Hawaii.

Our research shows that tiger sharks are constantly on the move and rarely "set up shop" at a specific spot. We have many instances of tagged sharks ranging up and down the entire length of the island chain and far out into open ocean. One of our tagged tiger sharks was caught by fishermen in Mexico! This constant mobility is why it makes no sense to try and catch the "culprit" after an attack — the shark will be long gone.

We have some indication that during fall, pregnant female tiger sharks congregate around the main Hawaiian islands to give birth to their pups. We do not know if this migration contributes to the rate of attacks at this time of year, or exactly where tiger sharks give birth — or even if they have specific birthing sites or depths.

Until they are about 8 feet long, tiger sharks feed mostly on reef fishes and invertebrates. As they get larger, tiger sharks eat bigger prey including other sharks, turtles and birds. Tiger sharks also scavenge carrion such as dead whales and carcasses carried down rivers — which is why you should stay away from murky river mouths after a rain.

Most shark attacks are not fatal. The best water safety tip is to not go in the water alone — go with a friend or seek out places where there are other people in the water. That way, if by some rare chance there is an attack, you can get help and someone can call for professional assistance.

Compared with other parts of the world, Hawaii is blessed with a remarkably low level of shark attacks. And we have to remember that the sharks that are in our waters are vital to the overall well being of our ocean environment.

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