Shark Bites Two: Possible Explanations for Attacks
Credit: Michael Rothschild | Shutterstock.com

A trip to the beach turned terrifying for two young people on Sunday when each was attacked by a shark while wading in waist-deep water off the coast of Oak Island, North Carolina. Experts in marine science say the closeness of the two attacks was quite unusual.

Both the 12-year-old girl and 16-year-old-boy survived their separate attacks, which occurred 2 miles (3.2 kilometers) apart and within 90 minutes of one another. These incidents follow on the heels of another shark attack that took place last Thursday (June 11) at Ocean Isle Beach, North Carolina, a barrier island about 15 miles (24 km) west of Oak Island, according to a report by the Associated Press.

There haven't been so many severe attacks, so close together, in decades, said Dan Abel, a professor of marine science at Coastal Carolina University in Conway, South Carolina. [Great White Shark High 5? Here's What Really Happened]

"There were some fatalities associated with shark bites in North Carolina and Virginia waters over a decade ago. But, typically, along this part of the coast, these kinds of interactions are rare," Abel told Live Science. However, non-life-threatening shark bites are not that uncommon in the waters off the southeast coast of the United States, he noted.

"Many shark biologists distinguish between a bite and an attack based on severity. Bites will usually occur with blacktip sharks in shallow water," Abel said. "The shark thinks it's pursuing a fish, but it bites your foot or your hand, and it immediately releases you."

However, the young people who were attacked on Sunday weren't just bitten and then released. In fact, each victim lost an arm to what Abel believes was a single bull shark. It's improbable that a low-likelihood event such as a shark attack would occur twice in such a short time frame and involve two different sharks, Abel said.

Bull sharks hunt in shallow waters all along the U.S. East Coast, and this is one of the shark species most often involved in fatal attacks against humans (behind only tiger sharks and great whites). Along with sand tiger sharks, tiger sharks, hammerheads and many other species of shark, bull sharks spend much of their time hunting for food close to the eastern coastline.

"The Atlantic coast in the summer is a fairly 'sharky' place," Abel said. "There are dangerous sharks in areas where there are rarely any attacks against humans. And there are areas where there may be fewer sharks but there are attacks."

Trying to figure out why a shark would attack several people off the coast of North Carolina while other swimmers a bit further north or south remained safe is, as Abel put it, "not good science." But there are factors that can contribute to the likelihood of a shark attack in a particular area, he said.

As the water warms in the summer months along the Atlantic coast, more people venture out into the water, providing more opportunities for humans and sharks to interact. There are also environmental factors to consider, Abel said. The attacks occurred just two days before a new moon, and during this lunar phase, the difference in water depth between low tide and high tide is greater than it is during most other lunar phases. Sharks may come closer to shore during the new moon phase, he said.

Both attacks occurred in the evening, during an incoming tide (i.e., when the water was rising up the beach). Biologists believe that more sharks may make their way closer to shore during incoming tides, and it's known that sharks are more active during what Abel called "transition times," or dusk and dawn.

Many who have commented on news reports regarding the attacks noted that sharks may have been drawn closer to the shore of Oak Island because fishermen were trying to lure in fish by throwing chum, or bait, into the water from a nearby pier. Other commenters noted that large schools of fish were seen off the coast in that area, which suggests that larger numbers of predators may have been close behind.

"Sharks have an array of senses that are very sensitive, but vision in murky water is not one of them. So they sense something as a potential prey item, and, if you're unfortunately the victim, it's not a happy result," Abel said.

Researchers are currently working on humane solutions for keeping sharks and swimmers apart from one another, Abel said. But in the meantime, he had a few suggestions to help individuals stay safer: Stay out of the water in the early morning and at dusk. Avoid swimming near piers or anywhere else where fishermen are baiting the water. If you see large schools of fish, you should exit the water, he said. And lastly, if you hear of a shark nearby, it's a good idea to wait a few hours (or even longer) before going for a swim.

However, he also said, "What scares me going to the beach is the sun.I'm more likely to get melanoma [than get eaten by a shark]." 

He continued, "I'm also afraid of jet skis, getting hit in the head with a surfboard, stepping on a sting ray, getting run over by an SUV in the parking lot or getting a bacterial infection from the wastewater that pollutes our [ocean]. Those kinds of things scare me more than the sharks do."

 Follow Elizabeth Palermo @techEpalermo. Follow Live Science @livescience, Facebook & Google+. Original article on Live Science.