Are sharks more active during the time of a full and new moon? According to Shark Attacks Survivors News Archive, which is updated daily, 63.5% of all reported shark attacks—625 reports in the last 5 and a half years—have occurred within a 4–5 day period of a full or new moon.
It starts with the relationship between the moon, the sun, the earth, and the earth’s ocean tides. “The high and low tides are caused by the gravitational forces between the earth and the moon,” states Yasar Safkan, a Ph.D. in Nuclear Physics at Massachusetts Institute of Technology.
“The ocean's surface rises and falls predictably due to changes in gravitational forces originating from the Moon and the Sun. These changes in ocean surface level are known as tides,” explains MarineBio, a nonprofit marine conservation and science education group. The sun and the moon each possess strong gravitational pulls on the earth and its oceans. Because the sun is further away from earth, its pull is only about 40% as strong as the moon, adds MarineBio. This means that the moon has a greater effect on the tides than the sun. But, when the moon and the sun align, also known as a full or new moon, the pull is at its strongest, causing the tides to be at their highest and lowest, which is known as a spring tide.
“With the greater tidal range during spring tides, you get faster tidal currents that would tend to entrain and concentrate plankton and nutrients on edges such as shorelines,” states Dr. Dean Grubbs, a scientist at Florida State University's Coastal and Marine Laboratory and frequent expert for Discovery Channel’s Shark Week. “This could lead to concentrations of forage fishes and therefore, higher prey abundance for sharks. Many marine animals spawn during spring tides also, which could have the same effect.”
“It’s thought that the spike in shark attacks occur during a new moon is because of the tidal effect on the fish that sharks eat,” says George Burgess, director of the International Shark Attack File at the University of Florida. When fish come in towards shore, sharks will come in towards shore. And because of the change from high tides to low tides and back again happens so quickly, sharks may move in areas closer to where people swim when the tide is low, Burgess adds.
“Most attacks occur in near shore waters, typically inshore of a sandbar or between sandbars where sharks feed and can become trapped at low tide,” states Ichthyology, the Florida Program for Shark Research. When tides get high, sharks have a chance at finding a sandbar closer to the shore. When sharks are closer to the shore and the tides quickly become low, people are closer to sandbars and the sharks are closer to them.
Grubbs adds that sharks tend to position themselves in sandbars because it a steep slope. Animals, such as plankton and fish, aggregate behind the current that is over the drop off, making it easy for sharks to catch their prey.
Other factors play a role in shark attacks. According to the National Parks Conservation Association, swimmers should avoid swimming at night, wearing flashy jewelry, swimming or standing in the water near sandbars and drop-offs, and swimming alone. It is always safest to swim in a large group of people, the Association adds.
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