How do great white sharks stalk their prey beneath the waves? A paper by Neil Hammerschlag and the late R. Aidan Martin, published in the latest Marine Biology Research Journal, reveals how the physics of light-scattering in ocean waters helps the great predator maintain stealth as it hunts its most common food source — seals.
Great White Sharks: AKA “White Death”
For their study, Hammerschlag and Martin chose the shallow waters surrounding Seal Island in False Bay, located in the extreme southwest of South Africa. Some 60,000 Cape fur seals call this small, elongated, rocky islet home. It is also one of the few places where great white sharks are listed as an endangered species.
According to Miami’s press release of December 09, 2011:
“Animal hunting in the ocean is rarely observed by humans,” said Hammerschlag, director of the RJ Dunlap Marine Conservation Program at UM. “The high frequency of attacks by white sharks on seals at our study site in South Africa provides a very unique opportunity to uncover new insights about predator-prey relationships.”
Torpedo-shaped, warm-blooded great white sharks are solitary creatures approaching 20 feet in length. Their 3000 razor-sharp teeth and powerful jaws literally produce tons of pressure. They possess an acute sense of smell and have specialized organs enabling the detection of minute electrical currents (as little as 0.005 millivolts) — which all living creatures generate. Their powerful, muscular bodies and large tails produce enormous bursts of energy. In a seal attack, a 5000-pound great white can breach the water surface at speeds reaching 25 miles an hour.
The Physics of the Attack
Cape fur seals regularly feed beyond the 25-mile wide mouth of False Bay. After several days at sea, they typically return solo or in groups of two or three. Great whites wait in the shallow waters of the Bay, and most often strike small groups of returning seals who are less than one year old.
The sharks typically attack the seals from below and behind. They prefer to hunt in the dim light of early morning. Here the play of sunlight in the water makes it difficult for a seal on the surface to see the shark below. Per Hammerschlag and Martin, the “wavelength-selective absorption of daylight” by the ocean waters results in “low-light conditions restricted to a narrow region of the spectrum.”
The deadly sharks lurk at a minimum depth of about 23 feet. When seals look down, they have great difficulty spotting their mortal enemies. Evolution has given great whites dark grey backs. Thus, as seen from above, they are camouflaged against the dark reef habitat below. “Back-scattered light is a function of depth” and an object’s reflectivity, Hammerschlag and Martin explain. Upwelling of backscattered light is such that “only about 0.01% as much light is backscattered upward“. (Backscatter is the deffuse reflection of light back to its source. In the case the reflection of sunlight from the shark’s back, backscatter returns light back up to the ocean surface.)
What does the shark see? He sees a Cape fur seal “backlit and silhouetted against the (ocean) surface“. Thus, due to the physics of light-scattering in ocean waters, seeing seals from below is much easier than seeing sharks from above.
Seals Fight Back
Cape fur seals, however, are far from helpless. They are constantly on watch, with unique abilities to avoid or outmaneuver great whites. They are more agile than their much heavier enemy — a seal’s turning radius is three to ten times shorter than a great white’s.
After the initial attack, according to Hammerschlag and Martin, a seal who has not been disabled often “leaps toward the shark’s back (away from jaws)” to escape. Seals have even been known to injure great whites in these encounters.
The predator/prey relationship in False Bay is natural selection at work. Shark attacks serve to eliminate weak and sick seals, strengthening future seal populations. Great whites help keep seal and sea lion population in check all over the world. Some marine scientists have even suggested the depletion of salmon populations off the California and Oregon coasts (seals feed on salmon) may be due to a lack of great whites to keep the number of seals down.
Martin, R. Aidan & Hammerschlag, Neil. Marine predator–prey contests: Ambush and speed versus vigilance and agility. (2011). Marine Biology Research. 8:1, 90-94. Accessed December 10, 2011.
Gonzalez, Barbra. New study illustrates the physics behind great white shark attacks on seals. University of Miami Rosenstiel School. (2011). Accessed December 10, 2011.
Extreme Science. Biggest Carnivorous Fish: Great White Shark. Accessed December 10, 2011.
AASharks. Great White Sharks. Accessed December 10, 2011.
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