On the Hunt: study looks at great white shark's ambush capabilities

If you have shown even a passing interest in great white sharks, you have probably heard or read descriptions of these powerful sharks as ambush predators which feed primarily on seals and sea lions. And you may have seen images of white sharks on the hunt, leaping out of the water as they hope to catch an unsuspecting seal - striking images of one of the ocean's truly most magnificent predators.

In a recent scientific paper, Neil Hammerschlag, Ph.D. of the University of Miami's RJ Dunlap Marine Conservation Program and R. Aidan Martin, Ph.D. of the University of British Columbia examined the nuts and bolts of what transpires when a great white shark is in a predation mode. "Marine predator-prey contests: ambush and speed versus vigilance and agility," published in Marine Biology Research, details the optical and physical advantages and disadvantages for both predator and prey.

Camouflage, Vision, and Speed
The researchers studied great white shark predatory behavior at South Africa's Seal Island, famed for its spectacular breaching white sharks. They observed several elements that play into the shark's ability to affect a successful ambush. First, the time of day: we have heard that many sharks like to hunt at early dusk or sunrise. This is a time period that provides the shark with a workable degree of light to see prey at the surface, but also provides maximum camouflage using its natural counter-shading (dark coloration on top, light underneath). In low light levels, the shark's gray upper body reflects very little light and easily blends in with the dark rocky bottom below.

I have seen this personally where, during bright sun, the shark is illuminated with dancing rays of sunlight and can take on a slight metallic sheen to its skin. But as the sun falls and the dappled light fades, the shark's skin becomes strikingly flat and non-reflective - its cloak of invisibility now at work.

The researchers also noted that the great white shark's ability to see a seal moving along the surface is linked to the shark's depth. A principle call Snell's Law restricts the shark's range of vision to a formula based on depth - a common depth exhibited by the sharks in gaining an ideal horizontal view of the surface (150 to 170m/490 to 550ft) is around 26 to 30m (85 to 98ft).

Finally, there is the great white shark's speed. The muscles of a great white are incredible storehouses of energy, capable of very powerful bursts that can hurtle the shark to speeds of 35km/h (22 mph) at the surface, over 1.5x the speed of breaching blacktip sharks and enough to lift a 2000 pound shark clear of the water. To reach that maximum velocity, the white shark requires a depth - a running start, as it were - of a little over 25m (82ft) and can cover that distance in less than 3 seconds! That leaves the seal with precious little time to escape.

An Opportunity for the Prey
Nature has certainly allowed the great white shark to evolve into an extraordinarily efficient hunter. But does its prey have anything up its sleeve? In the early dusk light, the seal's vision is limited to about 3m (9ft) in depth which can provide it with just a split second to spot an ambushing shark at top speed. However, with that small window of time, nature has provided the seal with a chance to escape. Seals are very agile in the water, with a very tight turning radius and the ability to change direction quickly. So if the seal is not injured in the initial charge, it can utilize its maneuverability to make a hasty getaway. The seal's vigilance and agility can give it an edge in survival. And as one can expect, those seals that are less experienced (ie: young) or slowed by illness or age are often prey for the great white shark because their awareness and agility is diminished.

We know the great white shark is a powerful hunter with capabilities that put it near the very pinnacle of the food chain. Hammerschlag's and Martin's study looks at the actual mechanics of those amazing abilities and makes for an interesting quick read. It proves, once again, how truly fascinating and magnificent these animals are.

Click here to download a PDF of the report.

Hawaiian Monk Seal: proposed protections need public support

When considering marine mammals, whales and dolphins are what come to mind for many. And that's understandable as these animals have been the subject of many conservation battles, from ongoing anti-whaling campaigns to inhumane treatment (as in my post on Wednesday of this week) to the tragedy of the Taji harvest as shown in the documentary, The Cove.

However, also included in the marine mammals group are seals and sea lions and while, perhaps, less severely threatened as whales and dolphins, they are not completely out of the woods. Once extensively hunted for their fur, hide, blubber (for making seal oil) and meat, the populations of many seals and sea lions declined rapidly. But large scale seal hunting has declined, much like whaling in the past, and the number of seals and sea lions have improved despite continued hunting by several countries - personified largely by the harp seal hunts that take place in Canada and Northern Europe and Scandinavia, although it does still take place worldwide.

Being based in Southern California as I am, I have had the opportunity to come in contact in the wild with California sea lions, harbor seals and, occasionally, northern elephant seals. Typically curious and playful, I have had them nipping on my dive fins for fun, sneaking up on my game bag (in my early days of diving when I would go for the occasional lobster or abalone), or I have been repeatedly dive bombed underwater, as many other local divers have experienced, either as playful exercise or as a means of shooing us away if we appeared to be a threat.

Unfortunately, even where hunting is not taking place or is even prohibited, seals and sea lions are still set upon with challenges: caught in fishing nets, tangled up and choked by plastic garbage, poisoned by pollution or naturally occurring substances like domoic acid from algal blooms, and even shot by the occasional fisherman who felt the animal was affecting his potential catch. Add to that, changes in their natural habitat from climate change, and we find seals and sea lions - animals that are important members of the ocean community and add to its natural balance - are in a perilous position not much different than whales and dolphins.

One species of seal I have not had the pleasure yet of seeing in the wild is the Hawaiian monk seal, unfortunately one of the most endangered of all marine mammals. Once heavily hunted, their numbers were reduced to a level that has had them continually at that tipping point of total population collapse. Designated as endangered under the Endangered Species Act, the number of Hawaiian monk seals continues to slowly decline to just a little over one thousand in number. Further reductions would certainly put it on a slippery slope towards extinction.

Several conservation organizations have taken the U.S. government to task to ensure that all is being done to give these ocean mammals a fighting chance. Kahea - The Hawaiian-Environmental Alliance and the Center for Biological Diversity (CBD) are petitioning concerned citizens to be the voice of the Hawaiian monk seal in demanding the Obama administration to adhere to the Endangered Species Act and grant the seal protected habitat not only on the northwestern islands where it is currently protected but on the main islands as well. Just this month, such a proposal was made, expanding the seal’s habitat to 11,000 square miles, including beaches and coastal waters on all the main Hawaiian Islands and increasing its protected habitat in the Northwestern Islands.

But it is not yet a done deal and so both Kahea and CBD continue to exert legal and public pressure (there's even a Facebook page dedicated to saving the Hawaiian monk seal) through public petitions and filings in the courts to make sure that the National Marine Fisheries Service makes good on the expanded habitat proposal. There is still a chance that one of Hawaii's unique species can be saved from extinction.

Learn about Kahea - The Hawaiian-Environmental Alliance.
Learn more about The Center for Biological Diversity and its Hawaiian monk seal campaign.