Thursday, May 27, 2010

Great White Sharks and Boats: do they stalk small boats?

Great white sharks are known for their ability to ambush and capture large prey, like seals, sea lions, or elephant seals - a common prey because of their high fat content that provides the shark with the energy it needs. But do they selectively hunt and attack boats?

This is the question that The Dorsal Fin blog was asking in response to a recent press release covering the upcoming trans-Atlantic voyage of Wave Vidmar as he prepares for a solo-rowboat expedition this summer. Apparently, previous solo boaters have reported being followed by white sharks for hours, even days, and have experienced shark bites on their vessels.

The press release stated, "
Typically Atlantic Great White sharks (Carcharodon carcharias) will follow the tiny ocean row boats for hours to days, then attack by biting the backs of the boats."

That's a pretty bold statement to make and one that The Dorsal Fin rightfully questioned. He received additional information from Wave Vidmar to clarify that four solo boaters had anecdotal experiences of white sharks following them, but that it was not necessarily "typical" behavior. Perhaps an over-ambitious public relations person felt that Vidmar's upcoming voyage needed a little extra sizzle, but let's put it in perspective based on what we do know about white shark predatory behavior.

First of all, great white sharks have two roles as predators: hunters and scavengers. As hunters, besides feeding on marine pinnipeds (seals) and large fish (like tuna), white sharks have been known to feed on cetaceans like dolphins, porpoises, and small whales on rare occasions. Studies have shown that their primary point of attack is on the caudal, or tail, area, as this will immobilize the cetacean.

As scavengers, white sharks have been shown to feed on a variety of cetacean carcasses including much larger whales. The sharks, attracted by the scent of the decomposing animal, will make a slow and careful investigation, and then commence feeding on the remains.

So, could either of these behaviors come into play involving a small boat? Perhaps. The shark may be first attracted to the vibrations given off by the rowing motion. Then the visual of a large dark body floating on the surface may further pique its curiosity - is this a floating carcass? The shark may very well follow the boat for some distance, making a determination as to whether this is viable prey. Bumping the boat or engaging in an investigative bite or nibble, often at the stern (the carcass' "tail"), is not uncommon. But a full-on rush from underneath, as when ambushing a seal, is highly unlikely.

From Great White Sharks: The Biology of Carcharodon carcharias, "While predation by sharks on cetaceans is relatively rare, many sharks scavenge dead cetaceans. White sharks are frequently observed feeding on carcasses of whales off Australia and the eastern United States."

I have had the personal experience of being bumped by a great white shark while sitting in a Zodiac inflatable, during filming. Of course, the "bump" from a 15-foot white shark is no little pat - the shark pushed the inflatable, and the two of us on board, a good foot out of the water. But the shark was merely checking out whether this black mass floating on the surface was a dead whale - it was in its scavenger mode and certainly was not trying to sink the inflatable to get after the two occupants aboard.

So, would a great white shark trail behind a boat and bump or bite it? It's possible. Would it track the boat for several hours? That's also possible; I have seen white sharks spend a considerable amount of time cautiously investigating a tuna head suspended in the water, used by shark divers as an attractant. Would a shark spend days tracking the boat? My gut feeling tells me that's a bit of a stretch. The boaters may be seeing more than one shark over a period of several days. Or it may be the same shark returning, its curiosity once again piqued. But the press release statement seems to imply that the shark is round-the-clock relentless in its pursuit. Sensational but unlikely.

Like the oceanic white tip shark I wrote about earlier, great white sharks are important ocean predators. But we must not think of them as only hunters, continually on the prowl. Their roles as scavengers is critically important and can be the source of their curiosity with surface objects like small boats.

Read the entire press release.
To get some detailed information on white shark hunting and scavenging behavior, read
Great White Shark: The Biology of Carcharodon carcharias, edited by Drs. Peter Klimley and David Ainley. Of particular interest is Chapter 27: White Shark Predation and Scavenging on Cetaceans in the East North Pacific Ocean.


Anonymous said...

There are several cases of Great Whites stalking small boats in Australia. If I recall it correctly, about two years ago, a GWS bumped a kayaker into the water off south Sydney. Then the tragic case in South Australia of the young man being towed on a board by a boat who was followed, attacked and devoured. The point should be made that the GWS has the ability to see above, as well as below the water. I once watched a shark follow a large and terrified mudcrab scrambling along a breakwater wall. The shark kept pace... waiting till it missed its footing.

RTSea said...

There certainly are cases of large sharks bumping kayaks and surfboards. In some cases involving white sharks, it was a full-on attack as the shark mistook the small board for a seal.

And many sharks do have good eyesight or will "skyhop" - pushing their heads above the surface to look around. But one must be careful in making implications, or having their descriptions of events misconstrued by others unfamiliar with shark behavior, that sharks are specifically selecting humans as their prey.

Trans-Atlantic rowers, like Wave Vidmar, use very high-tech vessels that are of a size that white sharks could investigate possibly as a whale carcass, as opposed to misidentifying it as a seal.

Interestingly, Wave Vidmar is actually hoping that a white shark will become curious with his rowboat, as he plans to tag a white shark for scientific tracking purposes.

The issue is whether a statement like a shark will track a small boat for days stretches the truth.