By Dr. E. K. Ritter
During my work in South Africa last February, I was asked to help in the
investigation of a surfboard accident with a great white shark (Carcharodon
carcharias). An analysis showed that the shark had attacked and grabbed the surfer
from behind and then returned at least twice. Unfortunately in this case the surfer
died. Based on available details of the incident, I concluded that the surfer could
not have been bitten by accident but that other factors must have played a role. Part
of my work as advisor for the "Global Shark Attack File (GSAF)" of the Shark Research
Institute in Princeton, New Jersey, USA (also see the report in
Shark Info 1/99),
includes investigating and testing possible causes for such accidents in addition to
examining and analyzing the sequence of events in such cases. When I returned to
South Africa in May to work on another project I also used the opportunity to test
some initial theories on the cause of this latest fatal event, the type of accident
which is usually explained by saying that the shark mistook the surfboard for a seal.
Our tests were intended to consciously provoke attacks by white sharks to enable an
analysis of the factors which triggered the above incident. Past accident statistics
show that white sharks usually bite surfers who sit or rest quietly on their board,
i.e. who do not actively move away. If one accepts the hypothesis that sharks mistake
surfers for seals, then sharks will find a dummy resting motionless in the water
interesting and sooner or later will grab or attack it. In order to increase the
sharks' curiosity we selected a dummy resembling a seal rather than a surfboard,
considering that seals are the white shark's preferred prey. The dummy was black (not
colored as most surfboards are) and was made of multilayered rubber.
alternative to the immobile dummy, a second experiment was made in which we pulled
the dummy at a constant speed of 5 knots (9.26 km) per hour at a distance of 15 m
behind the boat.
white shark cautiously approaches the motionless dummy.
© Shark Info
During two weeks of continuous experimentation we managed to attract up to 10 white
sharks per day to the motionless dummy in the water. However, only in rare cases did
they bite the dummy, and then only very reluctantly and never with the force and
speed observed when they pursued seals. The sharks always approached the dummy from
behind - swimming directly on the surface - or vertically from below. At no time did
the sharks accelerate their speed as they normally do with real prey. At high speeds,
sharks can even jump completely out of the water.
It thus appears that sharks do not
mistake this dummy for a seal, otherwise they would have directly attacked it. Only
the silhouette appears to have aroused their curiosity because it resembled the
familiar search image of a seal, an image which predators have to help them
concentrate on a target and react very quickly.
Only in one instance was
the dummy bitten in two.
© Shark Info
Here the situation took a different turn, for as we pulled the dummy
behind the boat in accordance with the second test approach we registered up to five
attacks per hour. Most sharks jumped out of the water when attacking. In some
instances the animals, which measured up to 5 m in length, were even seen completely
out of the water, indicating extremely high acceleration and attack speed. This
experiment was also interesting from another point of view since it showed that in
most cases biting was only a means of holding onto the prey. Only once was the dummy
bitten in two. Analysis of the videos showed that the sharks would drop the dummy as
they fell back into the water. It was also astonishing to note that the sharks do not
always approach from behind or below, as expected, but often also from the front,
directly into the dummy's path. Depending on the angle of approach the sharks jumped
fairly horizontally or vertically out of the water. At steep jump angles, they would
rotate once in the air, but not when swimming horizontal paths. Similar behavior has
also been observed when they attack seals, e.g. on Dyer Island (South Africa).
Attacks on a moving dummy are perhaps explainable by the white shark's hunting
behavior and the behavior of the seals. Seals are very agile and are thus not an easy
prey, in addition they can defend themselves astonishingly well by biting.
Furthermore, they often swim in the vicinity of cliffs and rocks and only seldom in
open water, preferring regions which serve as natural protection from sharks.
such skilled predators as the great white shark does not have access to abundant food
supplies since nourishment is usually fairly scarce in the ocean.
A seal swimming in
open waters is thus considered an easy meal and provokes a quick reaction or attack.
An older assumption, known as the exsanguination or bleeding theory, offers another
possible explanation why sharks have rarely bitten the dummy (only in one instance).
According to this theory white sharks attack to wound the prey, then retreat to
prevent a fight and from wounding themselves, waiting until the prey is weakened by
the bleeding wounds before reattacking at no risk to themselves.
It is thus possible
that the moving dummy comes very close to their search image of "here is a freely
swimming seal", thus triggering a fast reaction.
Another possible reason for the
attacks could be the low frequency vibrations stemming from a slowly operating boat
motor. Sharks presumably react to such vibrations because they may resemble those
stemming from wounded animals. However, additional investigations must be made to
verify this. Another reason why the shark can be provoked to an untypical quick
attack may be the search image "here is a wounded, freely swimming seal".
experiments the sharks appear to have mistaken the dummy for a seal (it was, after
all, a dummy seal), since the stimulus threshold eliciting an attack is very low in
such a tempting situation. Still, a white shark can in such an instance clearly
distinguish between a moving surfboard and a seal (or other appropriate dummy),
otherwise attacks on moving surfers would be more frequent from a statistical point
Still, it is too early to draw definite conclusions from these tests. In the Fall we
will begin systematically changing certain parameters of our experiments, including
the form, material and smell of the dummy as well as speed in order to determine
which stimuli elicit a response from the sharks and finally which factors really lead
to their attacking and biting surfers.
At the moment, one thing is certain: When it
comes to a motionless dummy in the water, great white sharks can differentiate quite
well between seals and surfboards and will only approach the surfboards with great
caution and curiosity.
* Dr. Erich K. Ritter is a shark biologist and adjunct assistant professor at Hofstra University, New York (USA)
May be published only by indicating the source: Shark Info / Dr. E. K. Ritter