Surf Board 59

Moving Dummy

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.

Even 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".

In our 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 of view.

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.