Batesian mimicry is a broad and somewhat complex area of evolutionary biology. It was first brought to light by an English naturalist called Henry Walter Bates,
To test whether or not mimicking the visual signal of an orca whale will reduce attack rates on potential prey items at the surface I am towing foam decoys around Seal Island in Mossel Bay. These decoys will simulate potential prey items at the surface and induce seal hunting behaviour from the white sharks. There are 4 decoys towed in pairs, separated 15 m apart in order to give enough space between them to make them independent tests due to the low visibility of the water.
The two pairs will consist of a plain black decoy paired with an orca decoy, which will have the ventral black and white pattern of an orca whale, and a plain black decoy paired with a black and white chequered decoy as a control against the specific biological pattern. If white sharks are afraid of the visual sign stimulus of orca whales it is expected that the orca decoy will be attacked less frequently than the other decoys.
The chequered decoy will test if the sharks are avoiding the orca decoy because it views it as a threat i.e. an orca, in which case the chequered decoy will be attacked significantly more than the orca decoy, or purely for the reason that it doesn’t look like the shark’s intended target i.e. a Cape fur seal, due to the contrasting colours and therefore both the chequer and the orca decoy will have a similar attack rate.
Underwater activity will also be recorded using GoPro cameras to show any withdrawal at the decoys that cannot be seen from the surface. If the orca pattern is seen as a deterrent by white sharks, it is expected that there will be a higher withdrawal rate on the orca decoy than the other decoys.
All four decoys towed at the same time to test the possible aposematic qualities of the ventral pattern of an orca whale in the same environmental conditions.
Environmental conditions also play a large part of white shark hunting behaviour. It is assumed that sharks hunting in clear calm waters will be able to distinguish the difference between a real prey item such as a seal, and a foam decoy. Therefore fewer attempts on the decoys in high visibility are anticipated. However, when there is a disturbed sea surface due to wind chop and the water visibility is poor due to increased debris and wave movement, sharks appear to mistake a decoy for a prey item more regularly, as witnessed in previous research. This is presumably down to a greater margin of error in identifying prey at the surface. My study will investigate whether there is any correlation between the frequency of attacks on the decoys and the abiotic factors they are towed in, specifically to the Mossel Bay area.
The overall outcome of this research project is to investigate whether it is possible to reduce surfer’s risk of attack from white sharks by replicating the ventral pattern of an orca whale on the bottom of surfboards to act as a deterrent. Furthermore, by recording the weather conditions and analysing correlations with attacks on the decoys, we can create better guidelines for swimmers and surfers specifically in the Mossel bay area (and also comparing it with other areas) making people more aware of higher risk times to be in the water.
Mike Barron - Msc. Candidate