South Africa has long been regarded as a white shark ‘hotspot’, but in 1991 it was necessary to protect the species due to increased demand for their jaws as trophies and evidence that some populations in other parts of the world were declining. Due to the rarity of these sharks being apex predators, coupled with a low reproductive output, white sharks are particularly vulnerable to exploitation. Ironically through this protection status, valuable biological information is ‘lost’ to science.
Traditionally, population status and diet are determined for shark and fish populations using catch statistics and analysis of stomach contents, but when it comes to white sharks we need to be more creative in the ways that we collect this data as we cannot (and don’t want to) catch and kill these sharks. Scientists use observation, stable isotope analysis and tracking. Unfortunately there are inherent biases in the data which need to be overcome, and ‘ground truthed’ against actual data. This is why we can learn a lot from specimens, especially from the Western Cape waters.
A year ago a juvenile white shark (2.7 meters TL) was found lying dying along the Mossel Bay coast. It is suspected that the animal was caught by a rock and surf angler and subsequently released, but that it eventually succumbed from the stress of the capture. Many people people that sharks are hardy and can be pulled to shore, manhandled, taken pictures of and then released safely, but the truth is that they are sensitive to this capture due to lactic acid build up and many sharks die as a result, even though they were released.
Two weeks ago Marine and Coastal Management invited me to participate in the dissection of this animal and although I was sad that it died, I was also excited to be able to learn more about the small shark’s physiology and what it was eating in our waters. White sharks are known to feed on a variety of prey items, like fish, other sharks, cephalopods and marine mammals, but although we know a lot about their taste for Cape fur seals, we know very little about what else they supplement their diet with (although we have our suspicions).
One of the most interesting things to see was how large the liver was, it measured over 1 meter in length and filled up the abdominal cavity. The liver’s function is two-fold: it stores energy to allow the sharks to survive for extended periods of time without food and it helps the shark regulate it’s buoyancy. Even more interesting was seeing the rete mirabilis for the first time. This mass of blood vessels is a heat exchange system that allows the white shark (and other members of the Lamnidae family) to retain heat generated by their muscles. This physiological adaptation has contributed to their success in cold -temperate waters and allows them to actively hunt large prey in cold waters.
It was then time to open the stomach. I was secretly guessing what we would find inside. Would it be seals, fish, sharks or something completely unexpected. I think I held my breathe while Mike Meyer (MCM) made the incision. Alas, the stomach was completely empty which was a huge disappointment. I suspect that either the shark hadn’t eaten in awhile or, more likely, it regurgitated it’s meal as a result of the stress it was under just before it died.
I am very fortunate to be able to work with these magnificents sharks on a regular basis, and don’t usually have to deal in their death. However, I have learnt a lot from this experience and their fascinating physiology. I believe this will help me on my journey to better understanding these ultimate predators.
Click here for more information on the white shark research project.
Click here for more information on the Save Our Seas Shark Centre.