A unique opportunity: The Dyer Island Conservation Trust dissect a Great White Shark carcass.

July 09, 2015 by dyertrust

By Alison Towner, DICT Marine Biologist.

On Friday afternoon the 26th of June, a local fisherman Pikkie Smal contacted the Dyer Island Conservation Trust (DICT) to report the carcass of a Great White Shark (Carcharodon carcharias). The shark was found washed up at De Gruis, towards the end of the Uilkraalmonds beach, in Franskraal. The DICT responded immediately with a team of their crew and biologists alongside CEO Wilfred Chivell. On arrival, the sun was setting, but the team managed to confirm that the shark was an adolescent female. The specimen quality was very poor and decomposed. The shark had likely been dead for a number of weeks as her skin pigmentation was badly bleached. The team measured her at 3.2meters, pre caudal length. Interestingly the tail was completely missing, with this intact she would likely have measured close to 3.5m total length (TL).

At sunrise the next morning the DICT team returned, together with Marine Dynamics International Marine Volunteers (IMV) and co ordinator Francois Swart - the more helping hands to move the heavy shark the better! She was loaded on to a 4x4 Samel- a vehicle specifically procured by the DICT for accessing difficult parts of the coastline. To prevent further damage whilst in transit, the shark was carefully wrapped in a trawl net. Once removed from the beach, she was taken to the nearby IMV lodge in Kleinbaai and offloaded onto an open grassy area. I joined the team at this point and on first inspection of the shark, three bite marks were visible on the underside as well as a traumatic boat propeller injury towards the tail area.

Given the decomposed nature of the shark, she would have degraded considerably in any attempt to transport her to Cape Town. It was agreed that freezing was not an option. The department of environmental affairs branch Oceans and Coasts therefore permitted Meredith Thornton, DICT research and IMV co coordinator and myself to perform the necropsy on site. Meredith and I had recently attended a government dissection of two smaller white sharks. Meredith has extensive experience with necropsies of aquatic animals. Marine biologists Georgia French and Simone Rizutto from Sussex and Stellenbosch University massively assisted in both the retrieval and dissection.

Inside the shark.

The first organ exposed when cutting into any shark, is the enormous liver. Aside from being the main detoxification unit, the liver is both a buoyancy aid as well as an energy reserve. White sharks break down nutrients from their food and store them in their liver as a form of oil called squalene. This female white sharks liver weighed 59kgs and was almost liquefied- it was by far the most grueling component to remove and weigh! The Necropsy was completed in five hours from start to finish. All the organs, measurements and necessary samples were collected, including the stomach which had the remnants of seal carcasses inside of it. The impressive jaw was cleaned down to the cartilage, then all of the samples were sent to government. The Dyer Island Conservation Trust is applying for a permit to display the jaw on site for educational purposes.

It is particularly sad when marine predators wash ashore deceased but we have to be mindful of the fact that it really does give us a unique opportunity to collect samples which would otherwise be difficult to obtain. The information collected from this female white shark will go towards several crucial studies improving our knowledge in areas such as genetics, diet, age/growth- just to mention a few. Simone Rizutto was able to take muscle samples for his eco toxicology work to assess levels of contaminants which have accumulated in the sharks muscles, and the stomach contents will go to Dr Malcolm Smale of Bayworld Museum to be used as part of a long term diet project.

White shark carcasses rarely wash ashore, it is more likely that they sink. The last white shark to wash ashore locally in Gansbaai was three years ago, in June 2012. It was a 3.8m (TL) male found on Dyer Island. The DICT successfully retrieved this specimen too, it was taken through to government for dissection, which revealed no obvious cause of death despite 6 cape fur seals in its stomach! (SEE LINK TO PREVIOUS DICT BLOG).

Cause of death?

Great White sharks are incredibly hardly animals and we have seen them recover rapidly from traumatic propeller injuries in the past. However, if a deep enough laceration occurs to a sharks tail, the forward momentum of the shark swimming could cause the lobe(s) to detach, and of course no shark can survive without its tail- it will simply sink and suffocate. With this female shark, the propeller laceration could potentially have caused a fatal blow- although this is only speculation and we will never truly know if the propeller and bite wounds occurred before or after death. The other possibility is transient Killer Whales (Orchinus orca) which have been documented to first bite the tail (fluke) from dolphins when hunting, however the evidence is too unclear to say. Interestingly, at the exact time the shark carcass appeared, cage diving boats were battling to find white sharks in the area. A decomposing white shark carcass could well signal to others not to stay in the area - anecdotal evidence exists of this elsewhere but exactly how this occurs remains unclear.

All in all, the dissection was a huge success. The process would not have been possible without the large amount of effort combined with the quick execution of appropriate equipment by the DICT- as well as the dedicated hard work of all involved. We would like to make a special thanks to Pikkie Smal. This was the second occasion within the last two months Pikkie has phoned in to notifiy the DICT of marine specimens, the previous occasion had been a bottlenose dolphin (Tursiops truncatus) at Dangerpoint. The DICT are exceptionally grateful for Pikkies vigilance and quick contact and we encourage anyone in the local area to phone in at any time if they sight something of interest along our local coasts.

Contact Numbers:
DICT 0829075607 or 0828018014African Penguin & Seabird Sanctuary Rescue Line 0725987117