CSI shark – inside the perfect predator
Shark Spotters is a unique organization that combines a direct safety service while promoting the conservation of our oceans and sharks through research and education to find a balance between the safety of water users and the conservation of a much feared, but threatened species, the white shark. Our science team conducts applied research focusing on in-shore white shark presence and behavioural ecology in Cape Town to inform public safety policy, management strategies and education and awareness.
Shark Spotters is committed to provide information to the public on sharks and shark issues which is fact-based, objective and aims to remove the “hype”. Research is a fundamental component of the programme. By understanding population trends, seasonal pattern of occurrence, environmental and biological factors related to high or low shark activity, high risk areas, and the typical behaviour of white sharks in Cape Town we can enhance public safety by incorporating the most relevant information into safety, education and awareness campaigns. We will be sharing more of our research activities and results over the course of the next few months.
The science is led by Alison Kock, our research manager, and supported by research scientist, Adrian Hewitt, a MSc student from UCT currently working on a population estimate for white sharks in False Bay using data collected by our team since 2004. The programme also facilitates post-graduate student projects such as those recently completed by Kay Weltz, an applied marine science student from UCT who completed her MSc earlier this year, where she related shark presence at Muizenberg and Fish Hoek to various environmental factors – the results are currently going through checks and balances and will be published soon!
In the meantime, for those interested in a shark physiology and diet, below is an account from Adrian on a recent white shark necropsy we participated in.
On June 19th 2012, a large adult male white shark (3.8 m total length, weighing 507 kilograms) washed up on the rocky shore of Dyer Island, Gansbaai, South Africa. The dead shark was retrieved by the Dyer Island Conservation Trust (DICT) and transferred into the possession of the Department of Environmental Affairs to await necropsy (animal autopsy) in storage.
On August 2nd, 2012, the necropsy was performed to try and determine cause of death. By all accounts this shark was in prime condition evidenced by its healthy robust physique, a perfect set of sharp dentures and a massively bulging stomach. These all indicated that he was healthy and at the top of his game as a predator. There were no external signs of injury on the body, which could indicate potential boat strikes, or any other kind of anthropogenic (human-related) impacts. There were a few old bite scars on its pectoral fin and on its body, but these had healed completely. So we got down to the dirty, smelly, but fascinating work of dissection and took a closer look inside.
To call this individual a top predator would be an understatement. In total, six, yes six!!!!, Cape fur seals (Arctocephalus pusillus pusillus), were removed from the shark’s stomach. This was a surprise to even the most senior scientists involved! Four of the seals were identified as young of the year pups, the other two were identified as two year olds. The mass of this seal smorgasbord was in the region of 100 kilograms!
Another fascinating part of the dissection was seeing the sheer size of the shark’s liver! The liver is made up of three lobes, two large (one on the left and one on the right hand side) and a smaller median lobe. Their combined weight was approx. 70 kg, which equates to roughly 14% of the sharks total body mass. For comparison, on average human liver makes up just 2% of the total body mass. In white sharks, the liver is thought to be an important organ for various reasons, including buoyancy and nutrition. The liver consists of fats and oils (typically squalene in sharks), which are positively buoyant in water. This prevents the shark from sinking and allows it to maintain its position in the water column, with no extra effort required. The large fins of the white shark aid with balance and they also provide lift, just like wings on an airplane.
Newly weaned seal pups (around 6 months old) usually have a blubber rich layer consisting of tightly packed fats/lipids. These are accumulated from the mother’s rich milk. Blubber is an essential adaptation to keep seals warm, especially when they swim in chilly waters, such as those of the Western Cape. However, unfortunately for the seals, this comes at a price. The energy rich blubber is highly sought after by large white sharks. As such seals are a preferred prey, particularly during the South African winter months when there is an abundance of the young and naïve and (read easier to catch) seals. The stomach contents of this necropsy illustrate perfectly how white sharks will provision or “stock up” on this seasonal abundance of prey when it is available. So how does this all relate to the liver? It is thought that the energy rich blubber from the pups is converted to lipids and stored in the shark’s huge liver. This energy store may then be utilized in the future when food is not as bountiful.
Despite the loss of such a magnificent creature, its body will be put to good use, assisting scientists in learning more about these elusive creatures. Scientists will investigate various aspects (e.g. age and maturity, diet, genetics, reproductive status (hormones), parasites, bacteriology and toxicology) of the biology, behavior and life-history to provide a better understanding of these sharks.
Next year after my MSc I will start a PhD which will investigate white shark reproductive biology through the analysis of hormones. This research field is known as behavioral endocrinology “the study of how hormones affect behavior”. Reproductive endocrinology is a new research discipline being applied to study white sharks and will hopefully allow scientists to determine the reproductive status of free swimming sharks for the first time. If successful, this research may hold the key to identifying when, where and how often reproduction occurs and how this influences movement patterns – information still lacking for the South African population.
Various organs, tissues and samples (i.e. blood, gonads (testes and vas deferens), sperm and even the brain) were removed from this individual to provide baseline information for future hormonal studies. These organs and tissues are known to be important sites of hormone production and regulation in the body. Hormonal research should shed some light on the reproductive riddle of this poorly understood species. Reproduction is a critical aspect of the white shark biology and one which must be clearly understood if we are to effectively manage and conserve these apex predators in South Africa.
At the end of the day, there were no obvious indications of the cause of death from the necropsy itself, which unfortunately, is all too often the typical outcome. We did however, as always, learn new important information from this rare and fascinating encounter.