Alison Kock , who for the past eight years has been researching the behaviour of the Great White Sharks in False Bay for her PHD, presented a talk entitled “Solving Cape Town’s Great White Shark puzzle: Can an improved understanding of shark habits help us to live together?” to a fascinated audience at the U3A Natural Science Group meeting at Meadowridge Library on Monday.
Between the years 1960 and 2012 there have been 29 reported shark incidents in Cape Town of which six have been fatal. All six fatalities have occurred in the last six years. “Incidents are rare, but they are very traumatising and have high impact on the local communities,” said Alison. “They have invariably involved a large number of witnesses and sensational reporting in the media causing anti-shark sentiment to run high. There is global coverage of the events, especially via the social media and a lot of misinformation about sharks is spread. Fish Hoek Lifesaving Club is the top lifesaving club in South Africa, but the three shark incidents in Fish Hoek Bay have caused a dramatic decline in club membership according to its members. Local businesses have also felt the impact with fewer visitors to the area. Surf schools in Muizenberg, one of the best places to learn to surf in the world, have suffered too. We therefore need to mitigate against these effects and find pro-active solutions to having sharks and humans sharing the same space for the benefit of both.”
West Australia has recently dealt with its shark “problem” in a radical way. Following six fatal attacks along its west coast in less than three years, a political decision was made to kill large white sharks close to popular beaches “deemed an imminent threat”. “This decision goes against available scientific information” says Alison. Great Whites are rare and are listed on CITES Appendix II and as Vulnerable on the IUCN Red List of Threatened Species. There is no global estimate of numbers. “Preliminary information indicates that a few hundred white sharks use False Bay on a regular basis, which is not many in comparison to other species.” Commercial trade in Great Whites is illegal.
“Great Whites play a vital role in ecosystems. They are the big cats of the sea, the apex predators. A wide range of creatures make up their diet: including seals, squid, yellowtail, cob, white steenbras, and other species of shark such as smooth hounds, soup fins and rays. There are 20 relatively common species of shark and ray in False Bay.”
Part of Alison’s research has revealed that False Bay is home primarily to the “teenage” group of the white shark population, with an average size of 3.6 meters. Measuring 1.5m at birth, the sharks only reach maturity at 3.6 meters or 8-10 years (males) and 4.5 meters or 12-15 years (females) “We generally don’t see many young of the year sharks or adults over 5 meters and still don’t know where the adults spend most of their time.” White sharks can reach maximum size of 6 meters, and live for 45-60 years.
Alison tagged 78 Great Whites which were monitored by more than 30 receivers deployed around False Bay for her PhD. From this she has been able to detect which sharks are in the bay, where and when and whether male or female. A very specific pattern has emerged, with peak shark activity at Seal Island during the autumn and winter months and an equally high incidence of sharks inshore, but during the spring and summer months when most people are using the water. She also found that it is only the female sharks that are coming inshore during spring and summer while the males seem to leave the bay at this time, or are found at the island. These results have major implications for conservation and management. It has often been suggested that traditional shark nets or drumlines which catch sharks should be deployed inshore. Had this been done in Cape Town, it could have drastically impacted the population by selectively removing females of the population.
“Cape Fur Seals, of which there are over 70 000 on Seal Island, give birth in December and January. The baby seals drink milk from their mothers and are unavailable to the sharks when born as they don’t enter the water. When they reach the age of 3 to 4 months they need to supplement their milk diet with fish and this is when the white sharks start arriving at the island. In spring the South Easter starts to blow, plankton blooms and there is a huge shift in the diversity of fish and sharks coming into False Bay. It seems that the female white sharks come inshore to take advantage of this increase in prey availability, at a time which also coincides with the young of the year seals now having extra experience in avoiding sharks around Seal island. As the Great Whites are opportunistic, they probably find it more productive to go inshore to feed where prey is abundant and easy to catch. It is a mystery as to where the males go,” said Alison.
In describing how sharks attack their prey, Alison said that the Great Whites do not generally treat humans as they do their natural prey. “They are designed to inflict the maximum amount of damage as quickly as possible to still their prey or lose it to a competitor. Over 70% of the attacks on people have involved bite and release without any feeding, although consumption does happen on rare occasions. When attacking seals, their approach is powerful, fast and unrelentless. As large, confident apex predators they are very curious animals and investigate all they see. Their teeth may also function as touch receptors. The younger sharks are less confident hunters and cannot inflict as much damage with their dagger-like teeth. They become more confident as they grow older. When they are 3 – 3.5m long their teeth become broader and work like a knife and fork – the knife for cutting at the top and the fork at the bottom for holding the prey. Their success rate in hunting seals at Seal Island is more than 50%. Because the animals are dark on top and because 70% of the time the sharks attack in low light situations, it is difficult for seals swimming above them to see them. However, seals are very agile and very clever and use tactics to avoid capture.”
The Great Whites are able to increase and maintain an elevated body temperature, generating heat in their muscles. “They have the physiology like a cheetah – built for short bursts of speed which cannot be sustained. They need to eat frequently.”
It is a mere 6 kms from Seal Island to Strandfontein. Through monitoring the movement of the sharks, Alison has discovered that the great majority of inshore sharks are found here, followed by the inshore areas of Muizenberg, Macassar, Simon’s Town and Fish Hoek. One of her recommendations from her work is for a consideration to increase the size of the current TMNP-MPA to include Seal Island and Strandfontein, two critical areas for sharks in the Bay.
Alison is the research manager for Shark Spotters which operate at eight beaches on the peninsula, of which five are manned throughout the year. The only beach on the Atlantic side which is monitored year round is De Hoek at Noordhoek. “Over 70% of the sightings at Muizenberg are behind the breaker line. The beach is not always closed when the sharks are in the vicinity. It is only closed if the sharks pose a risk in terms of their proximity to water users, but the movement of the animals is carefully monitored. Since the programme started in 2004 there have been > 1450 sightings. On average the sightings last for about 20 minutes after which the beach is opened again, once the shark has swum past. Three significant findings have been made by analysing the data collected by Shark Spotters at both Muizenberg and Fish Hoek:
- There is up to 8x more chance of a shark being close inshore at these beaches when the water temperature is above 18deg as opposed to when it is 14 deg. 18 degrees is the optimal temperature for many other shark and fish (potential prey) species. The Great White tolerates temperatures between 5 and 28 degrees C.
- There is a significant increase in sightings at New Moon versus Full Moon. Scientists do not fully understand the reason for this.
- There has been an increase in the number of sightings over the past three years at both Muizenberg and Fish Hoek. We cannot say if the increase indicates an increase in the number of sharks or a change in their distribution paths or whether these are the same sharks using the shoreline more often.”
Alison believes that it is important to give out as much information as possible about shark behaviour so that water-users can make informed decisions when entering the ocean at any time.
Questioned about the shark exclusion net which has been deployed six times at Fish Hoek beach, Alison said that it differs from the conventional shark nets which catch sharks and other sea creatures in mesh of 20cm diameter. Only two crabs have so far been caught (and released) in the net, which has a mesh diameter of 4cm. The net reaches down to the bottom of the sea and is withdrawn in the evening. She hopes to see the net being deployed on a daily basis this coming spring and summer to assess its efficacy as a shark barrier. ”Cape Town is at the forefront of in terms of living with sharks with world firsts and its pro-active and environmentally friendly approach to a highly emotive topic,” she said.
Referring to the Ocearch Global shark tracker programme, she said that adult sharks have been tracked up to Reunion, Madagascar and Mozambique. Over 1/3 of the tagged sharks have been picked up by acoustic receivers in Port St John’s and signals from Ragged Tooth sharks from Algoa Bay near Port Elizabeth have been picked up in False Bay in summer. “In collaboration with many institutions e.g. SAIAB and ATAP (Acoustic Tracking Array Platform) there are over 100 receivers along the coast – all the way to Mozambique.”
Alison is continuing research on the white sharks, but has also started doing research on the Seven Gill Sharks found in False Bay. Thirteen of these sharks have been tagged so far with transmitters that will last up to ten years.
Copyright Viv von der Heyden
Alison Kock is a marine scientist who “recognises the cultural, environmental and economic value of sharks” and is passionate about understanding and conserving them. She is the Research Manager for Shark Spotters.