Shark cage diving in False Bay (some photos)

Sunrise
Sunrise

We took a trip to Seal Island in False Bay to see the white sharks there, in late July. I’ve already posted my video footage from the cage. We also took some photos – mostly Tony. The trip entailed getting up very early, so as to be at Seal Island by sunrise. Once there, we scanned the horizon for predatory behaviour: typically, the white sharks here attack the juvenile seals from below, often launching their entire bodies out of the water in an explosive burst of energy.

Video still of one of the great white sharks we saw
Video still of one of the great white sharks we saw

It was a very rough day with a swell of about five metres, and from speaking to people who come to Seal Island often, I gather that the sharks tend to be less active on days like this. Their accuracy in striking the seals is reduced by the movement of the water column. Nonetheless we did see a couple of predation events, with the characteristic flock of seabirds waiting to pick up any leftovers, and the slick of “oily seal juices” (to quote Gary!) left on the surface afterwards. The sharks are so quick that if you’re looking the wrong way, it’ll all be over by the time you turn around.

After some time watching natural behaviour, a decoy (surprisingly realistic looking, made to resemble a young seal) is towed behind the boat, to try and elicit breaching behaviour from the sharks. We didn’t have much luck here, again probably because of the surgy seas, but one shark made a few investigations of the decoy before losing interest.

White shark next to the boat
White shark next to the boat

Finally sharks are attracted to the boat using chum, which is mostly fish oils and other fishy substances. A tuna head was splashed in the water near the boat, and when sharks came to investigate it they were visible from the cage. While in the cage we breathed off scuba regulators, which was great. Trying to breath-hold or snorkel while the sea was so choppy would have been next to impossible. The sound of the bubbles emanating from our regulators didn’t bother the sharks at all.

Bernita and some stormy seas
Bernita and some stormy seas

We spent about twenty minutes (or maybe more – I am not sure) in the cage, some of it just waiting for action, and some of it with our full attention focused on the enormous fish swimming by and looking at us with its black eyes. Five minutes of looking at a great white shark, eye to eye, gives sudden perspective on life and the natural world. I’ll recommend this experience to anyone who will listen!

 

Newsletter: Mellow and mild

Hi divers

Mellow and mild aptly describes the weather for the weekend. Day time temperatures of around 17 degrees celcius, water temperatures of around 14 degrees and viz of 5-6 metres or as Facebook will have you believe, anywhere between 3 and 12 metres… There is not much swell, very little wind and more sun than clouds (in the forecast) so the diving should be good.

This week’s dives

We spent some time in the pool (the visibility was good) and yesterday we had 5 metre viz at Long Beach. The bay has patches of clean and dirty water and the cage diving and whale watching boats report clean and dirty water scattered around the bay. This is most likely the rainwater runoff that has not moved too far as there has not been all that much wind. The fishermen report very clean water south of Miller’s Point.

Weekend dives

I have a lot of student dives to get through this weekend for both Open Water and Advanced courses so if you are keen to dive we can slot you in. We are launching tomorrow, Saturday and Sunday and will only decide the sites once out on the bay and we can see where the water is clean. Text me or reply to this email to book a spot.

The rudder of the Brunswick
The rudder of the Brunswick

I’ve had students with me all week so there are no underwater photos this week (no camera allowed), but Clare tracked down the rudder of the Brunswick at the Slave Lodge in town – there’s a photo of it above. It’s huge!

regards

Tony Lindeque
076 817 1099
www.learntodivetoday.co.za
www.learntodivetoday.co.za/blog/

Diving is addictive!

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Shark cage diving in False Bay

The opportunity to see great white sharks safely, on your own terms (that is, not by surprise while diving!), and in a way that isn’t harming the sharks or affecting their behaviour on a large scale, is amazing and unusual one. Living in Cape Town, South Africa, we are more fortunate than most people in having three excellent cage diving operations (Apex Predators, African Shark Eco-Charters, and Shark Explorers) on our doorstep in False Bay, and – for the summer months, when the False Bay season closes down – Gansbaai just a two hour drive away.

I have visited Seal Island on board the Shark Spotters research boat, but that wasn’t for getting in the water with the sharks – the scienfic data is collected from the surface (I watched – more here, here and here), but Tony has never been. Tony and I have tried to go together to visit the sharks at Seal Island on two occasions before. Once, the conditions were too poor so we ended up in Gansbaai (more on that here), and the second time we planned an overseas trip and had to cancel our cage diving booking. The operators can get booked up very far in advance during peak season, which is when we wanted to go, which is why the overseas travel ended up overlapping with the cage diving trip.

Third time lucky! Two of Tony’s former students, Tamsyn and Gary, work for African Shark Eco-Charters, Tamsyn taking bookings and Gary as Divemaster on the boat. We booked a trip with them for late July, which is during the best period to see white sharks at Seal Island. We were excited to be able to breathe off scuba regulators while in the cage, and this turned out to be a wonderful thing because it was a very rough day (big swell, wind – and rain!) when we ventured out. The Stugeron that Bernita and I had ingested did its wonderful work.

Here’s a video clip of some of what we saw while in the cage. I’ve slowed this video down to 35% of the actual speed, because it’s really bumpy – the cage was like a washing machine! Trying to snorkel would have been unpleasant.

[youtube=”http://www.youtube.com/watch?v=qLZBaZZW0XE&w=540″]

The shark in the video is a female white shark (she has no claspers – she obligingly shows us her big belly), and she was huge. It was lovely to have Bernita with us, and absolutely amazing to see our False Bay sharks up close. They are magnificent, remarkable animals worthy of our protection.

Maps about sharks (part II)

For the final assignment in week five of the Coursera MOOC I just completed, Maps and the Geospatial Revolution, we were tasked with creating a map that tells a story. Following on from the discussion I’d had with my classmates in week four, I decided to make the series of maps I’d talked about in the discussion, showing trends in human-shark interactions through time. I downloaded the entire International Shark Attack File database for South Africa from SharkAttackFile.info, and some census data from Statistics South Africa. I used the census data to normalise the ISAF data, so that we can look at rates of interaction and account for increasing coastal populations.

South African coastal provinces and sea surface temperature
South African coastal provinces and sea surface temperature
Colour scale for maps showing number of incidents
Colour scale for maps showing number of incidents

This series of maps shows the number of interactions (fatal and non-fatal, including bites, nudges, etc) between humans and sharks for each decade between 1951 and 2010, as well as the rate of interactions per million people. Its purpose is to determine

  • whether the widely-held perception in South Africa (and in the Western Cape province particularly) that shark attacks are increasing unchecked, is correct;
  • and whether the frequently-provided partial explanation, that increasing coastal populations – putting more people in the water and in the home of sharks – explains the perceived increase or not.

I think these maps are an improvement over the ISAF map for South Africa for a couple of reasons:

  • We can spot trends through time, which a single, static map does not allow.
  • The ISAF data quality has probably improved over the years, and plotting data from 1905 and 2005 on the same map is questionable. At least this way we can just look at the most recent maps if we want to know what things look like right now.
Colour scale for maps showing number of incidents per million people
Colour scale for maps showing number of incidents per million people

From the map series above, you can see that the number of encounters between humans and sharks each decade has remained fairly constant. A naïve reading of the data would suggest that there have been no advances in mitigating the risk of a shark bite. However…

The rate of encounters per decade (normalising the data for increasing coastal populations and water use – orange maps below) has mostly been decreasing, after peaking in 1970. In KZN, this can probably be attributed to increased shark netting at swimming beaches. In Cape Town, the Shark Spotters program (started in 2004) warns swimmers out of the water when a white shark is spotted near the backline of the surf, and notifies them when it is safe to return to the water. In the last 25 or so years scientific research has also shed light on the conditions that sharks tend to prefer, enabling water users to avoid the water at these times.

The water temperature off the Western Cape ranges from 10 to 22 degrees celcius, while off KwaZulu Natal (KZN) it varies between 20 and 25 degrees with the seasons. Warmer water temperatures draw more people into the water, where they meet sharks. For the first 40 years of my data set KZN has a relatively high rate of shark encounters because all it took was a bathing suit to enjoy the ocean there. The Western Cape only caught up to KZN with respect to human-shark interactions once thermal protection from wetsuits became commonplace and more affordable in the 1970s, and people were able to stay in the water for longer.

There is a lot of natural variation in the numbers. Detecting firm trends in time series derived from an ecosystem is tricky, as many factors influence the process under observation. Great white sharks have been protected in South Africa since 1994, but because there was no known baseline population figure for the animals at that stage, it is difficult to assess whether there has been an increase in shark numbers as well as bathers. The presence of a whaling station in Durban, that closed in 1975, probably also contributed to numbers of sharks close to the Durban beachfront (attracted by the offal that was pumped out into the ocean in front of the Bluff) that were larger than there would be otherwise.

Finally, the likelihood of you meeting a shark while in South African waters (unless you go out of your way to, on a cage or baited dive) is very, very small. The most recent numbers put it at a less than six in a million chance over ten years. We dramatically overestimate the risk of a shark incident, because they are so emotive and fear-inducing.

If you want to see these maps laid out nicely in sequence, click here to download a pdf of my assignment. It’s easier to follow, I promise!

Newsletter: Midwinter summer’s day

Hi divers

Wow! is about all I can say for the current summer day-like conditions. A short while ago I installed a weather station at home to help my inadequate weather forecasting, and this is a screen shot of today’s temperatures. Note the high of 26.5 degrees.

Screen shot from our home weather station console
Screen shot from our home weather station console

Weekend plans

To complement the great weather the bay is clean and blue ,and the water temperature is 15 degrees. All this good stuff from today is set to continue for the weekend, and we will launch the boat on both days. Neither day will deep dives as I have Open Water students, so we will pick the sites from a range of the following: Photographer’s Reef, Ark Rock, SAS Pietermaritzburg, Spaniard Rock or Caravan Reef. Why the loose arrangement? Well there are a few traces of red tide further north and further out in the bay, so we will dive where we have the best conditions.

If you want to dive, text me. Be quick, because the boat is already quite full!

The past week(end)

White shark at Seal Island
White shark at Seal Island

We did not really dive last weekend as the weather wasn’t all that great and most of this week has been spent on Divemaster training. Over the weekend there was rather a large swell and lots of wind and rain. We did however get wet on Sunday, above and below the surface, as we did a cage diving trip in False Bay to take a closer look at some of the rather large animals hunting around Seal Island. We had some sun, some rain and some time in a cage, and seeing the white sharks was absolutely remarkable.  Despite the swell the experience was most definitely a memorable one and if I won the lotto I would go back every other day for the entire season.

Great white shark in False Bay
Great white shark in False Bay

Training

We are working on some detail for a few Specialty Courses not offered much in Cape Town, namely Drift Diving, Research Diver and Equipment Specialty. If any of these courses tickle your fancy send me a mail and I will send you the details.

regards

Tony Lindeque
076 817 1099
www.learntodivetoday.co.za
www.learntodivetoday.co.za/blog/

Diving is addictive!

To subscribe to receive this newsletter by email, click here or use the form on this page!

Movie: Dark Tide

Dark Tide
Dark Tide

I’m going to go out on a limb here and say that this is the worst movie ever made. I don’t think a worse movie could be made; I’m willing to make this prediction even if the human race continues making movies in their current form until the end of civilisation. The fact that this is an appalling film shouldn’t put you off seeing it if you live in Cape Town, however. If you’re a Halle Berry fan, you will probably also be interested in this offering, and your enjoyment will probably be enhanced by viewing the film with the sound turned off. The movie was filmed a couple of years ago in Simon’s Town (False Bay Yacht Club, Bertha’s restaurant, and the jetty outside Bertha’s all feature) and False Bay. There is brief footage at (I think – it’s dark) Miller’s Point, Boulders Beach, and a fair amount shot at Seal Island. The underwater footage looked like it was shot in a kelp forest off Duiker Island in Hout Bay. Lots of seals. There was a lot of kelp – more than I remember there being at Seal Rock near Partridge Point. It could also have been shot at Seal Island (where it purports to be) in summer, but the water is quite clean which makes me unsure. There are about six characters, most of whom are played by local actors. We are treated to a variety of accents, sometimes several different ones from a single individual. There is a lot of supposedly endearing and humourous banter between Berry and her local staff members, which I just found patronising and offensive. Halle Berry’s character, Kate, freedives with white sharks. After causing the death of her safety diver (he was eaten), she retires from shark diving and takes people on boat tours to Boulders Beach to look at penguins and to Seal Island to look at seals. She can do this all in one short trip because Boulders is on the way to Seal Island when you sail out of Simon’s Town. Right? Right! (Another interesting fact I didn’t know about the geography of False Bay is that Seal Island is a 20 minute surface swim from Miller’s Point. The abalone poachers apparently do it often, but have a “less than 50% chance” of making it back.) It was fun to see Simon’s Town on film, and to identify that Kate’s office is actually the clubhouse for the kids’ dabchick sailing school at FBYC. A wealthy man of indeterminate nationality wants to swim with white sharks outside a cage. Kate is tricked (sort of) into taking him to do so. At seal island they see a couple of sharks, but the millionaire cannot follow instructions (“stay in the cage”) and Kate discovers that her boyfriend promised him a cageless dive without consulting her. After an INORDINATE amount of shouting and screaming on the boat, Kate loses her rag and decides to take the millionaire “around the point” to “Shark Alley” where the really big great white sharks can be found, to teach him a lesson. (Readers unfamiliar with Cape Town should know that there is a place here called Shark Alley, but it’s inside False Bay and no white sharks are found there… Only sevengill cowsharks.) Despite the worsening weather they make the trip, and at this point the movie becomes a cross between The Perfect Storm and Jaws. There is a lot more shouting on the boat. Lots of people get eaten by sharks. No doubt the NSRI is called. Not many of the characters make it home. To sum up, several people die in extremely violent and gory shark attacks. The blame for all of the deaths can be laid at Berry’s character Kate’s feet. She is immature, has a bad temper, and is incapable of assessing risk. Unfortunately she survives. Some of the shark footage is nice. An alternative title for the film could be “Shouting on a Boat” or “Halle Berry in Small and/or Tight Clothing”. If either of those appeal, by all means, be my guest. I hope the Department of Environmental Affairs, FBYC and STADCO made some nice money out of issuing permits and renting facilities for this film (really). It’s great that local venues are benefiting from the international film industry. SharkLife apparently sponsored a lot of the clothing worn in the film. Their logo was everywhere. I watched the credits with greater attentiveness than I did the rest of the movie, looking for familiar names among the stunt divers, skippers, cameramen and extras who featured. I found some! You can buy the DVD here if you’re in South Africa, otherwise here or here.

Haai nee man!

(An alternative title for this post could be “Stay classy, Gansbaai“… But I don’t think it’s just a Gansbaai thing.)

On one of the days we were in De Kelders it rained unpleasantly in the morning, so Tony and I visited the Danger Point lighthouse and explored Gansbaai harbour. In the fish shop there we found the following array of delectable treats:

There is a factory nearby that processes shark meat, and I suspect that the biltong, offcuts and other little bits of shark originated there.

It’s generally not a good idea to eat the ocean’s top predators such as whale, tuna and large sharks, but the vast majority of shark species are actually quite small and wouldn’t be prone to the mercury and toxic chemical buildup that occurs so prodigiously in the larger species at the apex of the food web.

Commercial shark fishing is prevalent in South Africa, and eating shark seems more common (among fishermen, at least) than you’d think judging by these two threads of discussion. It also seems to be something that happens by accident, when unscrupulous suppliers mislabel or deliberately obfuscate the identity of the fish they are selling.

What do you think about purposely going to buy some smelly little bits of dead shark to eat? And what do you think about being able to buy them in a town deriving much income from the massive shark eco-tourism industry that relies on live sharks, located just around the corner?

[“Haai nee man” is an Afrikaans expression said to someone as an expression of disbelief – “no really man!” where “man” is a commonly used casual South African form of address when one wishes to exhort or chide, and “nee” means “no”. “Haai” is untranslatable in this context, but also means “shark” in Afrikaans. So it’s funny. Of course, by explaining all this, the humour is lost. Oh well.]

Boat-based whale watching

Whale Whisperer, the Marine Dynamics vessel used for whale watching
Whale Whisperer, the Marine Dynamics vessel used for whale watching

On a recent visit to the Walker Bay area, Tony and I took a whale watching trip with Marine Dynamics, the same company we did a cage diving trip with last June. Marine Dynamics is associated with the Dyer Island Conservation Trust, which conducts research on whales, sharks and penguins, as well as running educational initiatives focused on marine conservation. In addition to a cage diving permit for the Dyer Island and Geyser Rock area, they have a boat-based whale watching permit covering the area from Danger Point (near Gansbaai) to Quoin Point.

Southern right whale
Southern right whale

The harbour at Kleinbaai (just beyond Gansbaai) has a treacherous little channel running between rocks and over a seemingly permanent set of rollers out to sea. Launches are thus scheduled according to tides and swell. We met around 9am at the Great White House, Marine Dynamics’ headquarters, in Geelbek Street in Kleinbaai. After a safety briefing we were fitted with waterproof jackets and life vests, and we took a short walk down to the harbour. Whale Whisperer, the Marine Dynamics whale viewing boat, is a Gemini rubber duck with two rows of benches running the length of the vessel. There is an observation deck, too, and up to eight people can stand upstairs to view the action from above.

The sea conditions were marginal, with a brisk wind and large swell, so “sails” on one side of the boat were let down to protect the passengers on that side from the spray. Without a permit, it is an offence to approach a whale closer than 300 metres. Whale watching is tightly regulated and self-regulated and operators in possession of a permit are allowed to approach to within 50 metres of whales provided they are not trapped against the shoreline.

Southern right whale as big as a bus
Southern right whale as big as a bus

The trip was over two hours long, and we saw many mother-calf pairs and one or two lone whales. All the whales about at this time of year are southern right whales. They have distinctive whitish callosities and exaggerated, bow-shaped mouths. When they exhale, their blow is usually in a V-shape. They come to southern African waters to mate, calve and socialise and are found here from June to December, with peak season in September and October.

Calf breaching
Calf breaching

The calves are curious and exuberant, ranging in size from 4.5 to 6 metres at birth (imagine!). Several times a mother whale steered her calf away from our boat with her body, as the calves sometimes made a beeline for our boat when it appeared on the horizon! The calves’ behaviour is characterised by joyful tail and fin slapping, spyhopping, and rolling over and over. We also saw one little chap leap out of the water, eight or ten times in succession. A hopeful flock of birds hovered nearby, waiting to feed on the parasites and bits of dead skin dislodged by this activity.

Southern right whale calf breaching
Southern right whale calf breaching

After looking at whales, we passed by the shark cage diving boats where they were anchored in the shallow water near Pearly Beach, looking for white sharks. We only saw one white shark – an active, very small chap under 2 metres in length – during our brief visit there.

The Marine Dynamics shark cage diving boat at anchor
The Marine Dynamics shark cage diving boat at anchor

The final leg of the trip was to Shark Alley, which separates the seabird haven of Dyer Island from the seal colony of Geyser Rock. The smell of seal was overpowering. There were far more seals in the water than there were when we visited Shark Alley last June (southern hemisphere winter) – they modify their behaviour according to where the sharks are. In winter, the sharks hang out in Shark Alley and feed on seals. In summer (they tell me that’s now!) the sharks go inshore to the highly oxygenated, warm shallow water where they may rest, socialise and possibly engage in breeding activities.

Feisty little white shark at the cage diving boat
Feisty little white shark at the cage diving boat

The trip to Shark Alley and back to Kleinbaai harbour was powerfully bumpy, and by the time the boat was retrieved from the water a powerful stench of vomit – and some visible streaks and smears – marred the deck of the boat. As we left, the staff hosed it down with disinfectant to remove both the smell and the fluids. I felt sorry for the people who were doing the third trip of the day!

Geyser Rock, tasting strongly of seal
Geyser Rock, tasting strongly of seal

White Sharks – Biology, Behaviour and Physiology (part III)

This post follows on from my review of Global Perspectives on the Biology and Life History of the White Shark. That book (a collection of scientific papers) is divided into three sections, and I’m going to highlight papers that I found particularly interesting in each of the sections. My notes here are mostly nuggets of information that grabbed me when I read the papers, and in some cases aren’t even related to the author’s main point. If the paper sounds interesting to you, I suggest you track it down to read in full. Hopefully these bits and pieces are interesting, too!

This is part III of a series of three posts on this topic. Here is part I and here is part II.

What we can learn from white shark cage diving operators

Boat-Strike Wound Healing in Carcharodon carcharias – Towner, Smale, Jewell

The authors of this paper are involved with Marine Dynamics, the cage diving operator in Gansbaai we dived with in June 2011. They describe the injury of a 2.3 metre male white shark that was struck by a fishing vessel near Dyer Island, causing a would 25 by 30 centimetres in extent, and 8.5 centimetres deep (absolutely enormous). The shark stayed at Dyer Island for two months, and then went elsewhere for nine months, after which it returned with a closed, pigmented scar where the wound had been, and looking a little the worse for wear (thinner than normal). Nearly two years after the boat strike, the shark bore no long-term damage other than a healed area where the wound had been. Its condition and behaviour matched that of other sharks. Comparing the rate at which this injury healed to that of other animal species, the paper concludes that white sharks heal relatively rapidly. These findings are important because increasing marine traffic puts larger pelagic creatures such as whales and sharks at risk from boat strikes, and also because some of the tagging and tissue sampling methods used on white sharks are fairly invasive.

Investigatory Behaviour towards Surface Objects and Nonconsumptive Strikes on Seabirds by White Sharks, Carcharodon carcharias, at Seal Island, South Africa (1997-2010) – Hammerschlag, Martin, Fallows, Collier, Lawrence

The “Fallows” in the author list above is Chris Fallows of Apex Predators, and the “Lawrence” is Rob Lawrence of African Shark Eco-Charters, both cage diving operators in False Bay. This paper is citizen science at its best. The cage diving operators have collected a huge database of in-the-field observations of white shark behaviour, recording the time of day, the species of bird involved, and whether the bird survived the encounter. In only one case was a bird (an African penguin) consumed by the shark – in the other 61 incidents recorded the birds were not eaten.

The authors speculate that many studies of white shark diet are biased towards marine mammals (seals, whales, dolphin) because it is easier to identify the relatively large remains of these creatures in the shark’s gut. They suggest that white sharks consume a far larger proportion of low-fat foods than current research indicates. They observe that most of the seabird strikes reported in their study took place in winter, in low light and choppy seas, and during times of intense competition among sharks for seals. This suggests a mistaken-identity element in many of the attacks, owing to environmental conditions, as well as pre-emptive strikes to avoid losing a feeding opportunity. They also suggest that seabirds may give off unpalatable chemicals, and that their feathers may be difficult to digest.

Comparisons Between White Shark-Pinniped Interactions at Seal Island (South Africa) with Other Sites in California – Fallows, Martin, Hammerschlag

This study compared 2,600 predatory interactions (data collected by Chris Fallows) of white sharks with Cape fur seals at Seal Island in False Bay, with previously published data regarding white shark predation at the Farallon Islands and off the central Californian coastline.

White shark-seal interactions at Seal Island in False Bay had the following characteristics:

  • They occurred mostly within an hour of sunrise, before 0830, dropping off progressively after that.
  • More attacks took place at high tide.
  • More attacks took place during northerly winds.
  • More attacks took place in low light.
  • Predations peaked in the months of June to August (southern hemisphere winter), when most seals were available.
  • Success rates (proportion of predations that resulted in a kill) were similar all year round.
  • Most attacks took place on solitary seals, though group sizes of targeted seals ranged from 1 to 15 individuals.
  • Most of the seals targeted were young of the year, newly weaned with a thick layer of blubber but with limited stores of energy (weighing 4.5 kilograms and about 70 centimetres long).
  • As the seals got larger, the white sharks’ success rate decreased.
  • Size of attacking sharks ranged from 2.1 to 4.5 metres, but sharks that were between 3.1 and 3.5 metres made significantly more attacks than other sized animals, and sharks longer than 3.6 metres were by far the most successful in their predations.

The prey available at the Farallon Islands, however, are northern elephant seals, which can weigh 2,200 kilograms when fully grown. (Male Cape fur seals are an order of magnitude smaller, and weigh up to 360 kilograms.) The Farallon Islands off California play host to much larger sharks, with sizes between 3.5 and 5.9 metres reported (compare to the False Bay sharks sizes: 2.2 metres to 4.6 metres). The absence of very large sharks from Seal Island in False Bay is probably related to the difficulty of keeping up with an agile, manoeuvrable little seal, which can turn as fast as a shark but in a fraction of the distance.

The success rate of white shark predations at Seal Island was measured to be 48%, compared to the Farallones’ 64% (the authors acknowledge that this may be an overestimate). Unlike at Seal Island, the Farallon Island predations occurred throughout the day, in all light levels, but are also targeted at juvenile seals. At both locations attacks took place mostly at high tide, close to the island, and in deep water. Other differences in attack patterns are related to differences in the shape and topography of Seal Island and the Farallones. The authors conclude that there are some significant differences in how white sharks prey on pinnipeds, mostly related to prey and attack site.

What we can learn from fishermen who catch white sharks

A Summary of Observations on the Maximum Size Attained by the White Shark, Carcharodon carcharias – Castro

This paper is an entertaining read – Castro does a historical study of reported “monster” white sharks, examines the evidence, takes measurements of their remains (teeth, vertebrae and jaws still exist today in some cases), analyses photographs, interviews fishermen in Mexico and California, and concludes that the largest white shark that has been reliably measured was a 6 metre specimen from Western Australia. So if someone tells you they need to get a bigger boat, or that seven metre white sharks roam the waters off our coast, just nod, smile, and offer to buy them another beer.

A trip to Seal Island (part II)

Seal Island, launchpad in the foreground
Seal Island, launchpad in the foreground

The first couple of hours of our time at Seal Island were spent watching for natural predations by white sharks on seals. I found it quite emotionally draining, and almost gave myself whiplash swivelling my head from side to side in an effort not to miss anything. After that the boat was anchored, and a small chum slick was created from the boat in order to draw sharks closer. A tuna head on a float attached to a rope was also used to create water movement and a scent trail to draw sharks closer for observation.

Whenever a shark approached, Adrian and Alison attempted to identify it (they recognise many of the sharks that frequent the island from their markings and scars), estimated its length, determined (if possible) its gender, and any other distinguishing marks. A GoPro camera, mounted on a pole between two laser lights enables the researchers to estimate the dimensions of each shark more accurately, if it comes close enough to the boat to be filmed underwater. This is a remarkable yet simple technique that enables precise calculations because the distance between the two lights, the position of the camera, as well as the cameras’ field of vision, is known. This can also be a way of enhancing the information-gathering potential of the BRUVs currently being tested in False Bay. Tissue biopsies are taken on some of the sharks, also using a long pole.

There was much excitement (from me, at least) when we saw one of the sharks tagged by the Ocearch expedition. A small piece of red fishing line was caught on its tag; Adrian reached over and removed it when the shark surfaced next to the boat. The tag was much smaller than I’d imagined. Unfortunately the magic confluence of enough time at the surface plus satellite overhead did not occur, and when I searched the tracking website for a False Bay ping on 25 July I was disappointed. So I don’t know the name of this shark, or where she was from, but I was very happy for the sighting!

The Ocearch-tagged white shark that visited us
The Ocearch-tagged white shark that visited us
The tagged shark on the surface
The tagged shark on the surface

Another visitor who caused great excitement was an unknown (i.e. not recognised by her distinguishing marks) female shark of absolutely tremendous proportions – she was nearly five metres long. Adrian said she was the largest white shark he’d seen in False Bay. She swam around the boat several times. The sharks were surprisingly hard to see from above, even with absolutely calm conditions and bright sunlight. The water clarity was fairly good, too. Their bodies are extremely well camouflaged from view.

A nearly 5 metre long female white shark next to the boat
A nearly 5 metre long female white shark next to the boat

While we were at anchor and looking at sharks from the surface, the cage diving boats were at anchor nearby, and looking at sharks underwater from their cages. They also spend a bit of time towing a decoy (cut out of a seal, made from rubber or carpet or similar material) behind their boats in order to elicit a predatory response from passing sharks.

After an enthralling procession of beautiful animals past the boat, it was time to raise the anchor and head back to shore.