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.
