This September, the Great White Sharks in False Bay enjoyed the luxury of a whale of a feast provided by a dead Brydes Whale at Seal Island. Seal Island is a favoured winter hunting ground for Great White Sharks who hunt young and inexperienced seals there with a good chance of success. Typically, with the change of season in September, the sharks disperse to hunt the frontrunners of the summer fish shoals moving into the Bay.
The feast fit for a shark was provided by a 11 m Brydes Whale carcass (Balaenoptera edeni) which was spotted drifting towards the shore at Partridge Point, near Simonstown. Had it gone ashore, it would have created a 10 ton ‘clean-up’ problem for Cape Town authorities.
Of bigger concern, is the risk associated with attracting sharks inshore. Two years ago, when a dead humpback whale calf washed ashore near Muizenberg beach, the Shark Spotters recorded significantly increased shark sightings resulting in the beaches being closed temporarily. This time, a quick decision was made to tow the dead whale to Seal Island – a big thank-you to the SA Navy for their support. At Seal Island the sharks could help solve the problem by simply doing what comes naturally to them as top predators – keeping our oceans in balance and healthy.
The floating whale carcass was spotted by Chris Fallows of Apex Expeditions. “One of the single biggest lessons in nature is to always be aware of the signals the environment is constantly projecting.” (Monique Fallows) During 4 previous sightings of dead whales at sea, the Fallows had seen Giant Petrels, the great scavenging birds of the oceans at the carcass. As soon as Chris spotted Giant Petrels circling a patch of sea, he knew what it signaled.
Respected shark scientist Dr Alison Kock, from the Save Our Seas Shark Centre at St James, describes below how the white sharks feasted on the whale carcass for 9 days until it was a mere 2m section of bone and gristle.
Brydes whales (pronounced Broo-das whales) are found all year round in False Bay, Cape Town. In our experience they are usually very shy and rarely approach boats, although during the annual sardine run, further up the South African coast, divers have had very special close encounters with them while they have been feeding on the schools of sardine. These whales reach lengths of over 12 meters and can weigh over 15 tons. It’s suspected that this whale’s death was not natural due to a large, very regular mark running alongside its body which may have been an indication of a boat strike or even perhaps being caught in a trawl net, a suggestion by Chris Fallows. While the death of the whale was very unfortunate, it allowed us to document an extreme example of the cycle of life. It also provided an unparalleled opportunity to document white shark behaviour and record the number of sharks in the area. Morne, Adrian and myself spent nine days at sea documenting the spectacle providing fascinating insight into white shark dining behaviour.
In the animal kingdom it’s all about balancing the ‘budget of life’ with survival being the ultimate goal. Everything the sharks do, like feeding, costs them energy, and to survive they need to make sure that the reward of capturing prey is more than the cost of catching it. White shark experts, like Dr. Peter Klimley, have theorised for many years that white sharks target high energy prey because of their own high energy budgets that comes with being able to thermo-regulate (one of the few sharks able to do this) and maintain an active lifestyle. At Seal Island, sharks often launch themselves completely out of the water to catch their seal prey, expending loads of energy doing so, but presumably with the reward being worth the effort. Whale carcasses present sharks with a different scenario and the opportunity of a high reward meal, but with very little cost to the shark in terms of energy expenditure – which creates an all round winning situation.
After getting over the initial excitement of seeing this mind-blowing event with so many sharks around the carcass, we were able to observe very interesting behaviour. The first thing that surprised us was the total number of white sharks that fed on the dead whale over the nine days. September in False Bay is a month of change with the sharks spending less time around the seal colony and many moving away from the area; in fact a few days before the carcass was towed to the island, we were not reliably sighting sharks there. A detailed examination is still needed of the data, but our preliminary observations, using video and photographs, indicate that over 30 different sharks fed on the whale while we were there. Many of the sharks I recognized from our studies, but there were also new sharks that I had never seen before. The first two days of the event were the busiest with the most sharks feeding, and every day afterwards the activity decreased till we recorded no sharks on day eight and nine. By this stage the sharks had reduced the carcass from 11 meters to a mere 2 meters of mostly bone and muscle – naturally solving the problem.
White sharks maintain a pecking order when around one another, most often with the largest animals dominating over the smaller ones. On the first day we recorded the largest sharks, over 4 meters, taking advantage of the best opportunity to feed on the ‘choice cuts’ available. Thereafter, we recorded the smaller sharks taking their opportunity. White sharks usually communicate their dominance using their size and body language, and even though at one stage we saw four white sharks attempting to feed simultaneously at the carcass, we recorded few actions of aggression towards one another. A couple of sharks sported fresh superficial bites from other white sharks, just a few puncture wounds from the top teeth of another, maybe indicating that shark forgot his manners (understandably perhaps) in the presence of the feast on offer. Over 80% of attacks on Cape fur seals at Seal Island are over in less than 1 minute, and we suspect given that the seals only provide a ‘meal for one’ that they finish it so quickly, so as not to lose their catch to another. However in the presence of such a large food source like a dead whale, the need for fierce competition was eliminated, allowing them to feed side by side.
By far the most interesting observation was that of how selective the sharks were. One might expect that they would simply eat as much as they could on whatever part of the carcass was available. But this wasn’t the case at all. They targeted the energy rich blubber, often making repeat “test bites” (where no flesh removed) and only removing flesh once they had determined it was what they wanted. The blubber of the whale has a much higher calorific value than the muscle, providing up to double the amount of energy. In this case we documented numerous examples showing that the sharks knew exactly what they wanted (the blubber) and if they got a mouthful of muscle they often spat it out.
This may provide some insight into why so many bites on humans (over 70% of white shark bites http://www.flmnh.ufl.edu/fish/sharks/isaf/isaf.htm ) are a bite and release only. It provides evidence that when they bite into a surfboard, or kayak or person wearing a wetsuit they can immediately determine whether it’s something they want to eat and in the case of the latter, it’s usually not. Unfortunately we weren’t able to stay out at night due to poor weather, but there were definitely indications that they fed extensively during the night as we documented a lot of new bites and a shrinking carcass from the time we left to the time we returned.
This was an extreme example highlighting the very important role sharks play in the ecosystem, that of recycling life, and of keeping our oceans healthy by removing dead and decaying animals like dead whales. It also provided an opportunity to show a different side to the white shark, a side that few people get to read about and are usually only exposed to sensationalistic shark attack stories. We have gotten such great feedback from all corners of the globe with the Daily Mail article, including from children absolutely enthralled with sharks and wanting to learn more. On top of that it will be a story I’ll be telling my grandchildren one day, a special opportunity I’ll treasure forever.
Compiled by KimK from accounts by Alison Kock and Monique Fallows. Oct 2010 Photos copyright Alison Kock of the Save Our Seas Foundation