Ocean News

Diving at Seal Island – home of the flying white sharks

5th November 2009

Our 2009 white shark season was the worst shark season, in terms of shark activity, I have experienced in all my time working at Seal Island. Not only did the sharks arrive later than usual this year, but they left earlier than usual too and the average number of sharks recorded per day was much lower compared to previous years. Strangely, this low activity was not mirrored further up the coast in nearby Gansbaai. However, a number of great sea days with the sharks were still enjoyed and we did get most of our winter field work done that was planned. However, the end of the season means diving out the receivers that have been monitoring the area for tagged sharks and the safest time of year to dive is now when shark activity is low.

The day started off badly when one of the boat’s batteries and battery connection were fried by the saltwater finding its way inside the box. Two hours later with a new battery connected we were on our way only to have the day turn worse when the south easterly wind that was predicted to die down – didn’t. Diving at the island is a logistical nightmare as we have strict codes of conduct and best practices, set out by our Department of Labour, that we need to adhere to when diving for work or research. There are many potential risks to consider, including the possibility of a shark bite. The receivers were deployed in the sharkiest waters at Seal Island to record maximum shark activity, but this means that they are located in the prime hunting areas around the island, 30 meters down.

In an area where white sharks are world renowned for their ambush attack strategy on seals, you can forgive the divers for feeling a little apprehensive getting into the water here. As a result we need to ensure that we drop the divers on the mark, as close as possible, so that searching for the instrument is reduced or even eliminated, this means dropping a shot line on the exact GPS position. This proved very tricky on the day due to the fact that the wind was coming from one direction and a 1.5 knot current was running in another. We need to deploy a safety cage that the divers can use when descending and ascending, but the cage also needs to be dropped on the exact location and be relatively stable otherwise it can be a liability rather than a safety measure.

Once in position (and on this day it took us nearly one hour just to get to this point) the divers descend as quickly as possible. The most vulnerable position for a diver is on the surface and in the middle of the water column and so getting to the bottom as quick as possible is vital. Once on the bottom the divers will locate the mooring and receiver and start to loosen the bolts holding the receiver in place. Good visibility makes the dive safer and easier, but on this day the visibility at the bottom was only about one meter adding to the stress. However, Morne and Steve, who were the primary divers, have been assisting the research this way since 2001 and 2004 respectively, and their experience makes all the difference here. Each receiver costs about USD1200 so we don’t want to lose one, but the data they collect over the season is priceless and we definitely don’t want to lose one of those. Ensuring that the old instrument is recovered and the new instrument is deployed correctly and securely is essential to the success of the project, not to mention determining how much sleep I get.

Every minute the divers are down I hold my breath and typically if they are longer than 10 minutes I start to sweat a little. Everyone on the boat has an important role to play, but none as important as lifting the cage once the divers are ready to ascend. Ascending at a rate greater than 18 meters / minute is dangerous and if a diver has a problem ascending, such as a reverse block, we need to know when to stop hauling the cage up. This means we need exceptional communication between the divers and crew on board. Lifting the cage is no easy task either, but luckily we had Brocq and Rob, two strong lads to do most of the lifting. Each dive done this way takes up nearly two hours.

Of course the most exciting thing for me is rushing home afterwards and hooking the receiver up to my laptop to download the data. The downloading time depends on how much data is stored on the instrument and when it takes a long time I know that we have some great data which makes everything worthwhile. The instruments revealed that a number of sharks tagged in 2008 returned to the island over the winter season, but they also recorded a 4 meter shark tagged in 2007 and surprisingly (and very cool) a shark I tagged in 2005 who was 4 meters at the time of tagging so must be in the 5 meter range now. We did not record both these sharks at the boat this season, highlighting once again the information collected from acoustic tagging and tracking provides us with information we would otherwise be oblivious too. Data like this is essential for understanding white shark activity patterns and using the information in conservation and management applications.

My deepest thanks to everyone on board, namely, Morne, Steve, Brocq, Rob, Megan and Sam for taking all the great photos.