Having arrived in Mossel Bay just a couple of days previously, Monday morning found our group of new interns chumming for white sharks just off Seal Island. Having been prepared to spend an hour or two waiting for our first glimpse of dorsal fin, we were almost unprepared when, within 10 minutes, the first shark appeared at our bait.
This arrival was greeted with a flurry of activity; recording data on the shark (particularly noting physical characteristics) and taking photo’s of the dorsal fin to record the pattern of notches that appear on the fins trailing edge – all to be used for identification should we encounter the same shark at a later date.
This shark was soon followed by a second female. It was decided that this second shark provided a good opportunity for tagging, as she was in the target range for the size of sharks wanted for data collection (at approximately 2.5 metres), and seemed interested in the bait enough to keep her around the boat long enough to get all three tags in her before she moved away. The shark was named Pinocchio as preparations for tagging were made.
The first tag was designed to measure the internal body temperature of the shark, from inside the stomach. The tag was prepared inside the body of a tuna that was then offered to Pinocchio. After a couple of cursory passes of the bait she took the tuna, effortlessly tearing it from the bait float.
The remaining two tags were applied simultaneously, via a modified spear gun, to the shark just below the dorsal fin. The first was also designed to give body temperature data, but this time from the white muscle tissue, and the third gave external (water) temperature as well as information on depth.
Data from the tags was collected from each tag, via a unidirectional hydrophone, to a box known as the VR100. This registered each data receipt with an audible ‘ping’ that would come to govern our lives over the next few days. The stronger the signal (dependant on the direction and distance of the tags, and hence the shark, relative to the hydrophone), the louder the ping – this was the information that allowed us to judge the sharks position and track her movements.
The first sets of data from the shark were confusing – the shark appeared to be in two places at once. It was apparent that rather than swallow the tuna, she had spat it out. We anxiously followed her movements with the remaining two tags and hoped that she would return to finish her meal, while a replacement stomach tag was prepared. Eventually, she returned to the boat and was offered a second tag laden tuna, which she duly took and consumed – resulting in a mass of high fiving and a very excited crew on the boat!
Having successfully tagged the shark, the personnel on the boat was swapped out. We returned to shore, and the first tracking shift kicked into gear. I went home to get some rest and prepare for my tracking shift, which was due to start at 2 o’clock the following morning.
2 am found us back at the tracking boat. Our shark had moved away from Seal Island, and was cruising slowly near the moth of Kleinbraak river. The high oxygen content of the water near the river mouth makes an ideal environment for a resting shark. The shark spent most our 7 hour shift cruising near the coast in shallow water.
Spending the hours of darkness on a small boat with the constant ping of the VR100 and the lights from shore was quite a surreal experience, interrupted occasionally by Enrico and his riddles! There were four of us on the boat, rotating between spending an hour tracking, piloting the boat, collecting positional data, and resting. The South African sunrise signalled that our shift was close to an end way before we felt ready to return to shore, but before long the next shift had been ferried out to our position and we were heading home.
2 am again, and our second shift saw us following Pinocchio back towards the island with some urgency – it must be time for breakfast! Our conclusions that he shark was hungry was backed up by the swimming pattern of the shark, as it took a yo-yo pattern moving from the surface to deeper water in it’s search for fish.
The shark circled Seal Island before we ‘witnessed’ it’s first meal since it took our tuna. I use the word ‘witnessed’ loosely as, although we did not visually observe any predation we were able to track it’s effects on the stomach temperature of the shark, which we saw take a dramatic drop as the shark ingested a mouthful of cold water with its meal. It may be rare to witness a shark predation, but how may people can say they’ve tracked the stomach temperature of a feeding white shark?
After breakfast, the shark returned to it’s river mouth, and to resting.
Morning three, and our ongoing tracking activities are threatened by an incoming storm. Pinocchio spends much of her time resting around the river mouth. At the end of the shift we are beginning to feel the effects of the storm and decide to continue tracking, rather than ferry out the next shift for potentially just an hours tracking before the boat is forced back to port. The shark seems to sense the changing water conditions also, and begins to move along the coast. Our additional hour tracking eventually turns into 6, but our extra efforts are rewarded as we catch up with the shark at the surface, and watch her swim by the side of the boat – the two external tags clearly visible against the dark back of the shark. We were now tracking visually as well as acoustically. Unfortunately, hampered by the rapidly worsening sea conditions we lost contact with the shark. A brief search proves fruitless, and the conditions mean that we could not track the shark again if we were to find her. We turned the boat round and headed back, through the growing swells, to a very well earned beer (or 2)!
Chris Pulham – Mossel Bay Oceans Research intern