(From the Marine Dynamics Newsletter, July 2009)
July has been a spectacular month for our shark sightings. Unexpectedly, our winter weather has not affected shark trips too drastically with most days spectacularly calm with flat seas.
July began with an average of 9-14 sharks a trip and remained that way the whole month. Some days at least 17-23 sharks were logged (on a single trip) at our boat which is just incredible. Liasing with other operators similar sightings were reported all around the Dyer Island area.
Communication between white sharks
Since the Great Whites have been around in such high abundance this month we have had the nice opportunity to make some spectacular behavioural observations. Often on trips there would be three sharks in view at our boat at a time- with dominance displays and jerky body displays happening regularly. Great White sharks are silent hunters and don't make noises to communicate but rather warn off con specifics by certain body language signals. Our underwater videographer and volunteers (who have had endless cage time this month) validated these body signals. Gestures such as the lowering of pectoral fins- arching of the back, flicking of their caudal fin and gaping are all signs that a shark is feeling uncomfortable and that someone/thing is in their personal space. Great Whites are such transient animals in Gansbaai we cannot really say they are territorial over specific areas of the reef. They are territorial however of personal space especially when there is a potential prey item in view!
Special sharks this month
Known as 'big girl' to other boats we nicknamed this beautiful shark 'Terry' after one of our British volunteers on board. What an animal she is! Terry arrived at the beginning of July and has remained in the area the entire month. Usually, we tend to see with the bigger sharks around our vessel that they circle us for a while and then move on. This shark however was definitely an exception to the rule. Measuring in at 3.8- possibly larger and with a relatively fresh scrape wound on of the leading edge of her dorsal this large female makes the most beautiful vertical approaches to our bait and endlessly lifts her head completely out of the water. Not only that, Terry loves the seal decoy and almost as if playing with it will continuously make attempts to grab it but without actually biting it- just nudging it. She has become a real favourite with clients and volunteers- probably for all the fantastic photo opportunities she has provided.
Just when we thought our notorious Slashfin and Nemo had left the bay both animals have been sighted in the last few days of July. Nemo definitely looks like she has gained a little bit of weight around the middle. Both sharks are real winter visitors to Gansbaai and its good to see both are still as spectacular as usual.
The majority of sharks have been sub adults measuring on average 2.8m. There have been so many 'characters' its too much of a task to name them all. Two Male sharks had the remainders of old tags lodged behind their dorsal and several sharks have had fishing hooks lodged in their mouths sadly. Interestingly, quite a few sharks have been spotted with the entrails of their breakfast seal pup streaming out of their gills- yummy! This is a nice reflection of natural predatory activity going well this month.
Tagged seals on Geyser Rock
At the beginning of July a tagging program was set up on Geyser rock- (Gansbaai's Cape fur seal colony) with Marine Coastal Management visiting the area to carry out the ground work. The Dyer Island Conservation trust (DICT) provided all the technical assistance and help needed to conduct the work. The DICT are the only organisation in the area with their own stranding kit and correct tools. In total, ten cows and ten pups were fitted with transmitters (two types of 'speaker' tags). Three receiver towers were deployed on Geyser rock in order to capture the signals from the seals. As far as we know the tags will stay on the seals for three months and the data will hopefully reveal how far and exactly where the seals head off to feed from the island on a daily basis. The results have been reported as very encouraging so far. We are eager to see the results. Last year a couple of our Penguins on Dyer island were tracked again with the technical assistance of the DICT and swam almost 60km in a day for their food- unexpectedly far distances for the impressive little birds!
Whale season
Southern Right Whales are back in our area- not in full force yet- but consistent mother and calf pairs also plenty of Humpback whales have been passing through the area. One day whilst on a shark trip- four humpbacks (two calves and two cows) swam so closely off our stern. It was an amazing experience to hear them calling to one another with a tremendous blow every so often- stunning animals and photo opportunity for our guests!
Stranded Risso's Dolphin
At the end of July a Risso’s Dolphin Grampus griseus was found washed up off Danger point just off the rocks at I&J Abalone Farm (Gansbaai). We got permission to go through and photograph the dolphin which measured just under 3m. The Risso’s dolphin is an offshore species which doesn't approach boats readily. Their particular prey preference is squid. These dolphins are a dusky grey/black-blue colour with a distinct rounded-melon shaped head. They have lots of white marks and scratches all over their bodies from either their squid prey- other dolphins biting them or parasites. Reasons in the past for the species stranding are bioaccumulation of toxins and internal parasites. There is a type of trematode (Parasitic worm) that reportedly eats away at the cranial nerve of the dolphin. Exact results of the stranding will hopefully be verified at the marine mammal institute at the museum in Cape Town by marine mammal expert Meredith Thornton. Natural death by old age or sickness is also a highly possible cause as the dolphin was believed to be a mature adult.
Sea birds- shark bitten penguin
As usual during winter months we sight spectacular sea birds out at sea in Gansbaai. We have been encountering some huge Giant Petrols n the area- some so heavy that they struggle to take off again once landed on the water! On the 29th we found a poor penguin floating in shark alley and took it back to shore. Unfortunately the bird had been dead for a while. Examining the animal closer a bite wound was clear- and the culprit a shark. Sharks kill penguins rarely in the area of Gansbaai and this is only the second time in three years we have collected a shark bitten animal on our vessel. White sharks are infrequent predators of sea birds at Dyer Island, perhaps due to an abundance of Cape fur seals (a preferred prey), anti predator behaviour by penguins, and sea birds not being a preferred prey type (Johnson et al 2005).
Regards
Alison Towner
