(What happened to them since)
By Carly Walker, A.A. Undergraduate Student, Seminole State College & Shark Angels Intern
Scientific Study: Trends in sightings and population structure of white sharks at Seal Island, False Bay, South Africa, and the emigration of subadult female sharks approaching maturity by Alison Kock and Adrian Hewitt.
Background
White sharks live in all the world’s oceans. Especially in False Bay, South Africa. You may have seen them leap out of the water on Shark Week. But that’s only scratching the surface of these incredible fish. What are they doing below the surface? Are they breeding in False Bay? Are they mating? Are they juveniles, adults, or both? Does False Bay have a distinct population of white sharks? And if the sharks do leave, where do they go? With the help and funding from the Save Our Seas Foundation, marine biologist, Alison Kock, was able to find the answers to these questions.
What and Why
Alison Kock tracked white shark migration around False Bay for nine years (2004-2012). But because of low shark sightings, only 34 months of data were used for this study. The goal of this study is to find out why white sharks are in the area, why they leave, and what maturity and sex are commonly around. The results will be used to help improve the management of critical habitats, shark safety strategies, and awareness campaigns.
Methods
Kock used photo identification of individual sharks to study their visitation patterns. She was able to identify 303 different sharks by taking pictures of their dorsal fins and comparing them to other sharks’ dorsal fins they had photos of. Out of the 303 individuals identified, 88 made a reappearance. Over those nine years, there were more shark sightings in 2004 and 2006 with a steady decline as the study progressed. It’s unclear if this decline is due to fishing or something else. More research needs to be done to figure out where they’re going if they are leaving the bay. It became clear that male sharks were present year-round, especially during spring and summer. Females and males are around consistently during the autumn and winter months. June and August of 2005 and 2006 had more than 20 sharks sighted with a max of 36 different individuals in June 2005.
Measuring sharks is a good way to guess the maturity of the individual. And while all the sharks that were used in the study weren’t all measured, more than half of those that were measured were subadult or juvenile, and less than 10% were mature. Male white sharks mature at a smaller size than females and most of the mature sharks sighted were male. The females that approached sexual maturity during this time and were frequent visitors for 2-3 years, were hardly seen again. Because most of the sharks are still immature, mating is an unlikely cause for the white sharks to aggregate in False Bay. But it could represent a pupping ground. White sharks have been protected in South Africa for about 25 years since this study was done, so there has been sufficient time for adult females to mate and come back to the area to pup. Unfortunately, there have been no reported sightings of pregnant females around the island or pups, so this theory might not go anywhere. If scientists can figure out where these mature females are going, they can then try to get that area protected and do further research to investigate more about the breeding and mating part of white sharks’ life cycles.
Findings
What the data suggests is that False Bay is a seasonal feeding ground for young white sharks. The juvenile white sharks feed on the inexperienced seals at Seal Island and when the sharks reach maturity, they seem to leave the waters. Where both males and females go, that is yet to be determined.
Update (March 2021)
For the last two years, great whites have not been seen in False Bay. This was problematic for all kinds of reasons. Kock said, “White sharks are important predators that have an impact on numerous other species in False Bay and thus influence the structure and function of the False Bay ecosystem. White sharks predate on numerous species, such as fish, other sharks, seal and dolphins, and influence these species directly by reducing their numbers, and removing the weak and sick, but they also do so indirectly by influencing the prey’s behaviour. For example, Cape fur seals are bolder when white sharks aren’t around, which in turn also has knock-on effects in the ecosystem, altering the way it looks and works.”
The reason for the disappearance- scientists believe its the presence of killer whales that have driven these great white sharks away. Researchers started finding the carcasses of sevengill sharks that appeared to have been killed by orcas bitten between their pectoral fins missing livers. Then in 2017, five dead great whites were found with similar wound patterns.
Good news though one was spotted in Jan 2021. Lets wait and see if they return.
Alison Kock is a marine biologist and research manager for Cape Town’s pioneering Shark Spotters program. She has been studying great white sharks in South Africa for over 20 years. Her role is to conduct applied research on the ecology and behaviour of Cape Town’s white shark population. So far her team have answered questions relating to the ‘when’ and ‘where’ of white shark occurrence. Although she has a good understanding of why white sharks visit the island, our understanding of why they spend so much time inshore is still limited. We do know that within the inshore area there are specific hotspots, like the northern shores of False Bay, which are used significantly more than other inshore areas. But we don’t know what makes the inshore area so attractive. We have also determined that most of the sharks are juveniles and sub-adults. Her research now focuses on the ‘why’. To achieve this goal she will use a combination of direct observation, photo-identification, acoustic monitoring and animal-borne cameras. With a hypothesis-driven approach, I will investigate the drivers of white shark presence in False Bay, both environmental (such as water temperature) and biological (such as prey availability). This will enable us to model which factors relate to high shark activity and can be incorporated into shark safety strategies.