When we think of great white hunting behavior, the iconic image that comes to mind is that of a shark breaching from the water, a seal clamped helplessly in its jaws. Great whites represent the archetypal ambush hunter to many of us, but a new study recently published in the scientific journal Functional Ecology suggests there may be more to their hunting techniques than initially meets the eye.
Research for the paper took place in Gaansbai, a scenic coastal town near the southern-most tip of South Africa. Renowned as the white-shark-diving capital of the world, and with one of the highest known densities of the species, Gaansbai was an obvious choice.
With the aim for the research project being to better understand the hunting behavior of Gaansbai’s white-shark population, the team attached acoustic tags to 15 white sharks. In order to impact the animals as little as possible, the tags were deployed at the base of the dorsal rather than on the fragile fin itself. The sharks were purposely chosen across a wide spectrum of sizes and sexes, in order to provide researchers with the most comprehensive insight possible.
Varied Hunting Methods
According to contributing biologist Alison Towner, “the sharks were actively tracked throughout all seasons at the scale of meters, from a small research vessel between 2010 and 2014. We then used powerful statistical modeling tools called Hidden Markov Models (HMMs) to analyze almost 500 hours of shark tracks.” This marks the first time that HMMs have been used to study shark movements, allowing researchers to establish when the sharks’ behavior changed, and in some cases, why.
Typically, predators employ one of two hunting methods. They either lie in wait for prey to come to them (in the manner of crocodiles), or actively patrol for their food (like cheetahs). The results of the Gaansbai research study show that great whites may do both, switching between the two hunting behaviors depending on a series of external factors, including the time of day, the activities of local shark-diving boats, and the gender of the shark itself.
The results showed differences between the behavior of male and female sharks, with females “less likely to display restricted movements than males.” The findings also suggest that specific sharks may have their own preferences when it comes to hunting techniques, with individuals using the same tactics over and over again. This groundbreaking discovery is a first, confirming the status of the great white as one of the ocean’s most versatile predators. The research also hints than human activity may have a greater impact on the sharks than previously understood.
Specifically, Dyer Island Conservation Trust founder Wilfrid Chivell comments that the research “shows [that the sharks] use a more costly swimming mode around cage-diving boats, yet after diving activities cease for the day, the sharks resumed their natural hunting state whilst in the area.” It is not yet clear why the boats affect the sharks’ behavior, whether it’s related to the fish oil they use to attract the sharks, or whether the boats’ presence itself is enough to affect the relationship between shark and seal.
Chivell underlines the importance of the study in shedding more light upon the effects of Gaansbai’s cage-diving business, saying “it is known that chumming does not impact migratory behavior of white sharks, but less is known about the short-term behavior when they visit regions along our coast. Studies are often limited to observations from anchored boats. This is [therefore] one of the most comprehensive studies on the natural behavior of white sharks in Gansbaai.”
Understanding more about the area’s sharks is important, because as Towner points out, “[their] behavior ultimately creates different pressures on other trophic levels, which is of particular concern in a complex and vulnerable marine system such as Dyer Island.” In other words, as the region’s key apex predator, changes in the white sharks’ hunting behavior have the power to affect other species further down the food chain and ultimately to impact the balance of the ecosystem as a whole. It’s hoped that the results of the recent study will help inform the ongoing protection not only of the sharks themselves, but also of the other species that share their home.