Picky Eaters
Updated: Apr 27, 2021
Despite what you might have seen in films like Jaws, great white sharks (Carcharodon carcharias) are not indiscriminate eaters! White sharks certainly do not like the taste of human flesh. They will NOT even eat sheep or cow carcasses and cannot be attracted to boats with their blood either. In fact, great whites are relatively picky eaters...
Great white sharks primarily hunt large fishes, other smaller sharks, rays, sea turtles and invertebrates, like octopi, squid and shellfish. They also scavenge on floating carcasses, especially whales. When they reach a critical size of 3 m in total length, white sharks undergo an "ontogenetic diet shift". This means that at this phase in their life, they naturally shift their diet to also hunt marine mammals, such as seals and dolphins. Therefore, white sharks are described as having a "generalist feeding strategy" meaning that they target more than one type of prey and are not particularly "specialist" in what marine animals they eat.
Ontogeny - development of an organism, from ὄντος (ontos) Ancient Greek = 'a being'
White sharks hunt using their acute sense of smell to detect prey over long distances. After tracking a food source and moving in closer to it, white sharks rely on their vision to inspect the prey. The shark will swim at depth, where their dark colouration will make them less visible through the water and will sneak underneath the target, before rushing vertically to strike the prey at the surface. This is known as an "ambush hunting strategy".
Visual and olfactory attractants can be used to lure sharks towards a boat. Baits are chunks of either frozen or fresh fish products, displayed at the end of a steel bait line- like fishing without a hook. Bait is a relatively short-range, primarily visual attractant, which can be used to draw a shark in closer for observation. Comparatively, chum is a mixture of fish blood and oils and sea water, spread into the ocean as a long-range olfactory attractant. The smell of chum can draw sharks towards a vessel from much longer distances. However, the effectiveness of these attractants depends on what ingredients you use (Becerril‑García et al, 2020).
A recent study sought to better understand which prey white sharks favour, by testing how easily they were drawn to a boat when using different attractants. The scientists studied white sharks at the Guadalupe Island Biosphere Reserve, off the coast of North America, where white sharks are seen year-round, with a peak in August. The researchers tested 3 different types of attractants: 1. frozen tuna bait 2. frozen tuna bait + fish tissue and blood chum and 3. fresh fish bait which had never been frozen (Becerril‑García et al, 2020).
They found that type of bait did not significantly affect the rate at which sharks were attracted to the boat (number of sharks sighted per hour), BUT they found that the type of attractant used affected the way that great white sharks behaved around the boat.
The researchers reported that when using frozen bait without chum, fewer aggressive behaviours, like horizontal and vertical strikes upon the bait, were recorded. Comparatively, when using fresh bait or chum, aggressive behaviours were observed in higher frequency (seen in red on the diagrams) (Becerril‑García et al, 2020) .
Scientists often visualise the succession of behaviours displayed by an individual animal on a diagram called an "ethogram". These images are designed to show how different behaviours go together and to summarise complex interactions. The researchers produced 3 ethograms for each attractant type:
a- frozen bait
b- frozen bait + chum
c- fresh bait
The researchers found that when using frozen bait only (diagram a), there was no defined pattern in the behaviours that the sharks displayed.
Comparatively, when using frozen bait with chum (diagram b), they found that the sharks often performed a vertical (VS) or horizontal strike (HS) on the bait and they were commonly successful in taking the bait and eating it. Also, when using fresh bait (diagram c), they found that horizontal strikes (HA) were commonly followed by close inspection of the bait (CLI).
They concluded that the use of chum and fresh bait elicited a feeding response, whereby the sharks behaved more violently when they
sensed a valuable food source. On the other hand, when using frozen baits, the smell and appearance of the food source was not as attractive. Therefore, the sharks displayed more behaviours related to curiosity, as opposed to aggression (Becerril‑García et al, 2020).
What was of particular importance, is that the research group also conducted an analysis of how the behaviours of the white sharks changed in response to baiting and chumming. When an animal learns to associate something with a food reward, this is known as "conditioning". For example, your pet cat or dog might begin behaving hungrily like clockwork if you feed them at a consistent time. There has been some concern that white sharks may be conditioned to associate boats with food rewards if they regularly encounter baits and chum used by shark diving companies. People have been worried that this could lead to increasing aggressive reactions from sharks outside of a research or tourism context and a subsequent rise in the number of unprovoked shark attacks. However, these scientists found that, whilst studying these sharks over several months, there was no evidence that they were being conditioned to the baiting and/or chumming activities. This suggests that white shark tourism and use of attractants for scientific research, should not be having an adverse affects on the behaviour of the great white sharks (Becerril‑García et al, 2020).
To learn more about how baiting and chumming affects great white sharks, you can check out Cage Diving & Shark Attack... Is There Cause for Concern? and How Does Cage Diving Affect Great Whites?.
References
Becerril‑García EE, Hoyos‑Padilla EM, Micarelli P, Galván‑Magaña F & Sperone E (2020). Behavioural responses of white sharks to specific baits during cage diving ecotourism. Scientific Reports, 10:1, DOI: 10.1038/s41598-020-67947-x. Access online.
By Sophie A. Maycock for SharkSpeak.