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Is it good-looking or does it smell good? Preliminary observations about great white shark's discriminatory patterns

  • Centro Studi Squali - Sharks Studies Centre - Scientific Institute


The White Shark Carcharodon carcharias L. is a top predator worldwide distributed. Its high survival and adaptation capacity is related with high developed senses, used to efficiently interact with a target. During daily activities of cage diving in the Marine Reserve of Dyer Island (South Africa) the behavioural patterns of the white sharks in front of different targets were observed. Previous studies showed that in an unnatural situation, as the cage diving is, the white shark displays a curiosity approach and it long investigate the situation before an attack. The aim of the present study was to understand if a white shark, in front of a still floating decoy similar to the natural prey (Cape fur seal, Arctocephalus pusillus pusillus) and a bloody piece of tuna fish, makes a choice based upon the sigh or the smell. The preliminary data analysis showed that the choice of the target could be related to size (and consequently) age of sharks. We also analyzed the type of approach (vertical, horizontal or diagonal) and observed that in this case the choice could be related to light and weather conditions. A correlation between type of approach and type of target (decoy or bait) was also observed: usually vertical tactics were more often used in presence of sealshaped decoy, while horizontal tactics were showed in presence of tuna bait. Our observations suggest that these patterns appear to be adapted for exploiting a challenging suite of surface-dwelling prey species and may be the basis of a speculative hunting strategy wherein individual sacrifice much of the possibility of identifying a potential prey item in exchange for an increased chance of capture.
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... Martin et al. [3] observed that Sea Island white sharks appear to select the age class of Cape fur seal Arctocephalus pusillus pusillus [19], a group with a defined size and a clear direction of movement, as well as a choice of hunting during times and in locations that maximize their probability for predatory success. Regarding the ontogenetic shift, French et al. [20] suggested that gender and individual specialization are also key drivers in white sharks' ecological variation, and that they remain important throughout ontogeny; in fact, individuals may learn a variety of different movement tactics for encountering and catching prey, and they may develop a preference for a particular tactic based on their experiences [21,22]. The individual surface behaviour of white sharks in the presence of a bait, in the same manner as their predatory [3,23,24] and social [25] behaviour, is not a simple stimulus response reflex, but rather a complex tactical situation with plastic responses [24]. ...
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Between the years 2008 and 2013, six annual research expeditions were carried out at Dyer Island (Gansbaai, South Africa) to study the surface behaviour of white sharks in the presence of two passive prey: tuna bait and a seal-shaped decoy. Sightings were performed from a commercial cage-diving boat over 247 h; 250 different white sharks, with a mean total length (TL) of 308 cm, were observed. Of these, 166 performed at least one or more interactions, for a total of 240 interactions with bait and the seal-shaped decoy. In Gansbaai, there is a population of transient white sharks consisting mainly of immature specimens throughout the year. Both mature and immature sharks preferred to prey on the seal-shaped decoy, probably due to the dietary shift that occurs in white sharks whose TL varies between 200 cm and 340 cm. As it is widely confirmed that white sharks change their diet from a predominantly piscivorous juvenile diet to a mature marine mammalian diet, it is possible that Gansbaai may be a hunting training area and that sharks show a discriminate food choice, a strategy that was adopted by the majority of specimens thanks to their ability to visualize energetically richer prey, after having been attracted by the odorous source represented by the tuna bait.
... Like VI, this behavioural unit may be a visual mechanism of prey selection. Micarelli et al. (2009) suggested that the use of TSt rather than VI is correlated with weather conditions: In particular , they reported that TSt was more often displayed on days with semi-cloudy and cloudless skies, whereas VI was more frequent on days with cloudy skies. Martin (2003) previously described the Spy Hop (SpH) behaviour and suggested that it could represent an investigative visual approach to the bait above the surface. ...
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