A new study published today in the journal Biology Letters gives us a glimpse beneath the waves from the perspective of a great white shark (Carcharodon carcharias). For years, shark biologists have studied the predator-prey dynamic between the great white sharks and Cape fur seals (Arctocephalus pusillus pusillus) of Gansbaai, South Africa. Surface observations have revealed that great whites typically ambush seals from underneath during twilight – or crepuscular – hours (Jewell et al., 2019). However, a different pattern began to emerge surrounding Dryer Island Marine Reserve. Within the marine reserve, the seals exhibit lower stress hormone levels than at other sites and the typical predatory behavior by the sharks at the surface is rare (Wcisel, O’Riain, de Vos, & Chivell, 2015). One possible hypothesis is that the abundance of kelp beds surrounding Dyer Island offers the seals some protection from the great whites, similar to the way it does sea otters in California (Nicholson et al., 2018).
To test this hypothesis, a team of researchers tagged eight great whites off Dryer Island using high-resolution cameras and motion sensors AVEDs in May of 2014. The cameras were attached to the sharks using a minimally invasive stainless steel fin clamp (Chapple, Gleiss, Jewell, Wikelski, & Block, 2015). Each camera was programmed to record during daylight hours for an 8 hour duration over 24 to 72 hours before popping off the shark. A total of 28.5 hours of video data was retrieved from the eight individuals tagged. Data was retrieved at the surface via telemetry (Jewell et al., 2019).
Surprisingly, all eight great whites ventured into the kelp forest surrounding Dryer Island during their recorded footage. The amount of time they spent in the kelp forest ranged from 1 to 60% of their footage. Seven of the sharks repeatedly moved into the dense areas of kelp, four of them made direct contact with the fronds and stripes (Jewell et al., 2019). The eighth shark appeared to have left the island system after tagging, as its tag was recovered 30 miles (50 km) away on a beach in Struisbaai. Two of the white shark even encountered other white sharks while in the kelp forests.
Astonishingly, the team even captured several interactions between Cape fur seals and a great white within the dense kelp forest. Overall, the team managed to capture ten encounters between a single shark and the seals. The seals were in groups of one to three individuals, and responded to the shark’s presence by blowing bubbles, hunkering down towards the sea floor, or swimming deeper into the kelp forest (Jewell et al., 2019). The shark, in turn, responded by turning sharply, pushing directly through the kelp and increasing activity. No active predations occurred within the kelp forest during the recorded observation (Jewell et al., 2019).
This is the first study to extensively document the repeated use of kelp forests by great white sharks. It is also the first study to describe predator- prey dynamics within this habitat. Dryer Island’s kelp forests habitat may serve as a possible daytime hunting ground for great white sharks, as an alternative to their twilight ambush predation strategy. The extent to which white sharks use these habitats, and the profitability of these alternative foraging strategies have yet to be determined and require further examination.
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Featured Image Source
Jewell et al. (Videographers). (2019). Supplementary Video from Cryptic habitat use of white sharks in kelp forest revealed by animal-borne video [Video Clip]. Retrieved from https://rs.figshare.com/
Chapple, T. K., Gleiss, A. C., Jewell, O. J., Wikelski, M., & Block, B. A. (2015). Tracking sharks without teeth: a non-invasive rigid tag attachment for large predatory sharks. Animal Biotelemetry, 3(1), 14.
Jewell, O. J. D., Gleiss, A. C., Jorgensen, S. J., Andrzejaczek, S., Moxley, J. H., Beatty, S. J., … Chapple, T. K. (2019). Cryptic habitat use of white sharks in kelp forest revealed by animal-borne video. Biology Letters, 15(4), 20190085.
Nicholson, T. E., Mayer, K. A., Staedler, M. M., Fujii, J. A., Murray, M. J., Johnson, A. B., … & Van Houtan, K. S. (2018). Gaps in kelp cover may threaten the recovery of California sea otters. Ecography, 41(11), 1751-1762.
Wcisel, M., O’Riain, M. J., de Vos, A., & Chivell, W. (2015). The role of refugia in reducing predation risk for Cape fur seals by white sharks. Behavioral ecology and sociobiology, 69(1), 127-138.