AVED Technology Reveals Cryptic Habitat Use of Kelp Forests by Great White Sharks

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).

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Benjamin, S. (Photographer). (2013). Kelp Seal [Digital Image]. Retrieved from https://sealsnorkeling.com/

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).

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Fig. 2 (a) A 2.7-m TL male with the CATS Diary tag already released, but the clamp still attached—see the two ends of the clamp arms. (b) First, the magnesium sleeve corrodes and one clamp arm falls away. (c) Then the clamp slides up the fin and (d) falls off completely leaving only slight abrasions on the fin (Chapple, Gleiss, Jewell, Wikelski, & Block, 2015).

 

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.

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Jewell et al. (Authors). (2019).  Figure S1 from Cryptic habitat use of white sharks in kelp forest revealed by animal-borne video [Digital Image]. Retrieved from https://rs.figshare.com/

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).

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Figure 2. Still-picture frames and a 10-min sub-sample of pseudo-track and data log of a white shark encountering Cape fur seals in kelp canopy. (a) AVED footage of a white shark (Shark 5) encountering Cape fur seals. (b) The seals respond to the shark’s presence by hunkering to the sea floor and blowing bubble streams as the shark passes overhead or swimming further into the kelp. (c) The shark swims through the bubbles, (d) then through kelp. (e) The shark pursues the seals, making contact with dense kelp fronds at several points and pushing through them. (f) At least three Cape fur seals (indicated by red arrows) are seen taking refuge in the canopy area of the kelp forest fronds and successfully avoiding the white shark. (g) A 10-min sub-sample of Shark 5’s dead-reckoned pseudo-track within dense kelp forest, including encounters with seals. (h) Time-series recordings of turning angle (°), activity (ODBA, g) and depth (m) for the corresponding time period; background colors represent low (teal), high (green) and no (white) density kelp observed.  (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.

New Project
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/

The new Ocean For Sharks Shop is open! There’s handmade ocean inspired plush animals, canvas paintings, and of course my children’s book, Winifred the Wondrous Whale Shark, available in print and PDF. Be sure to stop by. Remember proceeds benefit shark research and conservation with a donation to Project AWARE!

Conservation legislation needs public support in order to become law and help protect the environment and wildlife. Tell your representatives that you care about environmental and wildlife conservation.  It only takes a moment to make a change that will last a lifetime. Until next time finactics!

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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/

 

Literature Cited

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 Biotelemetry3(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. Ecography41(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 sociobiology69(1), 127-138.

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4 thoughts on “AVED Technology Reveals Cryptic Habitat Use of Kelp Forests by Great White Sharks

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  1. Another awesome piece, Amanda! 🤙🏽 I grew up surfing in Santa Cruz, California (a Marine Sanctuary). Tons of kelp beds, and protected wildlife. When I was younger I used to worry about the “men in white suits” (🦈)…now, I wonder if the eerie kelp beds I sat on top of while waiting for waves were actually protecting me? 🏄🏼‍♂️

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    1. There’s still a question as to whether white sharks use the kelp forests everywhere, or if there’s something about this particular region that makes this behavior exceptional, like their tendency to full body breach here but rarely do in California or Australia. More research is needed!

      Liked by 1 person

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