Journal of the Marine Biological Association of the United Kingdom



Research Article

Predatory behaviour of white sharks (Carcharodon carcharias) at Seal Island, South Africa


R. Aidan  Martin a1a2c1, Neil  Hammerschlag a2a3, Ralph S.  Collier a4 and Chris  Fallows a5
a1 Fish Museum, Zoology Department, University of British Columbia, 6270 University Boulevard, Vancouver, BC, V6T 1Z4, Canada
a2 ReefQuest Centre for Shark Research, PO Box 48561, 595 Burrard Street, Vancouver, BC, V7X 1A3, Canada
a3 Pew Institute for Ocean Science, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL 33149, USA
a4 Shark Research Committee, PO Box 3483, Van Nuys, CA 97407, USA
a5 Apex Expeditions, 14 Thibault Walk, Marina Da Gama, 7945 Cape Town, South Africa

Article author query
martin ra   [Medline] [Google Scholar
hammerschlag n   [Medline] [Google Scholar
collier rs   [Medline] [Google Scholar
fallows c   [Medline] [Google Scholar

Abstract

Between 1997 and 2003, there were 2088 natural predations by white sharks (Carcharodon carcharias) on Cape fur seals (Arctocephalus pusillus pusillus) and 121 strikes on towed seal-shaped decoys were documented from observation vessels at Seal Island, South Africa. White sharks at Seal Island appear to selectively target lone, incoming young of the year Cape fur seals at or near the surface. Most attacks lasted <1 min and consisted of a single breach, with predatory success rate decreasing rapidly with increasing duration and number of subsequent breaches. A white shark predatory ethogram, composed of four phases and 20 behavioural units, is presented, including four varieties of initial strike and 11 subsequent behaviour units not previously defined in the literature. Behaviour units scored from 210 predatory attacks revealed that, for both successful and unsuccessful attacks, Polaris Breach was the most commonly employed initial strike, while Surface Lunge was the most frequent second event, closely followed by Lateral Snap. Examination of video footage, still images, and tooth impressions in decoys indicated that white sharks at Seal Island bite prey obliquely using their anterolateral teeth via a sudden lateral snap of the jaws and not perpendicularly with their anterior teeth, as previously supposed. Analysis of white shark upper tooth morphology and spacing suggest the reversed intermediate teeth of white sharks occur at the strongest part of the jaw and produce the largest wound. White shark predatory success at Seal Island is greatest (55%) within one hour of sunrise and decreases rapidly with increasing ambient light; the sharks cease active predation on seals when success rate drops to ±40%; this is the first evidence of cessation of foraging at unproductive times by any predatory fish. At Seal Island, white shark predatory success is significantly lower at locations where frequency of predation is highest, suggesting that white sharks may launch suboptimal strikes in areas of greatest intraspecific competition; this is the first evidence of social influence on predation in any elasmobranch. Idiosyncratic predatory behaviours and elevated success rates of known individual white sharks at Seal Island suggest some degree of trial-and-error learning. A hypothetical decision tree is proposed that models predatory behaviour of white sharks attacking Cape fur seals at the surface.

(Received May 30 2005)
(Accepted September 8 2005)


Correspondence:
c1 e-mail: ram@elasmo-research.org