Theses and Dissertations

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Defense Date

2004

Document Type

Thesis - NSU Access Only

Degree Name

M.S. Marine Biology

Department

Oceanographic Center

First Advisor

R. Aidan Martin

Second Advisor

Edward O. Keith

Third Advisor

Mark I. Farber

Abstract

The present study identifies 12 biotic and abiotic factors that affect the frequency and success rate of predatory attacks by white sharks (Carcharodon carcharias) on Cape fur seals (Arctocephalus pusillus pusillus) at Seal Island in False Bay, South Africa. These factors (with particulars of greatest frequency and success identified in parentheses) are: time of daily seal movement about Seal Island (early morning), seal entry and exit points (southern terminus of Island), time of day in which attacks occur (early morning), total length of attacking shark (3.1 – 3.5 m), seal age class (juvenile), direction of seal locomotion relative to the Island (inbound), seal group size (single), location of predatory strikes (≤400 m from shore), water depth (26 – 30 m), abovesurface light intensity (low, - ≤200 μE) and wind direction (NN to NE). These factors support a white shark’s need for encountering its prey, remaining cryptic during approach and launching a vertical, undetected attack, resulting in a fatal initial strike. Specific factors affecting the frequency of white shark predation at Seal Island (prey age class, distance from island, and depths at which attacks range) are similar to those identified at the Farallon Islands, off California; however, most (time of year, time of day, prey group size, prey mass, location relative to the Island, distance from Island, depth, wind direction, prey movement, shark size and light intensity) are different or have not been identified. Seven factors affecting predatory success rate were identified at Seal Island (time of day, depth, prey group size, light intensity, location of attacks relative to island, wind direction, and distance from island); factors affecting white shark predatory success has not been assessed at the Farallons. Therefore, it is inappropriate to draw generalizations about white shark predatory behavior based on a single site.

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