A predator-prey behavioural game between white sharks and Cape fur seals.
Laroche, Karl*,1, Kock, Alison2, Dill, Lawrence1, 1 Simon Fraser University, Burnaby, BC, Canada2 University of Cape Town, Cape Town, WP, South Africa
ABSTRACT- We present evidence of a predator-prey game occurring between white sharks (Carcharodon carcharias) and Cape fur seals (Arctocephalus pusillus pusillus) around an island seal colony in South Africa. Sharks patrol waters adjacent to the island and attack seals as they move to or from it. The sharks′ main hunting tactic appears to involve swimming at depth and ambushing seals at the surface, a visual technique which requires at least some amount of light. As a result, a shark′s ability to detect and catch a seal is likely at its lowest overnight. Correspondingly, seals most often depart immediately after sunset in very large surface-porpoising groups from the South of the island. They also return over the course of the entire night, although from all directions and in much smaller groups, often hugging the ocean floor. These tactics appear adapted to minimize predation risk as a function of differing bathymetry around the island, varying light levels, and achievable group size (large while departing, small when returning). Shark behaviour was also influenced by light level, as they were present in the greatest density shortly before sunrise, with numbers decreasing over the day, reaching a low after sunset which was maintained through most of the night. Mean shark swimming depth was approximately 13m around the entire island, regardless of bottom depth, and they spent the most time to the South, where the majority of seal movement occurred. Thus, contrary to the seals, shark strategies appear adapted to maximize predation opportunities as a function of seal presence and light levels. Data were used to calculate a lethality index for various times and areas around the island from a seal′s perspective. Results indicate that a seal′s chance of being attacked is approximately equal in all areas of the island despite differing bathymetry and shark presence, although this varies dramatically depending on time of day. We are in the process of completing similar calculations for the sharks, and predict similar results, where, at any given time, an individual animal′s chances of killing a seal would be equal around the entire island, but with most opportunities occurring early in the morning, when seal movement is maximal and light levels are optimal.
Key words: game theory, predator-prey, Carcharodon carcharias, Arctocephalus pusillus pusillus
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