Encounters between white sharks and Cape fur seals in a shallow channel

Abstract

This communication presents observations of predatory and non-predatory encounters between white sharks and Cape fur seals in a shallow (3–6 m) channel between Geyser Rock and Dyer Island, South Africa. Within the channel Cape fur seals raft extensively for thermo-regulatory purposes, to play, or due to terrestrial competition for space. The channel's physical environment effectively limits a white shark's approach orientation to the horizontal plane, thus inhibiting it to effectively utilize depth, and associated stealth, to capture pinnipeds. In the absence of effective camouflage, sharks may patrol this area in search of unaware, incapacitated or dead seals. Here, predator mobbing is a behavioural strategy adopted by Cape fur seals to lower predation risk. Specific benefits of mobbing may include: (a) perception advertisement to sharks; (b) intra-specific communication of a shark's locality; (c) driving the shark away from the area; (d) increased vigilance; (e) advertisement of a mobber's good health to a shark; and (f) possibly learning about a predator's behavioural capabilities by inexperienced prey. Mobbing expression is further promoted by the channel's shallow nature which enhances a seal's ability to visually detect the shark, and therefore makes it easier for a seal to evade it (reducing immediate predation risk). This environment thus promotes the widespread use of mobbing amongst Cape fur seals when confronted with a patrolling white shark.